Vulnerability in Python Software Foundation CPython

[ Description ]Official description

ssl.SSLContext.wrap_bio() didn't require the server_hostname argument to not be None if ssl.SSLContext.check_hostname was set. Due to a missing parameter check in SSLObject, if the server_hostname argument isn't supplied then hostname verification would be silently skipped. This defect could lead to programs where certificate hostname verification *appeared* to be succeeding with SSLContext.check_hostname = True and no ValueError being raised due to misconfiguration. If the program passes a server_hostname value that isn't an empty string or None to any of these APIs then certificate hostname verification proceeds as expected and the program is not affected by this vulnerability. Mitigating this vulnerability doesn't require updating Python or applying the patch. To mitigate, pass a valid non-None and non-empty server_hostname value to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() and certificate hostname verification will proceed as expected. Upgrading to the latest version of Python or applying the patch only changes the behavior from silently skipping hostname verification to raising a ValueError, similar to SSLContext.wrap_socket(), when server_hostname isn't supplied.

[ 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/1697ea386c707142555d98a1263176bbbc014a96
• github.com/python/cpython/commit/5867d4e4ae6d1062352baf6b497a4026e8578ccf
• github.com/python/cpython/commit/641390146a16a38e6701923f4ee4f1940ae77082
• github.com/python/cpython/commit/869069d52ce0efab2f8c38197e92cdaaa312f1ed
• github.com/python/cpython/commit/966bf426d0b6c31c1b0a255ff14a17143a466ced
• github.com/python/cpython/commit/bdebbf9b366ec91e9cd9daa0b3510c9e84b60b80
• github.com/python/cpython/commit/f4e43ba525187282f2011da0e6ffc0d2b08d8062
• github.com/python/cpython/issues/156793
• github.com/python/cpython/pull/158503
• mail.python.org/archives/list/security-announce@python.org/thread/QNZRG3YOAMTHDCMVCICXG…
• www.openwall.com/lists/oss-security/2026/09/30/16