CVE-2026-27026: pypdf possibly has long runtimes for malformed FlateDecode streams

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-20 · updated 2026-06-17
Weakness: CWE-770

Description

pypdf is a free and open-source pure-python PDF library. Prior to 6.7.1, an attacker who uses this vulnerability can craft a PDF which leads to long runtimes. This requires a malformed /FlateDecode stream, where the byte-by-byte decompression is used. This vulnerability is fixed in 6.7.1.

Affected products

py-pdf pypdf: before 6.7.1 — fixed in 6.7.1

How to fix

Upgrade to a fixed version: py-pdf pypdf 6.7.1. Check the vendor advisory for mitigations.

References

https://github.com/py-pdf/pypdf/commit/7905842d833f899f1d3228af7e7467ad80277016
https://github.com/py-pdf/pypdf/pull/3644
https://github.com/py-pdf/pypdf/releases/tag/6.7.1
https://github.com/py-pdf/pypdf/security/advisories/GHSA-9mvc-8737-8j8h

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