GeezerVison

1. Introduction

This section is not normative.

Certain classes of vulnerabilities occur when a web application takes a value from an attacker-controlled source (e.g. the document URL parameter, or postMessage channel) and passes that value, without appropriate sanitization to one of the injection sinks - various Web API functions with powerful capabilities.

These types of issues are traditionally difficult to prevent. Applications commonly call those injection sinks with attacker-controlled values without authors realizing it, since it’s not clear if the input was attacker-controlled when invoking the injection sink. Due to the dynamic nature of JavaScript it’s also difficult to ascertain that such pattern is not present in a given program. It is often missed during manual code reviews, and automated code analysis. As an example, if aString contains untrusted data, foo[bar] = aString is a statement that potentially can trigger a vulnerability, depending on a value of foo and bar.

This document focuses on preventing DOM-Based Cross-Site Scripting that occurs when attacker-controlled data reaches § 2.1.1 DOM XSS injection sinks, as that eventually causes execution of the script payload controlled by the attacker. DOM XSS is prevalent in the web applications as there are over 60 different injection sinks (e.g. Element.innerHTML, or Location.href setters).

This document defines Trusted Types - an API that allows applications to lock down injection sinks to only accept non-spoofable, typed values in place of strings. These values can in turn only be created from application-defined policies, allowing the authors to define rules guarding dangerous APIs, reducing the attack surface to small, isolated parts of the web application codebase, which are substantially easier to safeguard, monitor and review.

1.1. Goals

• Minimize the likelihood of client-side vulnerabilities that occur when calling powerful Web APIs with untrusted data - for example, minimize the likelihood of DOM XSS.

• Encourage a design in which security decisions are encapsulated within a small part of the application.

• Reduce security review surface for complex web application codebases.

• Allow the usability-preserving detection of vulnerabilities similar to how regular programming errors are detected and surfaced to the developers, with the assist of dynamic and static analysis tools.

1.2. Non-goals

• Prevent, or mitigate the result of injections into server-side generated markup, in specific reflections into the body of the scripts running in a document. To address server-side XSS vectors, we recommend existing solutions like templating systems or CSP script-src.

• Address resource confinement, e.g. to prevent data exfiltration, or connecting to external sources via [Fetch].

• Control subresource loading. Trusted Types aim to allow the authors to control loading resources that can script the current document, but not other subresources.

• Prevent cross-origin JavaScript execution (for example, Trusted Types don’t guard loading new documents with JavaScript code via data: URLs).

• Prevent malicious authors of the web application’s JavaScript code from being able to bypass the restrictions; attempting to protect against malicious authors would result in an overly complex and not-practical design.

1.3. Use cases

• An author maintains a complex web application written in a framework that uses a secure templating system to generate the UI components. The application also depends on 3rd party client-side libraries that perform auxiliary tasks (e.g. analytics, performance monitoring). To ensure that none of these components introduce DOM XSS vulnerabilities, author defines a Trusted Type policy in the templating library and enables the enforcement for the § 2.1.1 DOM XSS injection sinks.

• A website uses § 2.1.1 DOM XSS injection sinks. The website-developer adds trusted types to it and monitors violations by using the Content-Security-Policy-Report-Only header field. Violations are iteratively fixed by refactoring the code to use only safe methods. Afterwards, no § 2.1.1 DOM XSS injection sinks are called anymore. Hence, no trusted types are required anymore. The developer switches the report-only mode off and disables trusted type policies with the trusted-types and the require-trusted-types-for directives. The website’s functionality was never impaired during the refactorings.

• A large team maintains a complex client-side application. They create a number of Trusted Types policies that satisfy the security requirements for the application. The team consolidates the policy implementations and the safe abstractions that use them in a small number of heavily reviewed files and requires extra approval for commits that affect these files.

  The need to create trusted values to affect injection sinks, combined with additional scrutiny on changes that affect policy code, incents developers to use safe abstractions instead of writing ad-hoc string composition code when interacting with § 2.1.1 DOM XSS injection sinks.

  When considering the risk of DOM XSS, security auditors find a small attack surface; they focus on the small amount of code that crafts the CSP header and provides the safe abstractions, and ignore the bulk of client-side application code.

• An existing web application interacts with the DOM mostly using XSS-safe patterns (i.e. without using § 2.1.1 DOM XSS injection sinks). In a few places, however, it resorts to using risky patterns like loading additional script using JSONP, calling into innerHTML or eval.

  Review finds that those places do not cause XSS (e.g. because user-controlled data is not part of the input to those sinks), but it’s hard to migrate the application off using these patterns.

  As such, CSP cannot be enforced on this application (without resorting to an unsafe version using 'unsafe-eval' 'unsafe-inline'). Additionally, it’s possible some codebase with DOM XSS flaws was not included in a review, or will be introduced in the future.

  To address this risk, the author converts the reviewed parts to using Trusted Types, and enables Trusted Type enforcement. Additional places using the injection sinks, should they exist in the future, are correctly blocked and reported.

• A security team is tasked with assuring that the client-side heavy application code does not contain XSS vulnerabilities. Since the server side code is homogeneous (it’s mostly an API backend), and the application enforces Trusted Types, the review only focuses on the Trusted Type policies and their rules. Later on the reviewed policy names are allowed in the 'trusted-types' CSP directive, safe for the developers to use.

  Any additional code, including the code of often-changing dependencies, can be excluded from the review, unless it creates a Trusted Type policy. Without it, the code cannot cause a DOM XSS.

2. Framework

2.1. Injection sinks

This section is not normative.

An injection sink is a powerful Web API function that should only be called with trusted, validated or appropriately sanitized input. Calling the injection sink with attacker-controlled (i.e. injected) inputs has undesired consequences and is considered a security vulnerability.

Note: The exact list of injection sinks covered by this document is defined in § 4 Integrations.

It’s difficult to determine if a given application contains such a vulnerability (e.g. if it is vulnerable to DOM XSS) only by analyzing the invocations of injection sinks, as their inputs (usually strings) do not carry the information about their provenance. For example, while the application might intentionally call eval() with dynamically created inputs (e.g. for code obfuscation purposes), calling eval() on strings supplied by the attacker is definitely a security vulnerability - but it’s not easy to distinguish one from the other.

This document organizes the injection sinks into groups, based on the capabilities that sinks in a given group have. Enforcement for groups is controlled via trusted-types-sink-group values.

2.1.1. DOM XSS injection sinks

This section is not normative.

DOM XSS injection sinks evaluate an input string value in a way that could result in DOM XSS if that value is untrusted.

