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.
References10
- github.com/python/cpython/commit/34a53dce8174da2fceb12fe084a4de…
- github.com/python/cpython/commit/46133cd57d309652139ada74014aca…
- github.com/python/cpython/commit/63fab143d94cafae71850831acfb52…
- github.com/python/cpython/commit/b12968cefe69ca1dcb8606c832ff73…
- github.com/python/cpython/commit/cd7e51e7d4563866fbaa1e2521ae69…
- github.com/python/cpython/commit/d8717ed01717a9641686e6e6f83f0a…
- github.com/python/cpython/commit/ec44b5a3258cbda947d5e07242ee56…
- github.com/python/cpython/issues/156293
- github.com/python/cpython/pull/158504
- mail.python.org/archives/list/[email protected]/thre…
Metrics
- CVSS 4.0
-
9.2
CRITICAL
Score source: CNA
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 45 · as of 10 November 2026
- CWE
- CWE-416
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.