CVE-2026-27459

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.

What this means for your business

It affects Pyopenssl. It matters if your company, or a supplier that handles your data, runs it.
An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 0.7% chance of attack attempts being seen in the next 30 days, ranking above 51% of all known flaws.

What to do

1Check whether your company or your suppliers run Pyopenssl, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Scoring

CVSS: 9.8 (v3.1)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS v4: 7.2
Weakness: CWE-120
Assigned by: security-advisories@github.com

Dates

Published: 2026-03-18
Last modified: 2026-09-10
Sources: NVD

Affected products

Pyopenssl Pyopenssl22.0.0 - 26.0.0

As listed in the NVD configuration data. Not a statement about your estate.