Examples include:

• Setters for Element attributes that accept a URL of the code to load like HTMLScriptElement.src,

• Setters for Element attributes that accept a code to execute like HTMLScriptElement.text,

• Functions that execute code directly like eval,

• Navigation to 'javascript:' URLs.

Since HTML parsers can create arbitrary elements, including scripts, and set arbitrary attributes, DOM XSS injection sinks also include HTML parsing sinks:

• Functions that parse & insert HTML strings into the document like Element.innerHTML, ShadowRoot.innerHTML, and Element.outerHTML setters, or Document.write.

• Functions that create a new same-origin Document with caller-controlled markup like parseFromString().

Guarding DOM XSS injection sinks is controlled by the trusted-types-sink-group named 'script'.

2.2. Trusted Types

To allow the authors to control values reaching injection sinks, we introduce § 2.2 Trusted Types. The following list of Trusted Types indicating that a given value is trusted by the authors to be used with an injection sink in a certain context.

Note: Trusted in this context signifies the fact that the application author is confident that a given value can be safely used with an injection sink - she trusts it does not introduce a vulnerability. That does not imply that the value is indeed safe.

Note: This allows the authors to specify the intention when creating a given value, and the user agents to introduce checks based on the type of such value to preserve the authors' intent. For example, if authors intend a value to be used as an HTML snippet, an attempt to load a script from that value would fail.

Note: All Trusted Types wrap over an immutable string, specified when the objects are created. These objects are unforgeable in a sense that there is no JavaScript-exposed way to replace the inner string value of a given object - it’s stored in an internal slot with no setter exposed.

Note: All Trusted Types stringifiers return the inner string value. This makes it easy to incrementally migrate the application code into using Trusted Types in place of DOM strings (it’s possible to start producing types in parts of the application, while still using and accepting strings in other parts of the codebase). In that sense, Trusted Types are backwards-compatible with the regular DOM APIs.

2.2.1. TrustedHTML

The TrustedHTML interface represents a string that a developer can confidently insert into an injection sink that will render it as HTML. These objects are immutable wrappers around a string, constructed via a TrustedTypePolicy’s createHTML method.

[Exposed=(Window,Worker)]
interface TrustedHTML {
  stringifier;
  DOMString toJSON();
};

TrustedHTML objects have an associated string data. The value is set when the object is created, and will never change during its lifetime.

toJSON() method steps and the stringification behavior steps of a TrustedHTML object are to return the associated data value.

2.2.2. TrustedScript

The TrustedScript interface represents a string with an uncompiled script body that a developer can confidently pass into an injection sink that might lead to executing that script. These objects are immutable wrappers around a string, constructed via a TrustedTypePolicy’s createScript method.

[Exposed=(Window,Worker)]
interface TrustedScript {
  stringifier;
  DOMString toJSON();
};

TrustedScript objects have an associated string data. The value is set when the object is created, and will never change during its lifetime.

toJSON() method steps and the stringification behavior steps of a TrustedScript object are to return the associated data value.

2.2.3. TrustedScriptURL

The TrustedScriptURL interface represents a string that a developer can confidently pass into an injection sink that will parse it as a URL of an external script resource. These objects are immutable wrappers around a string, constructed via a TrustedTypePolicy’s createScriptURL method.

[Exposed=(Window,Worker)]
interface TrustedScriptURL {
  stringifier;
  USVString toJSON();
};

TrustedScriptURL objects have an associated string data. The value is set when the object is created, and will never change during its lifetime.

toJSON() method steps and the stringification behavior steps of a TrustedScriptURL object are to return the associated data value.

2.2.4. TrustedHTMLParserOptions

The TrustedHTMLParserOptions interface represents options that a developer can confidently pass into an injection sink that will parse HTML. These objects are immutable wrappers around a SetHTMLUnsafeOptions dictionary, constructed via a TrustedTypePolicy’s createParserOptions method.

[Exposed=Window]
interface TrustedHTMLParserOptions {
};

TrustedHTMLParserOptions objects have an associated boolean runScripts and an associated SanitizerConfig-or-null sanitizer configuration. These values are set when the object is created, and will never change during its lifetime.

2.3. Policies

Trusted Types can only be created via user-defined and immutable policies that define rules for converting a string into a given Trusted Type object. Policies allows the authors to specify custom, programmatic rules that Trusted Types must adhere to.

const sanitizingPolicy = trustedTypes.createPolicy('sanitize-html', {
  createHTML: (input) => myTrustedSanitizer(input, { superSafe: 'ok'}),
});

myDiv.innerHTML = sanitizingPolicy.createHTML(untrustedValue);

Note: Trusted Type objects wrap values that are explicitly trusted by the author. As such, creating a Trusted Type object instance becomes a de facto injection sink, and hence code that creates a Trusted Type instances is security-critical. To allow for strict control over Trusted Type object creation we don’t expose the constructors of those directly, but require authors to create them via policies.

Multiple policies can be created in a given Realm, allowing the applications to define different rules for different parts of the codebase.

const cdnScriptsPolicy = trustedTypes.createPolicy('cdn-scripts', {
  createScriptURL(url) {
    const parsed = new URL(url, document.baseURI);
    if (parsed.origin == 'https://mycdn.example') {
      return url;
    }
    throw new TypeError('invalid URL');
  },
});

myLibrary.init({policy: cdnScriptsPolicy});

Note: Trusted Type objects can only be created via policies. If enforcement is enabled, only the policy code can trigger an action of an injection sink and hence call-sites of the policies' create* functions are the only security-sensitive code in the entire program with regards to the actions of the injection sinks. Only this typically small subset of the entire code base needs to be security-reviewed - there’s no need to monitor or review the injection sinks themselves, as User Agents enforce that those sinks will only accept matching Trusted Type objects, and these in turn can only be created via policies.

The createPolicy function returns a policy object which create* functions will create Trusted Type objects after applying the policy rules.

Note: While it’s safe to freely use a policy that sanitizes its input anywhere in the application, there might be a need to create lax policies to be used internally, and only to be called with author-controlled input. For example, a client-side HTML templating library, an HTML sanitizer library, or a JS asynchronous code plugin loading subsystem each will likely need full control over HTML or URLs. The API design facilitates that - each policy may only be used if the callsite can obtain a reference to the policy (a return value from createPolicy()). As such, policy references can be treated as capabilities, access to which can be controlled using JavaScript techniques (e.g. via closures, internal function variables, or modules).

(async function renderFootnote() {
  const unsafePolicy = trustedTypes.createPolicy('html', {
    createHTML: input => input,
  });
  const footnoteContent = await fetch('/footnote.html').then(r => r.text());
  footNote.innerHTML = unsafePolicy.createHTML(footnoteContent);
})();

2.3.1. Using together with a sanitizer

While trusted types are built in a way that allows custom sanitizers to be used, the createParserOptions also allows for using them as a way to enforce using the built-in sanitizer API.

