CVE-2026-19553
Due to a missing parameter check in SSLObject, if the server_hostname argument isn't supplied then hostname verification would be silently skipped.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.55%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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. 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.
- CVSS 4.0
- 7.6 HIGHCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
- EPSS
- 0.55% probability · 46th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-297
- Source
- cna@python.org
EPSS moved from 0.5% on 1 October 2026 to 0.5% on 26 October 2026.
References
- https://github.com/python/cpython/commit/1697ea386c707142555d98a1263176bbbc014a96
- https://github.com/python/cpython/commit/5867d4e4ae6d1062352baf6b497a4026e8578ccf
- https://github.com/python/cpython/commit/641390146a16a38e6701923f4ee4f1940ae77082
- https://github.com/python/cpython/commit/869069d52ce0efab2f8c38197e92cdaaa312f1ed
- https://github.com/python/cpython/commit/966bf426d0b6c31c1b0a255ff14a17143a466ced
- https://github.com/python/cpython/commit/bdebbf9b366ec91e9cd9daa0b3510c9e84b60b80
- https://github.com/python/cpython/commit/f4e43ba525187282f2011da0e6ffc0d2b08d8062
- https://github.com/python/cpython/issues/156793
- https://github.com/python/cpython/pull/158503
- https://mail.python.org/archives/list/security-announce@python.org/thread/QNZRG3YOAMTHDCMVCICXGY6YEFPY2VDL/
- http://www.openwall.com/lists/oss-security/2026/09/30/16
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.