Bug report

Problem:

In _PySSL_msg_callback (SSL3_RT_HEADER branch, Modules/_ssl/debughelpers.c:55),
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.

version = cbuf[1] << 8 | cbuf[2];

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.

version = ((unsigned char)cbuf[1] << 8) | (unsigned char)cbuf[2];

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-161472: Fix signed char handling in SSL record header version #161474