(async function renderFootnote() {
  const footnoteContent = await fetch('/footnote.html').then(r => r.text());
  const policy = trustedTypes.createPolicy('html', {
    createParserOptions: options => {
      const sanitizer = new Sanitizer(options.sanitizer?.get() ?? {});
      sanitizer.removeUnsafe();
      return {
        sanitizer,
      };
    }
  });
  footNote.setHTMLUnsafe(footnoteContent, policy.createParserOptions({}));
})();

Note: When enforcing Trusted Types on HTML injection sinks that parse markup (such as setHTMLUnsafe() or innerHTML), a TrustedHTMLParserOptions object (obtained from createParserOptions or provided by a default policy) allows raw strings to be parsed directly, without requiring createHTML or a TrustedHTML instance. Policy authors typically configure a sanitizer in createParserOptions so that untrusted markup is sanitized during parsing. For string-only injection sinks that do not support parser options (such as srcdoc or document.write()), or for XML documents, raw strings must still be converted via createHTML.

2.3.2. TrustedTypePolicyFactory

TrustedTypePolicyFactory creates policies and verifies that Trusted Type object instances were created via one of the policies.

Note: This factory object is exposed to JavaScript through trustedTypes property on the global object - see § 4.1.1 Extensions to the WindowOrWorkerGlobalScope interface.

[Exposed=(Window,Worker)] interface TrustedTypePolicyFactory {
    TrustedTypePolicy createPolicy(
        DOMString policyName, optional TrustedTypePolicyOptions policyOptions = {});
    boolean isHTML(any value);
    boolean isScript(any value);
    boolean isScriptURL(any value);
    readonly attribute TrustedHTML emptyHTML;
    readonly attribute TrustedScript emptyScript;
    DOMString? getAttributeType(
        DOMString tagName,
        DOMString attribute,
        optional DOMString? elementNs = "",
        optional DOMString? attrNs = "");
    DOMString? getPropertyType(
        DOMString tagName,
        DOMString property,
        optional DOMString? elementNs = "");
    readonly attribute TrustedTypePolicy? defaultPolicy;
};

A TrustedTypePolicyFactory object has an associated TrustedTypePolicy default policy. Its value is initially null.

A TrustedTypePolicyFactory object has an associated ordered set of strings created policy names. Its value is initially « ».

anElement.innerHTML = trustedTypes.emptyHTML; // no need to create a policy

// With native Trusted Types support eval(trustedTypes.emptyScript) will execute and return falsy undefined.
// Without it, eval(trustedTypes.emptyScript) will return a truthy Object.
const supportsTS = !eval(trustedTypes.emptyScript);

eval(supportsTS ? myTrustedScriptObj : myTrustedScriptObj.toString());

trustedTypes.defaultPolicy === null;  // true
const dp = trustedTypes.createPolicy('default', {});
trustedTypes.defaultPolicy === dp;  // true

2.3.3. TrustedTypePolicy

Policy objects implement a TrustedTypePolicy interface and define a group of functions creating Trusted Type objects. Each of the create* functions converts a string value to a given Trusted Type variant, or throws a TypeError if a conversion of a given value is disallowed.

[Exposed=(Window,Worker)]
interface TrustedTypePolicy {
  readonly attribute DOMString name;
  TrustedHTML createHTML(DOMString input, any... arguments);
  TrustedScript createScript(DOMString input, any... arguments);
  TrustedScriptURL createScriptURL(DOMString input, any... arguments);
  [Exposed=Window] TrustedHTMLParserOptions createParserOptions(optional SetHTMLUnsafeOptions options = {}, any... arguments);
};

Each policy has a name.

Each TrustedTypePolicy object has an associated TrustedTypePolicyOptions options object, describing the actual behavior of the policy.

2.3.4. TrustedTypePolicyOptions

This dictionary holds author-defined functions for converting string values into trusted values. These functions do not create Trusted Type object instances directly - this behavior is provided by TrustedTypePolicy.

dictionary TrustedTypePolicyOptions {
   CreateHTMLCallback createHTML;
   CreateScriptCallback createScript;
   CreateScriptURLCallback createScriptURL;
   CreateParserOptionsCallback createParserOptions;
};
callback CreateHTMLCallback = DOMString? (DOMString input, any... arguments);
callback CreateScriptCallback = DOMString? (DOMString input, any... arguments);
callback CreateScriptURLCallback = USVString? (DOMString input, any... arguments);
callback CreateParserOptionsCallback = SetHTMLUnsafeOptions? (SetHTMLUnsafeOptions options, any... arguments);

2.3.5. Default policy

This section is not normative.

One of the policies, the policy with a name "default", is special; When an injection sink is passed a string (instead of a Trusted Type object), this policy will be implicitly called by the user agent with the non trusted string value, Trusted Type of the sink and the sink type, respectively.

This allows the application to define a fallback behavior to use instead of causing a violation. The intention is to allow the applications to recover from an unexpected data flow, and sanitize the potentially attacker-controlled string "as a last resort", or reject a value if a safe value cannot be created. Errors thrown from within a policy are propagated to the application.

If the default policy doesn’t exist, or if its appropriate create* function returns null or undefined, it will cause a CSP violation. In the enforcing mode, an error will be thrown, but in report-only the original value passed to the default policy will be used.

Note: This optional behavior allows for introducing Trusted Type enforcement to applications that are still using legacy code that uses injection sinks. Needless to say, this policy should necessarily be defined with very strict rules not to bypass the security restrictions in unknown parts of the application. In an extreme case, a lax, no-op default policy defeats all the benefits of using Trusted Types to protect access to injection sinks. If possible, authors should resort to a default policy in a transitional period only, use it to detect and rewrite their dependencies that use injection sinks unsafely and eventually phase out the usage of the default policy entirely.

Note: See § 3.5 Get Trusted Type compliant string for details on how the default policy is applied.

// Content-Security-Policy: trusted-types default; require-trusted-types-for 'script'

trustedTypes.createPolicy('default', {
  createScriptURL: (value, type, sink) => {
    console.log("Please refactor.");
    return value + '?default-policy-used&type=' + encodeURIComponent(type) +
          '&sink=' + encodeURIComponent(sink);
  }
});

aScriptElement.src = "https://cdn.example/script.js";
// Please refactor.
console.log(aScriptElement.src);
// https://cdn.example/script.js?default-policy-used&type=TrustedScriptURL&sink=HTMLScriptElement%20src

2.4. Enforcement

Note: Enforcement is the process of checking that a value has an appropriate type before it reaches an injection sink.

