Vulnerability in Python Software Foundation CPython

[ Why it matters ]Priority signals

• Critical severity: CVSS 9.2

[ Description ]Official 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.

[ Fixes ]Fixes and patches

• Security bulletin: Python QMQIUQB6WGGC3MI7I3WKQXOYOBDSPPS3Vendor patch or bulletin · CERT-FR · mail.python.org/archives/list/security-announce@python.org/thread/QMQI, cited by CERT-FR CERTFR-2026-AVI-1243
• Security bulletin: Python QNZRG3YOAMTHDCMVCICXGY6YEFPY2VDLVendor patch or bulletin · CERT-FR · mail.python.org/archives/list/security-announce@python.org/thread/QNZR, cited by CERT-FR CERTFR-2026-AVI-1243

[ Advisories ]Government CERT advisories

• BSI CERT-Bund1 Oct 2026WID-SEC-2026-3716

  CPython: Multiple vulnerabilities Translated from German

  Risk highCVE-2026-19445CVE-2026-19553
• CERT-FR1 Oct 2026CERTFR-2026-AVI-1243

  Multiple vulnerabilities in CPython Translated from French

  Multiple vulnerabilities have been discovered in CPython. They allow an attacker to cause remote denial of service and a security policy bypass.

  CVE-2026-19445CVE-2026-19553

[ References ]References

• github.com/python/cpython/commit/34a53dce8174da2fceb12fe084a4def02a10053d
• github.com/python/cpython/commit/46133cd57d309652139ada74014aca7665ac552b
• github.com/python/cpython/commit/63fab143d94cafae71850831acfb52041ba44af7
• github.com/python/cpython/commit/b12968cefe69ca1dcb8606c832ff73ee7dbf4ba8
• github.com/python/cpython/commit/cd7e51e7d4563866fbaa1e2521ae69b45daf3698
• github.com/python/cpython/commit/d8717ed01717a9641686e6e6f83f0ab8af235e2c
• github.com/python/cpython/commit/ec44b5a3258cbda947d5e07242ee562ed05ef24b
• github.com/python/cpython/issues/156293
• github.com/python/cpython/pull/158504
• mail.python.org/archives/list/security-announce@python.org/thread/QMQIUQB6WGGC3MI7I3WKQ…
• www.openwall.com/lists/oss-security/2026/09/30/17