ssl.SSLContext.wrap_bio() didn't require the server_hostname argument to not be None if ssl.SSLContext.check_hostname was set.
CVE-2026-19553 · published 30 Sept 2026
How serious is it
- Severity: high, CVSS score 7.6 out of 10.
- No exploit prediction (EPSS) score yet.
- Not on the CISA list of exploited vulnerabilities.
- No public exploit in Exploit-DB.
Is your app affected by CVE-2026-19553?
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Who is affected
- Python Software Foundation CPython: < 3.10.22, 3.11.0 to < 3.11.17, 3.12.0 to < 3.12.15, 3.13.0 to < 3.13.16
How to check
- Find out whether you run the affected product and which version, then compare it with the versions above.
How to fix
Copies a prompt for Claude Code, Cursor or Codex that asks it to check whether you run this product.
- Apply the vendor's update or mitigation from the references below. If there is none yet, limit exposure of the affected service until there is.
- If a vulnerable version was live, check your logs and rotate the secrets that server could read.
The advisory
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.
Check your whole app
Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.
References
- github.com/python/cpython/commit/1697ea386c707142555d98a1263176bbbc014a96
- github.com/python/cpython/commit/5867d4e4ae6d1062352baf6b497a4026e8578ccf
- github.com/python/cpython/commit/641390146a16a38e6701923f4ee4f1940ae77082
- github.com/python/cpython/commit/869069d52ce0efab2f8c38197e92cdaaa312f1ed
- github.com/python/cpython/commit/966bf426d0b6c31c1b0a255ff14a17143a466ced
- github.com/python/cpython/commit/bdebbf9b366ec91e9cd9daa0b3510c9e84b60b80
- github.com/python/cpython/commit/f4e43ba525187282f2011da0e6ffc0d2b08d8062
- github.com/python/cpython/issues/156793
- github.com/python/cpython/pull/158503
- mail.python.org/archives/list/security-announce@python.org/thread/QNZRG3YOAMTHDCMVCICXGY6YEFPY2VDL/
- www.openwall.com/lists/oss-security/2026/09/30/16
Merged from NVD. Last updated 24 Oct 2026. All vulnerabilities · RSS · JSON