The JavaScript API that allows authors to create policies and Trusted Types objects from them is always available (via trustedTypes). Since injection sinks stringify their security sensitive arguments, and Trusted Type objects stringify to their inner string values, this allows the authors to use Trusted Types in place of strings.

To secure the access to injection sinks, on top of the JavaScript code using the Trusted Types, the user agent needs to enforce them i.e. assert that the injection sinks from a given group are never called with string values, and Trusted Type values are used instead. This section describes how authors may control this enforcing behavior.

Authors may also control their policies by specifying rules around policy creation.

2.4.1. Content Security Policy

Applications may control Trusted Type enforcement via configuring a Content Security Policy. This document defines new directives that correspond to Trusted Types rules. The require-trusted-types-for directive specifies the injection sinks groups, for which the types should be required. The trusted-types directive controls how policies can be created.

Note: Using CSP mechanisms allows the authors to prepare their application for enforcing Trusted Types via using the Content-Security-Policy-Report-Only HTTP Response header.

Note: Most of the enforcement rules are defined as modifications of the algorithms in other specifications, see § 4 Integrations.

2.4.2. Sanitization enforcement with createParserOptions

While createHTML relies on sanitizing strings before they are injected to the parser using a JS sanitizer, the createParserOptions allows the page’s default policy to enforce HTML sanitization.

A createParserOptions callback in a default policy runs whenever an HTML fragment sink is invoked with unvetted parser options (or default options), allowing the policy to vet caller-supplied options and enforce sanitization even when the input markup is already an instance of TrustedHTML.

Content-Security-Policy: require-trusted-types-for 'script'

(async function renderFootnote() {
  trustedTypes.createPolicy('default', {
    createParserOptions: options => {
      const sanitizer = new Sanitizer(options.sanitizer?.get() ?? {});
      sanitizer.removeUnsafe();
      return { ...options, sanitizer };
    }
  });

  const trustedOptions = trustedTypes.createPolicy('trusted', {
    createParserOptions: options => options
  }).createParserOptions({});
  const untrustedFootnoteContent = await fetch('/footnote.html').then(r => r.text());

  // This HTML will be sanitized with a "safe" HTML sanitizer, as the default policy injects
  // a sanitizer with unsafe markup removed.
  footNote.innerHTML = untrustedFootnoteContent;
  // or:
  footNote.setHTMLUnsafe(untrustedFootnoteContent, {});
  const trustedMainContent = await fetch("main.html").then(r => r.text());
  main.setHTMLUnsafe(trustedMainContent, trustedOptions);
})();

(async function renderFootnote() {
  trustedTypes.createPolicy('default', {
    createHTML: string => string,
    createParserOptions: options => {
      return { ...options, runScripts: false };
    }
  });

  // The default policy would override the runScripts value to false.
  main.setHTMLUnsafe(someHTMLWithScripts, {runScripts: true});

  // This applies to Range.prototype.createContextualFragment()
  const fragment = new Range().createContextualFragment("<script>alert(1)</" + "script>");
  const div = document.createElement('div');
  // The scripts in the fragment would not be executed, as the default policy overrides runScripts.
  div.appendChild(fragment);
})();

2.4.3. Summary of input enforcement

This section is not normative.

The following table summarizes the behavior of get trusted type compliant input for various combinations of arguments and default policy callbacks under enforcing Content Security Policy:

TrustedHTML, runScripts, and sanitization: Sinks enforcing Trusted Types handle options and scripts as follows:

• When input is a TrustedHTML instance, a default policy’s createParserOptions still vets any caller-supplied options dictionary, allowing the policy to inspect, configure, or override the sanitizer configuration (as well as control script execution) despite the input already being TrustedHTML. If createParserOptions throws or returns null or undefined, the operation is rejected. When createParserOptions is absent, an author-supplied options dictionary that needs vetting is rejected, whether input is a TrustedHTML instance or a string converted by createHTML. Options that a sink constructs itself, like createContextualFragment()’s runScripts set to true, are not rejected.
• When options is a TrustedHTMLParserOptions, script execution and sanitization are governed by the policy that created the options object. When options are passed as a dictionary, a default policy’s createParserOptions determines the final script execution behavior, overriding the caller’s requested runScripts setting.
• XML fragment parsing does not support sanitization; any sanitizer configuration is ignored for XML sinks.
• For streaming HTML fragment parsing, get trusted type compliant parser options is invoked directly with throwIfMissing set to true. In that context, if createParserOptions is missing, throws, or returns null or undefined, a TypeError is thrown (or rethrown) immediately.

3. Algorithms

3.1. Create a Trusted Type Policy

To create a trusted type policy, given a TrustedTypePolicyFactory (factory), a string (policyName), TrustedTypePolicyOptions dictionary (options), and a global object (global) run these steps:

1. Let allowedByCSP be the result of executing should Trusted Type policy creation be blocked by content security policy? algorithm with global, policyName and factory’s created policy names value.

2. If allowedByCSP is "Blocked", throw a TypeError and abort further steps.

3. If policyName is default and the factory’s default policy value is not null, throw a TypeError and abort further steps.

4. Let policy be a new TrustedTypePolicy object.

5. Set policy’s name property value to policyName.

6. Set policy’s options value to «[ "createHTML" -> options["createHTML"], "createScript" -> options["createScript"], "createScriptURL" -> options["createScriptURL"], "createParserOptions" -> options["createParserOptions"] ]».

7. If the policyName is default, set the factory’s default policy value to policy.

8. Append policyName to factory’s created policy names.

9. Return policy.

3.2. Create a Trusted Type

To create a trusted type given a TrustedTypePolicy policy, a type name trustedTypeName, a string or dictionary value and a list arguments, perform the following steps:

1. Let policyValue be the result of executing get trusted type policy value with the same arguments as this algorithm and additionally true as throwIfMissing.

2. If the algorithm threw an error, rethrow the error and abort the following steps.

3. If trustedTypeName is "TrustedHTMLParserOptions":

   1. If policyValue is null, throw a TypeError.

      Note: Returning null from a named policy’s createParserOptions callback throws a TypeError here, whereas returning null or undefined from createHTML, createScript, or createScriptURL yields the empty string.

   2. Let sanitizerConfig be the result of getting a sanitizer configuration from policy’s relevant realm and policyValue.

   3. Let runScripts be policyValue["runScripts"] with default false.

   4. Return a new instance of TrustedHTMLParserOptions in policy’s relevant realm with its sanitizer configuration set to sanitizerConfig, and its runScripts set to runScripts.

4. Let dataString be the result of stringifying policyValue.

5. If policyValue is null or undefined, set dataString to the empty string.

6. Return a new instance of an interface with a type name trustedTypeName, with its associated data value set to dataString.

