What’s Vulnerable › Vulnerabilities

CVE-2026-19445: Use-after-free of a server-side SSLContext when sni_callback switches contexts

• Severity: critical (CVSS 9.2 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-30 · updated 2026-10-03
• Weakness: CWE-416

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.

Affected products

• Python: before 3.10.22; from 3.11.0 before 3.11.17; from 3.12.0 before 3.12.15; from 3.13.0 before 3.13.16; from 3.14.0 before 3.14.8; from 3.15.0a1 before 3.15.0rc3 — fixed in 3.10.22, 3.11.17, 3.12.15, 3.13.16, 3.14.8, 3.15.0rc3

How to fix

Upgrade to a fixed version: Python 3.10.22, 3.11.17, 3.12.15, 3.13.16. Check the vendor advisory for mitigations.

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/
• http://www.openwall.com/lists/oss-security/2026/09/30/17

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