Bug report

Bug description:

http.cookiejar.DefaultCookiePolicy applies its domain rules only to cookies whose version is 0 (Netscape) or positive (RFC 2965). A Set-Cookie header with a negative Version attribute matches neither case, so the cookie's Domain is never compared with the host that sent it: the cookie is stored for that domain, replaces an existing cookie with the same domain, path and name, and is returned to that domain.

Any server a program contacts with a shared CookieJar controls the header, and can set or overwrite cookies for an unrelated domain (anything with an embedded dot); no cookies are disclosed to it.

import urllib.request
from http.cookiejar import CookieJar
from email.message import Message

class Response:
    def __init__(self, set_cookie):
        self.msg = Message()
        self.msg["Set-Cookie"] = set_cookie
    def info(self):
        return self.msg

for version in "", "; Version=-1":
    jar = CookieJar()
    req = urllib.request.Request("http://www.a.example/")
    jar.extract_cookies(
        Response("k=v; Domain=b.example; Path=/" + version), req)
    req = urllib.request.Request("http://www.b.example/")
    jar.add_cookie_header(req)
    print(repr(version), "->", req.get_header("Cookie"))

Output on main:

'' -> None
'; Version=-1' -> k=v

Expected None in both cases. The code is unchanged since the module was added, so all maintained branches behave the same. This is the set-side counterpart of bpo-35121.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-160300: Reject cookies with a negative version in http.cookiejar #160301
• [3.15] gh-160300: Reject cookies with a negative version in http.cookiejar (GH-160301) #160645
• [3.14] gh-160300: Reject cookies with a negative version in http.cookiejar (GH-160301) #160646
• [3.13] gh-160300: Reject cookies with a negative version in http.cookiejar (GH-160301) #160647
• [3.12] gh-160300: Reject cookies with a negative version in http.cookiejar (GH-160301) #160648
• [3.11] gh-160300: Reject cookies with a negative version in http.cookiejar (GH-160301) #160649

ANT-2026-TA36I7V7