3.3. Get Trusted Type policy value

To get trusted type policy value given a TrustedTypePolicy policy, a type name trustedTypeName, a string or dictionary value, a list arguments, and a boolean throwIfMissing, perform the following steps:

1. Let functionName be a function name for the given trustedTypeName, based on the following table:

   Function name	Trusted Type name
   "createHTML"	"TrustedHTML"
   "createScript"	"TrustedScript"
   "createScriptURL"	"TrustedScriptURL"
   "createParserOptions"	"TrustedHTMLParserOptions"

2. Let function be policy’s options[functionName].

3. If function is null, then:

   1. If throwIfMissing throw a TypeError.

   2. Else return null.

4. If trustedTypeName is "TrustedHTMLParserOptions":

   1. Set value to the result of creating a parser options callback argument given policy’s relevant realm and value.

5. Let args be « value ».

6. Append each item in arguments to args.

7. Let policyValue be the result of invoking function with args and "rethrow".

8. Return policyValue.

3.4. Get Trusted Type compliant input

Note: See § 2.4.3 Summary of input enforcement for a summary of this algorithm’s behavior for various combinations of arguments and default policy callbacks.

To get trusted type compliant input given a global object global, a string or TrustedHTML input, a TrustedHTMLParserOptions, ParseHTMLUnsafeOptions or SetHTMLUnsafeOptions options, a string sink, an optional "html" or "xml" documentType (default "html"), and an optional boolean optionsFromAuthor (default true):

1. Let compliantOptions be options.

2. If documentType is "html":

   1. Set compliantOptions to the result of calling get trusted type compliant parser options, passing global, options, false, sink, and optionsFromAuthor.

      Note: Sinks that construct options themselves, rather than taking them from the author, pass false for optionsFromAuthor. For example, createContextualFragment() sets runScripts to true.

   2. If compliantOptions is an instance of TrustedHTMLParserOptions, return (stringified input, compliantOptions).

3. Let compliantHTML be the result of calling get trusted type compliant string, passing TrustedHTML, global, input, sink, and "script".

4. If the algorithm threw an error, rethrow the error.

5. Return (compliantHTML, compliantOptions).

3.5. Get Trusted Type compliant string

This algorithm will return a string that can be used with an injection sink, optionally unwrapping it from a matching Trusted Type. It will ensure that the Trusted Type enforcement rules were respected.

To get trusted type compliant string a TrustedType type (expectedType), a global object (global), TrustedType or a string (input), a string (sink) and a string (sinkGroup), run these steps:

1. If input is an instance of expectedType, return stringified input and abort these steps.

2. Let requireTrustedTypes be the result of executing Does sink type require trusted types? algorithm, passing global, sinkGroup, and true.

3. If requireTrustedTypes is false, return stringified input and abort these steps.

4. Let convertedInput be the result of executing Process value with a default policy with the same arguments as this algorithm.

5. If the algorithm threw an error, rethrow the error and abort the following steps.

6. If convertedInput is null or undefined, execute the following steps:

   1. Let disposition be the result of executing should sink type mismatch violation be blocked by content security policy? algorithm, passing global, sink, sinkGroup, and stringified input as source.

   2. If disposition is “Allowed”, return stringified input and abort further steps.

      Note: This step assures that the default policy rejection will be reported, but ignored in a report-only mode.

   3. Throw a TypeError and abort further steps.

7. Assert: convertedInput is an instance of expectedType.

8. Return stringified convertedInput.

3.6. Get Trusted Type compliant parser options

Note: throwIfMissing is false when called from get trusted type compliant input. Even then, author-supplied options that need vetting require a default policy with createParserOptions. For synchronous HTML fragment injection sinks, a TrustedHTMLParserOptions in the resulting options can satisfy Trusted Types enforcement without requiring createHTML on the input string. When streaming or in contexts where the HTML string cannot be validated up front, callers pass true for throwIfMissing to ensure a default policy with createParserOptions is required. For XML documents, parser options cannot satisfy Trusted Types enforcement; XML fragment sinks always require compliant string input via get trusted type compliant string.

To get trusted type compliant parser options given a global object global, input, which is either a TrustedHTMLParserOptions, a SetHTMLUnsafeOptions, or a ParseHTMLUnsafeOptions, a boolean throwIfMissing, a string sink, and an optional boolean optionsFromAuthor (default true):

1. If input is an instance of TrustedHTMLParserOptions, return input.

2. Let requireTrustedTypes be the result of executing Does sink type require trusted types? algorithm, passing global, "script", and true.

3. If requireTrustedTypes is false, return input.

4. Let requirePolicy be throwIfMissing.

5. If optionsFromAuthor is true and input needs vetting, set requirePolicy to true.

   Note: Markup vetted as TrustedHTML can become unsafe when parsed with an unvetted sanitizer (e.g., one that uses replaceWithChildrenElements) or with script execution enabled. Such options need to come from a TrustedHTMLParserOptions or a default policy’s createParserOptions.

6. Let defaultPolicy be the value of global’s trusted type policy factory’s default policy.

7. If defaultPolicy is null:

   1. If requirePolicy is true:

      1. Let disposition be the result of executing should sink type mismatch violation be blocked by content security policy? algorithm, passing global, sink, "script", and the empty string as source.

      2. If disposition is “Allowed”, return input.

      3. Throw a TypeError.

   2. Return input.

8. Let function be defaultPolicy’s options["createParserOptions"].

9. If function is null:

   1. If requirePolicy is true:

      1. Let disposition be the result of executing should sink type mismatch violation be blocked by content security policy? algorithm, passing global, sink, "script", and the empty string as source.

      2. If disposition is “Allowed”, return input.

      3. Throw a TypeError.

   2. Return input.

10. Let argument be the result of creating a parser options callback argument given global’s relevant realm and input.

11. Let policyValue be the result of invoking function with « argument, "TrustedHTMLParserOptions", sink » and "rethrow".

12. If the algorithm threw an error, rethrow the error and abort the following steps.

13. If policyValue is null or undefined:

    1. Let disposition be the result of executing should sink type mismatch violation be blocked by content security policy? algorithm, passing global, sink, "script", and the empty string as source.

    2. If disposition is “Allowed”, return input.

       Note: This step assures that the default policy rejection will be reported, but ignored in a report-only mode.

    3. Throw a TypeError.

14. Let runScripts be policyValue["runScripts"], with default false.

    Note: When invoked as a default policy, the createParserOptions callback determines the final script execution behavior, overriding the caller’s requested runScripts option.

