CVE-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, non-empty server_hostname value to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() so certificate hostname verification proceeds as expected.
Event History
Frequently Asked Questions
Which applications are exposed to this issue?
Applications are affected when SSLContext.check_hostname is enabled but they call SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() without a server_hostname value, or with None or an empty string. Applications that pass a non-empty, non-None server_hostname value are not affected.
What can be done if Python cannot be updated immediately?
Pass a valid, non-empty, non-None server_hostname value to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls(). This causes certificate hostname verification to proceed as expected without requiring a Python update or patch.
What behavior changes after applying the patch or upgrading Python?
The corrected behavior raises ValueError when hostname checking is enabled and server_hostname is missing, rather than silently skipping hostname verification. This aligns the behavior with SSLContext.wrap_socket().