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

Medium 6.9 Published 20 Feb 2026 Updated 24 Feb 2026

An attacker can cause long runtimes and potential denial of service by crafting a PDF with a malformed FlateDecode stream in the pypdf library.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2026-27026?

CVE-2026-27026 is a medium-severity denial of service vulnerability in Pypdf Project Pypdf, published on 20 Feb 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official 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.

Weakness: CWE-770

Which versions are affected by CVE-2026-27026?

3 open source packages ship an affected version, including pypdf. It affects 1 product.

Package	Affected	Fixed in
`pypdf` PyPI	All versions	`6.7.1`
`pypdf` PyPI	All versions	`6.7.1`
`pypdf` PyPI	All versions	`6.7.1`

Products include Pypdf Project Pypdf.

How do I fix CVE-2026-27026?

Upgrade pypdf to 6.7.1 or later.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-27026 being exploited?

There is no known exploitation of CVE-2026-27026 in the wild and no public exploit code.

All Pypdf Project Pypdf vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-27026, GHSA-9mvc-8737-8j8h, PYSEC-2026-3015, ROOT-APP-PYPI-CVE-2026-27026, UBUNTU-CVE-2026-27026.