15. Return a new TrustedHTMLParserOptions in global’s relevant realm whose sanitizer configuration is set to the result of getting a sanitizer configuration from global’s relevant realm and policyValue, and whose runScripts is set to runScripts.

A SetHTMLUnsafeOptions or ParseHTMLUnsafeOptions options needs vetting if options["runScripts"] exists and is true, or if options["sanitizer"] exists and is not an empty SanitizerConfig.

Note: An empty SanitizerConfig does not change the parsed markup.

To create a parser options callback argument given a realm realm and a dictionary dictionary:

1. Let runScriptsInput be dictionary["runScripts"], with default false.

2. Let argument be the ordered map «[ "runScripts" → runScriptsInput ]».

3. Let sanitizerInput be the result of getting a sanitizer from realm and dictionary.

4. If sanitizerInput is not null, set argument["sanitizer"] to sanitizerInput.

5. Return argument.

To get a sanitizer from dictionary given a realm realm and a dictionary dictionary:

1. Let sanitizerInput be dictionary["sanitizer"], defaulting to null.

2. If sanitizerInput is null, return null.

3. Let configuration be null.

4. If sanitizerInput is a SanitizerPresets:

   1. Assert: sanitizerInput is "default".

   2. Set configuration to the built-in safe default configuration.

5. Otherwise, if sanitizerInput is a Sanitizer:

   1. Set configuration to sanitizerInput’s configuration.

6. Otherwise, if sanitizerInput is a SanitizerConfig:

   1. Set configuration to sanitizerInput.

7. Assert: configuration is not null.

8. Let sanitizer be a new Sanitizer in realm.

9. Configure sanitizer given configuration and true.

10. Return sanitizer.

To get a sanitizer config copy from dictionary given a realm realm and a dictionary dictionary:

1. Let sanitizer be the result of getting a sanitizer from realm and dictionary.

2. If sanitizer is null, return null.

3. Return sanitizer’s configuration.

3.7. Process value with a default policy

This algorithm routes a value to be assigned to an injection sink through a default policy, should one exist.

To process value with a default policy given a TrustedType type (expectedType), a global object (global), TrustedType or a string (input), and a string (sink), run these steps:

1. Let defaultPolicy be the value of global’s trusted type policy factory’s default policy.

2. Let policyValue be the result of executing get trusted type policy value, with the following arguments:

   • defaultPolicy as policy

   • stringified input as value

   • expectedType’s type name as trustedTypeName

   • « trustedTypeName, sink » as arguments

   • false as throwIfMissing

3. If the algorithm threw an error, rethrow the error and abort the following steps.

4. If policyValue is null or undefined, return policyValue.

5. Let dataString be the result of stringifying policyValue.

6. Return a new instance of an interface with a type name trustedTypeName, with its associated data value set to dataString.

3.8. Prepare the script text

To prepare the script text given an HTMLScriptElement or SVGScriptElement (script), perform the following steps:

1. Let sink be "HTMLScriptElement text" if script is an HTMLScriptElement; otherwise "SVGScriptElement text".

2. If script’s script text value is not equal to its child text content, set script’s script text to the result of executing get Trusted Type compliant string, with the following arguments:

   • TrustedScriptURL as expectedType,

   • script’s Document’s relevant global object as global,

   • script’s child text content attribute value as input,

   • sink,

   • 'script' as sinkGroup.

   If the algorithm threw an error, rethrow the error.

3.9. Get Trusted Type compliant attribute value

To get trusted type compliant attribute value given a string attributeName, string attributeNs, Element element and TrustedType or a string newValue, perform the following steps:

1. If attributeNs is the empty string, set attributeNs to null.

2. Set attributeData to the result of get Trusted Type data for attribute algorithm, with the following arguments:

   • element

   • attributeName

   • attributeNs

3. If attributeData is null, then:

   1. If newValue is a string, return newValue.

   2. Assert: newValue is TrustedHTML or TrustedScript or TrustedScriptURL.

   3. Return value’s associated data.

4. Let expectedType be the value of the fourth member of attributeData.

5. Let sink be the value of the fifth member of attributeData.

6. Return the result of executing get trusted type compliant string with the following arguments:

   • expectedType

   • newValue as input

   • element’s node document’s relevant global object as global

   • sink

   • 'script' as sinkGroup

If the algorithm threw an error, rethrow the error.

3.10. Get Trusted Type data for attribute

To get Trusted Type data for attribute given element, attribute, attributeNs, perform the following steps:

The event handler content attribute concept used below is ambiguous. This spec needs a better mechanism to identify event handler attributes. See https://github.com/w3c/trusted-types/issues/520.

1. Let data be null.

2. If attributeNs is null, « HTML namespace, SVG namespace, MathML namespace » contains element’s namespace, and attribute is the name of an event handler content attribute:

   1. Return (Element, null, attribute, TrustedScript, "Element " + attribute).

3. Find the row in the following table, where element is in the first column, attributeNs is in the second column, and attribute is in the third column. If a matching row is found, set data to that row.

   Element	Attribute namespace	Attribute local name	TrustedType	Sink
   HTMLIFrameElement	null	"srcdoc"	TrustedHTML	"HTMLIFrameElement srcdoc"
   HTMLScriptElement	null	"src"	TrustedScriptURL	"HTMLScriptElement src"
   SVGScriptElement	null	"href"	TrustedScriptURL	"SVGScriptElement href"
   SVGScriptElement	XLink namespace	"href"	TrustedScriptURL	"SVGScriptElement href"

4. Return data.

4. Integrations

typedef (TrustedHTML or TrustedScript or TrustedScriptURL) TrustedType;

4.1. Integration with HTML

Window and Worker objects have a trusted type policy factory, which is a TrustedTypePolicyFactory object.

4.1.1. Extensions to the WindowOrWorkerGlobalScope interface

This document extends the WindowOrWorkerGlobalScope interface defined by HTML:

partial interface mixin WindowOrWorkerGlobalScope {
  readonly attribute TrustedTypePolicyFactory trustedTypes;
};

The trustedTypes getter steps are to return this’s relevant global object’s trusted type policy factory.

4.1.2. Enforcement for scripts

4.1.2.1. Slots with trusted values

An HTMLScriptElement and SVGScriptElement have:

an associated string script text.

A string, containing the body of the script to execute that was set through a compliant sink. Equivalent to script’s child text content. Initially an empty string.

4.1.2.2. Update script trustedness algorithm

To update script trustedness for HTMLScriptElement script given string value:

1. Set script’s script text value to value.

4.1.2.3. Setting slot values from parser

This document modifies the HTML parser to set the script text value when the script is created.

Modify the The text insertion mode algorithm as follows:

An end tag whose tag name is "script"

...

Set script’s script text value to its child text content.

