CVE-2026-19553: Python Software Foundation CPython vulnerability

1. In CISA KEV: not seen
2. Exploited (reported): not seen
3. Scanning observed: not seen
4. Proof of concept public1 public GitHub repository (unverified), listed by PoC-in-GitHub2 Oct
5. Not established: not seen

DescriptionNVD

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.

Exploitation

Unlikely (under 1%). EPSS under 1%, or no score yet.

CISA KEV

Not listed

Public check

No public nuclei template

Weakness

Improper Validation of Certificate with Host Mismatch · Cryptography

Vendor

Python Software Foundation

Published

September 2026

Band

Unlikely (under 1%), September 2026

1. 2 Oct
   PoC-in-GitHub lists 1 public GitHub repository (unverified) PoC-in-GitHub

• NVD entrynvd.nist.gov
• Vendor advisorygithub.com
• CVE recordcve.org
• Weakness CWE-297cwe.mitre.org