What’s Vulnerable › Vulnerabilities

CVE-2026-27459: pyOpenSSL DTLS cookie callback buffer overflow

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-18 · updated 2026-09-10
• Weakness: CWE-120

Description

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to `set_cookie_generate_callback` returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.

Affected products

• pyca pyopenssl: from 22.0.0 before 26.0.0 — fixed in 26.0.0

How to fix

Upgrade to a fixed version: pyca pyopenssl 26.0.0. Check the vendor advisory for mitigations.

References

• https://github.com/pyca/pyopenssl/blob/358cbf29c4e364c59930e53a270116249581eaa3/CHANGELOG.rst
• https://github.com/pyca/pyopenssl/commit/57f09bb4bb051d3bc2a1abd36e9525313d5cd408
• https://github.com/pyca/pyopenssl/security/advisories/GHSA-5pwr-322w-8jr4
• https://access.redhat.com/errata/RHSA-2026:10754
• https://access.redhat.com/errata/RHSA-2026:11856
• https://access.redhat.com/errata/RHSA-2026:11916
• https://access.redhat.com/errata/RHSA-2026:11996
• https://access.redhat.com/errata/RHSA-2026:13508
• https://access.redhat.com/errata/RHSA-2026:13512
• https://access.redhat.com/errata/RHSA-2026:13545
• https://access.redhat.com/errata/RHSA-2026:13553
• https://access.redhat.com/errata/RHSA-2026:14835

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