If the active speculative HTML parser is null, then prepare the script element script. This might cause some script to execute, which might cause new characters to be inserted into the tokenizer, and might cause the tokenizer to output more tokens, resulting in a reentrant invocation of the parser.

...

The above algorithm doesn’t account for the case when the script element’s content is changed mid-parse. Implementors must ensure they protect against this case. See https://github.com/w3c/trusted-types/issues/507.

There’s no proper definition for the processing of SVG script elements. However, implementations must apply a similar change to the processing of SVGScriptElements.

4.1.2.4. Slot value verification

The first few steps of the prepare the script element algorithm are modified as follows:

1. If el’s already started is true, then return.

2. Let parser document be el’s parser document.

3. Set el’s parser document to null.

   This is done so that if parser-inserted script elements fail to run when the parser tries to run them, e.g. because they are empty or specify an unsupported scripting language, another script can later mutate them and cause them to run again.

4. If parser document is non-null and el does not have an async attribute, then set el’s force async to true.

   This is done so that if a parser-inserted script element fails to run when the parser tries to run it, but it is later executed after a script dynamically updates it, it will execute in an async fashion even if the async attribute isn’t set.

5. Execute the Prepare the script text algorithm on el. If that algorithm threw an error, then return.

6. Let source text be el’s child text content. script text value.

7. ...

There’s no proper definition for the processing of SVG script elements. However, implementations must apply a similar change to the processing of SVGScriptElements.

4.2. Integration with Content Security Policy

4.2.1. require-trusted-types-for directive

This document defines require-trusted-types-for - a new Content Security Policy directive.

require-trusted-types-for directive configures the Trusted Types framework for all the injection sinks of certain groups in a current realm. Specifically, it defines what should be the behavior when a string value is passed to an injection sink of a given group (i.e. should the type-based enforcement be enabled for such sinks).

Note: Currently, only the enforcement for § 2.1.1 DOM XSS injection sinks is specified.

The syntax for the directive’s name and value is described by the following ABNF:

directive-name = "require-trusted-types-for"
directive-value = trusted-types-sink-group-keyword *( required-ascii-whitespace trusted-types-sink-group-keyword)
trusted-types-sink-group-keyword = "'" trusted-types-sink-group "'"
trusted-types-sink-group = "script"

Content-Security-Policy: require-trusted-types-for 'script'

4.2.1.1. require-trusted-types-for Pre-Navigation check

Given a request (request), a string navigation type and a policy (policy), this algorithm returns "Blocked" if a navigation violates the require-trusted-types-for directive’s constraints and "Allowed" otherwise. This constitutes the require-trusted-types-for directive’s pre-navigation check:

Note: This algorithm assures that the code to be executed by a navigation to a javascript: URL will have to pass through a default policy’s createScript function, in addition to all other restrictions imposed by other CSP directives.

1. If request’s url’s scheme is not "javascript", return "Allowed" and abort further steps.

2. Let urlString be the result of running the URL serializer on request’s url.

3. Let encodedScriptSource be the result of removing the leading "javascript:" from urlString.

4. Let convertedScriptSource be the result of executing Process value with a default policy algorithm, with the following arguments:

   • TrustedScript as expectedType

   • request’s clients’s global object as global

   • encodedScriptSource as input

   • "Location href" as sink

   If that algorithm threw an error or convertedScriptSource is not a TrustedScript object, return "Blocked" and abort further steps.

5. Set urlString to be the result of prepending "javascript:" to stringified convertedScriptSource.

6. Let newURL be the result of running the URL parser on urlString. If the parser returns a failure, return "Blocked" and abort further steps.

7. Set request’s url to newURL.

   Note: No other CSP directives operate on javascript: URLs in a pre-navigation check. Other directives that check javascript: URLs will operate on the modified URL later, in the inline check.

8. Return "Allowed".

4.2.2. trusted-types directive

This document defines trusted-types - a new Content Security Policy directive. The trusted-types directive controls the creation of Trusted Type policies.

The syntax for the directive’s name and value is described by the following ABNF:

directive-name = "trusted-types"
directive-value = serialized-tt-configuration
serialized-tt-configuration = ( tt-expression *( required-ascii-whitespace tt-expression ) )
tt-expression = tt-policy-name  / tt-keyword / tt-wildcard
tt-wildcard = "*"
tt-policy-name = 1*( ALPHA / DIGIT / "-" / "#" / "=" / "_" / "/" / "@" / "." / "%")
tt-keyword = "'allow-duplicates'" / "'none'"

Content-Security-Policy: require-trusted-types-for 'script'; trusted-types one two

Content-Security-Policy: trusted-types; require-trusted-types-for 'script'

The keyword 'none' may be used to explicitly express the above:

Content-Security-Policy: trusted-types 'none'; require-trusted-types-for 'script'

Keyword 'allow-duplicates' allows for creating policies with a name that was already used.

Content-Security-Policy: trusted-types foo bar 'allow-duplicates'

If the policy named default is present in the list, it refers to the default policy. All strings passed to injection sinks will be passed through it instead of being rejected outright.

Content-Security-Policy: trusted-types one two default

4.2.3. Does sink type require trusted types?

This algorithm returns true if the injection sink requires a Trusted Type, and false otherwise.

The does sink type require trusted types? algorithm, given a global object (global), a string (sinkGroup) and a boolean (includeReportOnlyPolicies), performs these steps:

1. For each policy in global’s CSP list:

   1. If policy’s directive set does not contain a directive whose name is "require-trusted-types-for", skip to the next policy.

   2. Let directive be the policy’s directive set’s directive whose name is "require-trusted-types-for"

   3. If directive’s value does not contain a trusted-types-sink-group which is a match for sinkGroup, skip to the next policy.

   4. Let enforced be true if policy’s disposition is "enforce", and false otherwise.

   5. If enforced is true, return true.

   6. If includeReportOnlyPolicies is true, return true.

2. Return false.

4.2.4. Should sink type mismatch violation be blocked by Content Security Policy?

This algorithm returns "Blocked" if the injection sink requires a Trusted Type, and "Allowed" otherwise.

The should sink type mismatch violation be blocked by content security policy? algorithm, given a global object (global), a string (sink), a string (sinkGroup) and a string (source), performs the following steps:

1. Let result be "Allowed".

2. Let sample be source.

3. If sink is "Function", then:

   1. If sample starts with "function anonymous", strip that from sample.

   2. Otherwise if sample starts with "async function anonymous", strip that from sample.

   3. Otherwise if sample starts with "function* anonymous", strip that from sample.

   4. Otherwise if sample starts with "async function* anonymous", strip that from sample.

4. For each policy in global’s CSP list:

   1. If policy’s directive set does not contain a directive whose name is "require-trusted-types-for", skip to the next policy.

