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

2026-09-28T18:32:07.733915+00:00

Affected: py-pdf pypdf

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.

GHSA-9mvc-8737-8j8h — pypdf possibly has long runtimes for malformed FlateDecode streams

2026-09-28T18:32:07.734153+00:00

Affected: PyPI: pypdf

### Impact

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.

### Patches

This has been fixed in [pypdf==6.7.1](https://github.com/py-pdf/pypdf/releases/tag/6.7.1).

### Workarounds

If you cannot upgrade yet, consider applying the changes from PR [#3644](https://github.com/py-pdf/pypdf/pull/3644).

PYSEC-2026-3015 — pypdf possibly has long runtimes for malformed FlateDecode streams