CVE-2026-19553

/// Technical 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. Up

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References