   2. Let directive be the policy’s directive set’s directive whose name is "require-trusted-types-for"

   3. If directive’s value does not contain a trusted-types-sink-group which is a match for sinkGroup, skip to the next policy.

   4. Let violation be the result of executing Create a violation object for global, policy, and directive on global, policy and "require-trusted-types-for"

   5. Set violation’s resource to "trusted-types-sink".

   6. Let trimmedSample be the substring of sample, containing its first 40 characters.

   7. Set violation’s sample to be the result of concatenating the list « sink, trimmedSample « using "|" as a separator.

   8. Execute Report a violation on violation.

   9. If policy’s disposition is "enforce", then set result to "Blocked".

5. Return result.

4.2.5. Should Trusted Type policy creation be blocked by Content Security Policy?

This algorithm returns "Blocked" if the TrustedTypePolicy should not be created, and "Allowed" otherwise.

The Should Trusted Type policy creation be blocked by content security policy? algorithm, given a global object (global), a string (policyName) and a list of strings (createdPolicyNames), performs the following steps:

1. Let result be "Allowed".

2. For each policy in global’s CSP list:

   1. Let createViolation be false.

   2. If policy’s directive set does not contain a directive which name is "trusted-types", skip to the next policy.

   3. Let directive be the policy’s directive set’s directive which name is "trusted-types"

   4. If directive’s value only contains a tt-keyword which is a match for a value 'none', set createViolation to true.

      Note: Like in other CSP directives, 'none' keyword will be ignored if other keywords or policy names are present.

   5. If createdPolicyNames contains policyName and directive’s value does not contain a tt-keyword which is a match for a value 'allow-duplicates', set createViolation to true.

      Note: trusted-types policyA policyB 'allow-duplicates' allows authors to create policies with duplicated names.

   6. If directive’s value does not contain a tt-policy-name, which value is policyName, and directive’s value does not contain a tt-wildcard, set createViolation to true.

      Note: trusted-types * allows authors to create policies with any unique names. To allow for multiple policies with the same name, use trusted-types * 'allow-duplicates' or don’t set the trusted-types directive at all.

   7. If createViolation is false, skip to the next policy.

   8. Let violation be the result of executing Create a violation object for global, policy, and directive on global, policy and "trusted-types"

   9. Set violation’s resource to "trusted-types-policy".

   10. Set violation’s sample to the substring of policyName, containing its first 40 characters.

   11. Execute Report a violation on violation.

   12. If policy’s disposition is "enforce", then set result to "Blocked".

3. Return result.

5. Security Considerations

Trusted Types are not intended to protect access to injection sinks in an actively malicious execution environment. It’s assumed that the application is written by non-malicious authors; the intent is to prevent developer mistakes that could result in security bugs, and not to defend against first-party malicious code actively trying to bypass policy restrictions. Below we enumerate already identified vectors that remain risky even in environments with enforced Trusted Types.

5.1. Cross-document vectors

While the code running in a window in which Trusted Types are enforced cannot dynamically create nodes that would bypass the policy restrictions, it is possible that such nodes can be imported or adopted from documents in other windows, that don’t have the same set of restrictions. In essence - it is possible to bypass Trusted Types if a malicious author creates a setup in which a restricted document colludes with an unrestricted one. In an extreme case, the restricted document might create a Blob from strings and navigate to it.

CSP propagation rules (see Content Security Policy 3 § 7.8 CSP Inheriting to avoid bypasses partially address this issue, as new local scheme documents will inherit the same set of restrictions, so - for example - script-src restrictions could be used to make sure injections into Blob contents would not execute scripts. To address this issue comprehensively, other mechanisms like Origin Policy should be used to ensure that baseline security rules are applied for the whole origin.

5.2. Deprecated features

Some long-deprecated and rarely used platform features are not subject to Trusted Types, and could potentially be used by malicious authors to overcome the restrictions:

• HTML imports

5.3. Script gadgets

While Trusted Types logic is called on many operations that results in creating DOM trees from string, it should not be treated as a mechanism for guarding all DOM tree creation in a document. This is important especially in the presence of script gadgets, where an application reacts to contents of usually benign DOM elements or attributes. Developers using DOM API directly can trigger such gadgets without using Trusted Types. However, in order for the gadget to trigger DOM XSS, it needs to obtain a Trusted Type value via a policy. Authors need to ascertain that the data passed to Trusted Type policies is indeed trustworthy, if the policy rules don’t enforce constraints or validate the data themselves.

5.4. Best practices for policy design

Trusted Types limit the scope of the code that can introduce vulnerabilities via injection sinks to the implementation of policies. In this design, insecure policies can still expose injection sinks to untrusted data. Special emphasis needs to be taken by use policies that are either secure for all possible inputs, or limit the access to insecure policies, such that they are only called with non-attacker controlled inputs.

As policies are custom JavaScript code, they may be written in a way that heavily depends on a global state. We advise against this. The policies should be self-contained as much as possible. All objects that may alter security decisions a policy makes effectively become the policy, and should be guarded & reviewed together.

A createParserOptions callback that returns a configuration that does not remove unsafe markup (e.g., scripts or event handlers) provides no protection against DOM XSS. Policy authors should also carefully inspect any caller-supplied sanitizer in the argument["sanitizer"] passed to createParserOptions. Blindly trusting or passing through a caller’s sanitizer allows unvetted configurations to govern parsing. Because createParserOptions executes even when the input markup is an existing TrustedHTML instance, the default policy ensures that unvetted caller options cannot bypass application-wide sanitization or script execution constraints. Without such a callback, TrustedHTML cannot be combined with a caller-supplied non-empty sanitizer or runScripts set to true, since those could make vetted markup unsafe.

Refer to the external document on secure policy design.

6. Privacy Considerations

The specification may partially observe and alter the behavior of scripts running within the application, e.g. causing certain operations on injection sinks to fail, or monitoring and changing their effect with a default policy. However, early-running scripts already have this capability by overriding appropriate property descriptors.

It is possible for the application to report violations of Trusted Types restrictions. Violation reports would include the trimmed-down payload passed to the injection sink (40 characters, including the sink name). These feature is reusing the Content Security Policy reporting mechanisms.

7. Implementation Considerations

7.1. Vendor-specific Extensions and Addons

Restriction imposed by Trusted Types SHOULD NOT interfere with the operation of user-agent features like addons, extensions, or bookmarklets. These kinds of features generally advance the user’s priority over page authors, as espoused in [html-design-principles]. Specifically, extensions SHOULD be able to pass strings to the injection sinks without triggering default policy execution, violation generation, or the rejection of the value.