CVE-2026-55586

6.6 MEDIUM 0.2% exploit odds Fix listed Memory safety OS & kernel

What it is

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.

At a glance

Exploitability: Can be exploited locally on the machine without logging in, but needs a user to act. High risk of data tampering.
Exploit odds: 0.2% chance of exploitation in the next 30 days (EPSS).
Fix: A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

2026-08-20 17:18 UTC PublishedDisclosed publicly on CVE.org and NVD
2026-10-18 20:10 UTC Last updatedMost recent change to the record

Why it ranks here

Exploited in the wild: no, CISA does not list it.
Exploit odds: 0.2% in the next 30 days (EPSS).
Severity: 6.6 MEDIUM (CVSS).
Fix: a patch or advisory is linked.

Details

Published: 2026-08-20 17:18 UTC
Last modified: 2026-10-18 20:10 UTC
Exploit probability: 0.2% · EPSS, next 30 days
Weakness: CWE-119 · Buffer errors
Record status: Deferred

Attack profile CVSS 3.1 · 6.6

Attack vector: Local
Complexity: Low
Privileges needed: None
User interaction: Required
Scope: Unchanged
Confidentiality: Low
Integrity: High
Availability: Low

Raw vector

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:L

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-55586.md and follow it. Check whether this project is affected by CVE-2026-55586. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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Fix and advisories

References

NVD record · CVE.org record