How severe is CVE-2026-27459?

No CVSS score has been assigned to CVE-2026-27459 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-27459?

CVE-2026-27459 affects the following packages: pyopenssl (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-27459?

Update pyopenssl to 26.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27459 is resolved across your whole dependency graph.

How do I detect CVE-2026-27459 in my PyPI dependencies?

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.

How do I mitigate CVE-2026-27459 if there is no patch (or I can't update yet)?

Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Is CVE-2026-27459 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-27459 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-27459?

CVE-2026-27459 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 59th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-27459?

CVE-2026-27459 is classified as CWE-120 (CWE-120). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-27459 published, and has it been updated?

CVE-2026-27459 was published on March 17, 2026 and was last updated on October 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-27459 — pyopenssl

Fix: pyca/pyopenssl@57f09bb

CVE-2026-27459 is a CWE-120 vulnerability in pyopenssl. A fix is available for pyopenssl — see the affected versions and patch details below.

pyOpenSSL DTLS cookie callback buffer overflow

Also known asGHSA-5pwr-322w-8jr4PYSEC-2026-2269

Published

Mar 17, 2026

Updated

Oct 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 14, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
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-2026-27459.

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk+0.03%

Lower risk than most CVEs59th percentile — riskier than 59% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍pyopenssl

Real-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 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 Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`pyopenssl`	≥ 22.0.0&&< 26.0.0	26.0.0`pip install --upgrade 'pyopenssl==26.0.0'`

Affected Products

1 product · 1 configurations

Application

pyopensslpyopenssl

≥ 22.0.0 && < 26.0.0

range

Detection & mitigation playbook

Open-source dependency

Detect
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-2026-27459 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

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.

Red HatImportant

This flaw is only exploitable when an application using the pyOpenSSL library provides a custom callback to the set_cookie_generate_callback function. For the buffer overflow to occur, the callback function must return a cookie string or byte sequence longer than 256 bytes, limiting the exposure of this issue. Due to…

Workaround published by Red Hat

To mitigate this flaw, ensure the callback provided to the set_cookie_generate_callback function strictly limits the returned cookie string or byte sequence to under 256 bytes.

Source: Red Hat security advisory for CVE-2026-27459 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat Ansible Automation Platform 2.5 for RHEL 8	`python3.12-pyOpenSSL-0:26.0.0-1.el8ap`	RHSA-2026:13512
Red Hat Ansible Automation Platform 2.6 for RHEL 9	`automation-controller-0:4.7.11-2.el9ap`	RHSA-2026:13508
Red Hat Satellite 6.16 for RHEL 8	`python-pyOpenSSL-0:24.1.0-2.el8pc`	RHSA-2026:14874
Red Hat Satellite 6.17 for RHEL 9	`python-pyOpenSSL-0:25.1.0-0.3.el9pc`	RHSA-2026:14873
Red Hat Satellite 6.18 for RHEL 9	`python3.12-pyOpenSSL-0:26.0.0-2.el9pc`	RHSA-2026:14835
RHUI 4 for RHEL 8	`python-pyOpenSSL-0:24.1.0-2.el8ui`	RHSA-2026:10754
Red Hat Ansible Automation Platform 2.5	`ansible-automation-platform-25/ee-supported-rhel8:1777398315`	RHSA-2026:13553
Red Hat Ansible Automation Platform 2.6	`ansible-automation-platform-26/ansible-dev-tools-rhel9:1777390240`	RHSA-2026:13545

UbuntuHIGH

USN-8115-1Ubuntu

Frequently Asked Questions

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.

O3 Security · Impact-Aware SCA

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