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GeezerVison is showing github.com — gh-161472: Fix signed char handling in SSL record header version by samitistfobos-dev · Pull Request #161474 · python/cpython

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@samitistfobos-dev

@samitistfobos-dev samitistfobos-dev commented Nov 6, 2026 •
edited by bedevere-app Bot
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Fixes #161472

Problem:
In _PySSL_msg_callback (SSL3_RT_HEADER branch, Modules/_ssl/debughelpers.c), the TLS record header version bytes are read through a signed char pointer (const char *cbuf) and combined as version = cbuf[1] << 8 | cbuf[2]. OpenSSL invokes the message callback for a received record header before the version is validated, so both bytes are controlled by the remote peer. When cbuf[1] has its high bit set, a negative value is shifted left (undefined behaviour in C11); when cbuf[2] has its high bit set, sign extension clobbers the upper byte, and the Python msg_callback receives a wrong (negative or sign-extended) version value.
Solution: cast both header bytes to unsigned char before the shift and the OR, so the version is always assembled as an unsigned 16-bit value.

  • Issue: Fix signed char handling in SSL record header version #161472

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@samitistfobos-dev
samitistfobos-dev requested review from gpshead and picnixz as code owners November 6, 2026 10:58
@python-cla-bot

python-cla-bot Bot commented Nov 6, 2026 •
edited
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@bedevere-app bedevere-app Bot added the awaiting review label Nov 6, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 6, 2026
Fix signed char handling in SSL record header version #161472
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@bedevere-app

bedevere-app Bot commented Nov 6, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@samitistfobos-dev
Fix: INTEGER_OVERFLOW.CHECKED
41d571f 
Problem:
In _PySSL_msg_callback (SSL3_RT_HEADER branch, Modules/_ssl/debughelpers.c),
the TLS record header version bytes are read through a signed char pointer
(const char *cbuf) and combined as version = cbuf[1] << 8 | cbuf[2]. OpenSSL
invokes the message callback for a received record header before the version
is validated, so both bytes are controlled by the remote peer. When cbuf[1]
has its high bit set, a negative value is shifted left (undefined behaviour
in C11); when cbuf[2] has its high bit set, sign extension clobbers the
upper byte, and the Python msg_callback receives a wrong (negative or
sign-extended) version value.
Solution: cast both header bytes to unsigned char before the shift and the
OR, so the version is always assembled as an unsigned 16-bit value.
Signed-off-by: Aleksandr Burlakov <[email protected]>
@samitistfobos-dev
samitistfobos-dev force-pushed the fix/pyssl_msg_callback_integer_overflow branch from 1049efc to 41d571f Compare November 6, 2026 12:47
@bedevere-app

bedevere-app Bot commented Nov 6, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@samitistfobos-dev

samitistfobos-dev commented Nov 6, 2026

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This change has little impact for Python users, because it's limited to how TLS record header versions are reported through the SSL message callback. Normal TLS connections and record processing are unaffected, and the issue only manifests when handling malformed record headers containing bytes with the high bit set.
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@picnixz

picnixz commented Nov 6, 2026

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No this still requires a NEWS entry.
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@picnixz

picnixz commented Nov 6, 2026

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Please add a test if possible as well
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@blurb-it
📜🤖 Added by blurb_it.
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@samitistfobos-dev

samitistfobos-dev commented Nov 6, 2026

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I'll provide the test soon
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@samitistfobos-dev

samitistfobos-dev commented Nov 6, 2026

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Please add a test if possible as well

Provided
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@samitistfobos-dev
pythongh-161472: Add regression test for SSL record header versions
f10388f
@samitistfobos-dev
samitistfobos-dev force-pushed the fix/pyssl_msg_callback_integer_overflow branch from 6b1b06a to f10388f Compare November 6, 2026 14:33
@samitistfobos-dev

samitistfobos-dev commented Nov 12, 2026

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@picnixz Can you look again?
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@gpshead gpshead Awaiting requested review from gpshead gpshead is a code owner
@picnixz picnixz Awaiting requested review from picnixz picnixz is a code owner

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Fix signed char handling in SSL record header version

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@samitistfobos-dev @picnixz
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