Python SSLContext.wrap_bio() hostname verification bypass <3.12.15,<3.13.16,<3.14.8
CVE-2026-19553 Published on September 30, 2026

SSLContext.wrap_bio() missing validation of server_hostname parameter
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.

Vendor Advisory NVD

Vulnerability Analysis

CVE-2026-19553 can be exploited with network access. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity and availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
PASSIVE

Weakness Type

Improper Validation of Certificate with Host Mismatch

The software communicates with a host that provides a certificate, but the software does not properly ensure that the certificate is actually associated with that host.


Products Associated with CVE-2026-19553

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Affected Versions

Python Software Foundation CPython: