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

2026-09-28T20:55:58.882377+00:00

Affected: py-pdf pypdf

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.

GHSA-m449-cwjh-6pw7 — pypdf's LZWDecode streams be manipulated to exhaust RAM

2026-09-28T20:55:58.882466+00:00

Affected: PyPI: pypdf

### 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](https://github.com/py-pdf/pypdf/security/advisories/GHSA-jfx9-29x2-rv3j) to align the default limit with the one for *zlib*.

### Patches This has been fixed in [pypdf==6.4.0](https://github.com/py-pdf/pypdf/releases/tag/6.4.0).

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

```python pypdf.filters.LZW_MAX_OUTPUT_LENGTH = 75_000_000 ```

PYSEC-2026-1832 — pypdf's LZWDecode streams be manipulated to exhaust RAM