CVE-2026-27448 — pyopenssl
Fix: pyca/pyopenssl@d41a814CVE-2026-27448 is a CWE-636 vulnerability in pyopenssl. A fix is available for pyopenssl — see the affected versions and patch details below.
pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27448.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
pyopensslReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 0.14.0 and prior to version 26.0.0, if a user provided callback to set_tlsext_servername_callback raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it. Starting in version 26.0.0, unhandled exceptions now result in rejecting the connection.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pyopenssl | ≥ 0.14.0&&< 26.0.0 | 26.0.0pip install --upgrade 'pyopenssl==26.0.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pyopenssl, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pyopenssl to 26.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27448 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27448 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27448. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw is only exploitable when an application using the pyOpenSSL library provides a custom callback to the set_tlsext_servername_callback function. For the handshake to proceed incorrectly, the callback must raise an unhandled exception, limiting the exposure of this issue. Due to these reasons, this…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Hardened Images | pyopenssl-main-26.0.0-1.1.hum1 | RHSA-2026:7224 |
Frequently Asked Questions
Is CVE-2026-27448 in your dependencies?
O3 Security finds CVE-2026-27448 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.