CVE-2026-19553

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.