---
id: CVE-2026-55586
title: SumatraPDF is a multi-format reader for Windows
summary: >-
  SumatraPDF is a multi-format reader for Windows. In 3.6.1 and earlier, a
  crafted CHM file can supply malformed LZX Huffman code lengths to
  make_decode_table in ext/CHMLib/lzx.c. In the long-code branch, the function
  writes new internal n…
severity: medium
cvss: 6.6
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:L'
cwe:
  - CWE-119
  - CWE-787
published: '2026-08-20'
updated: '2026-09-18'
sourceUpdated: '2026-09-18T20:09:01.757'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-55586'
references:
  - url: >-
      https://github.com/sumatrapdfreader/sumatrapdf/commit/13b3d4204dd12d93d426f2157b157b149edc29bf
    label: security-advisories@github.com
  - url: >-
      https://github.com/sumatrapdfreader/sumatrapdf/security/advisories/GHSA-m423-rp8p-whj8
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00127
epssPercentile: 0.02708
ingestedAt: '2026-09-18T20:51:25.641Z'
---

## Overview

SumatraPDF is a multi-format reader for Windows. In 3.6.1 and earlier, a crafted CHM file can supply malformed LZX Huffman code lengths to make_decode_table in ext/CHMLib/lzx.c. In the long-code branch, the function writes new internal nodes through next_symbol before validating that the canonical Huffman table has overflowed. The PRETREE case can write beyond the 104-entry PRETREE_table into adjacent heap state in struct LZXstate when reached through chm_open, chm_retrieve_object, LZXdecompress, and BUILD_TABLE. This produces heap memory corruption in the parser process, while arbitrary code execution has not been demonstrated. No fixed version is available as of this review.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.