BIT-python-2026-19553: SSLContext.wrap_bio() missing validation of server_hostname parameter
ssl.SSLContext.wrapbio() didn't require the serverhostname argument to not be None if ssl.SSLContext.checkhostname was set. Due to a missing parameter check in SSLObject, if the serverhostname 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.checkhostname = True and no ValueError being raised due to misconfiguration.
If the program passes a serverhostname 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 serverhostname value to SSLContext.wrapbio(), asyncio.createconnection(), or asyncio.loop.starttls() 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.wrapsocket(), when serverhostname isn't supplied.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
Pass a valid, non-None and non-empty server_hostname value to ssl.SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() so certificate hostname verification proceeds as expected.