A remote, unauthenticated TLS client can make a server crash or call through a freed pointer if its…

Why it is Amber

• CVSS 9.2 critical
• Remotely exploitable without logging in

References

• https://github.com/python/cpython/commit/34a53dce8174da2fceb12fe084a4def02a10053d
• https://github.com/python/cpython/commit/46133cd57d309652139ada74014aca7665ac552b
• https://github.com/python/cpython/commit/63fab143d94cafae71850831acfb52041ba44af7
• https://github.com/python/cpython/commit/b12968cefe69ca1dcb8606c832ff73ee7dbf4ba8
• https://github.com/python/cpython/commit/cd7e51e7d4563866fbaa1e2521ae69b45daf3698
• https://github.com/python/cpython/commit/d8717ed01717a9641686e6e6f83f0ab8af235e2c
• https://github.com/python/cpython/commit/ec44b5a3258cbda947d5e07242ee562ed05ef24b
• https://github.com/python/cpython/issues/156293
• https://github.com/python/cpython/pull/158504
• https://mail.python.org/archives/list/security-announce@python.org/thread/QMQIUQB6WGGC3MI7I3WKQXOYOBDSPPS3/

Original description

A remote, unauthenticated TLS client can make a server crash or call through a freed pointer if its sni_callback assigns a different context to SSLSocket.context (the documented way to select a certificate per server name) and nothing else keeps the original ssl.SSLContext alive. Typical cases are servers that create an SSLContext per connection or replace it while connections are open; servers that wrap their listening socket with it are not affected. Mitigation: keep a reference to every SSLContext that sets sni_callback for the lifetime of the server. TLS clients are not affected.