pypdf's LZWDecode streams be manipulated to exhaust RAM

MEDIUM 6.6CVE-2025-66019 · GHSA-m449-cwjh-6pw7 · PYSEC-2026-1832 · Published 26/11/2025 · updated 19/10/2026

In short

CVE-2025-66019 is a MEDIUM (CVSS 6.6) vulnerability affecting pypdf.
Chance of exploitation in the next 30 days (EPSS): 0.39%, higher than 32.8% of all vulnerabilities.
Fix: update to 6.4.0 or later.

Package	Affected versions	Patched versions
pypdf PyPI	`< 6.4.0`	`6.4.0`

Description

Impact

An attacker who uses this vulnerability can craft a PDF which leads to a memory usage of up to 1 GB per stream. This requires parsing the content stream of a page using the LZWDecode filter.

This is a follow up to GHSA-jfx9-29x2-rv3j to align the default limit with the one for zlib.

Patches

This has been fixed in pypdf==6.4.0.

Workarounds

If users cannot upgrade yet, use the line below to overwrite the default in their code:

pypdf.filters.LZW_MAX_OUTPUT_LENGTH = 75_000_000

NVD description

pypdf is a free and open-source pure-python PDF library. Prior to version 6.4.0, an attacker who uses this vulnerability can craft a PDF which leads to a memory usage of up to 1 GB per stream. This requires parsing the content stream of a page using the LZWDecode filter. This issue has been patched in version 6.4.0.

References

py-pdf/pypdf security advisory · WEB
py-pdf/pypdf security advisory · WEB
nvd.nist.gov/vuln/detail/CVE-2025-66019 · ADVISORY
py-pdf/pypdf commit 9618672 · WEB
aydinnyunus.github.io/2025/12/20/cve-2025-66019-pypdf-lzw-dos · WEB
py-pdf/pypdf · PACKAGE
py-pdf/pypdf release 6.4.0 · WEB
aydinnyunus.github.io/2025/12/20/cve-2025-66019-pypdf-lzw-dos
pypi.org/project/pypdf · PACKAGE
GitHub Advisory GHSA-m449-cwjh-6pw7 · ADVISORY

More vulnerabilities in pypdf

See all →

Running one of these products? Scan your site to see whether it exposes a vulnerable version.

Scan my site free

Package

Affected versions

Patched versions

pypdf

PyPI

< 6.4.0

6.4.0