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CVE-2025-66019: pypdf manipulated LZWDecode streams can exhaust RAM

• Severity: medium (CVSS 6.6 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-11-26 · updated 2026-10-19
• Weakness: CWE-400

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.

Affected products

• py-pdf pypdf: before 6.4.0

References

• https://github.com/py-pdf/pypdf/commit/96186725e5e6f237129a58a97cd19204a9ce40b2
• https://github.com/py-pdf/pypdf/releases/tag/6.4.0
• https://github.com/py-pdf/pypdf/security/advisories/GHSA-m449-cwjh-6pw7
• https://aydinnyunus.github.io/2025/12/20/cve-2025-66019-pypdf-lzw-dos/

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