CVE-2026-55586

MEDIUMPre-NVD 6.66.6—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 6.6 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.1%, top 97% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
6.6EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 6.6Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
6.6
EG Score
6.6(medium)
EG Risk
30(Track)
EG Risk 30/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity66% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
3%
KEV
Not listed

Published

August 20, 2026

Last Modified

August 21, 2026

Advisory Details (2)

Auto-updated Aug 20, 2026
🔬 Proof of concept available. Patch available. Sources: github, github_commit.
github_commit

commit 13b3d4204dd1 (sumatrapdfreader/sumatrapdf)

Fix landed in sumatrapdfreader/sumatrapdf commit 13b3d4204dd1 — awaiting tagged release

https://github.com/sumatrapdfreader/sumatrapdf/commit/13b3d4204dd12d93d426f2157b157b149edc29bf
github Patch Available🟡 PoC Available

Heap out-of-bounds write in vendored CHMLib LZX Huffman table construction reachable from crafted CHM files · Advisory · sumatrapdfreader/sumatrapdf · GitHub

https://github.com/sumatrapdfreader/sumatrapdf/security/advisories/GHSA-m423-rp8p-whj8

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 11× in last 7d / 19× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-30 01:22 UTCEPSS rescore
  2. 2026-08-29 17:57 UTCEG score recompute
  3. 2026-08-28 21:41 UTCEPSS rescore
  4. 2026-08-27 19:01 UTCEG score recompute
  5. 2026-08-27 14:25 UTCEPSS rescore
  6. 2026-08-26 19:30 UTCEG score recompute
  7. 2026-08-26 14:46 UTCEPSS rescore
  8. 2026-08-25 20:05 UTCEG score recompute
  9. 2026-08-25 13:49 UTCEPSS rescore
  10. 2026-08-24 20:38 UTCEG score recompute
  11. 2026-08-23 21:14 UTCEG score recompute
  12. 2026-08-23 00:19 UTCEPSS rescore
  13. 2026-08-22 21:50 UTCEG score recompute
  14. 2026-08-21 23:49 UTCEPSS rescore
  15. 2026-08-21 22:26 UTCEG score recompute
  16. 2026-08-21 22:00 UTCEG score recompute
  17. 2026-08-20 17:33 UTCEG score recompute
  18. 2026-08-20 16:52 UTCEG score recompute
  19. 2026-08-20 16:51 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-55586?
CVE-2026-55586 is a medium vulnerability published on August 20, 2026. 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 makedecodetable in ext/CHMLib/lzx.c. In the long-code branch, the function writes new internal nodes through nextsymbol before validating that the canonical…
When was CVE-2026-55586 disclosed?
CVE-2026-55586 was first published in the National Vulnerability Database on August 20, 2026, with the most recent update on August 21, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-55586 actively exploited?
CVE-2026-55586 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 97.4% of all scored CVEs.
What is the CVSS score of CVE-2026-55586?
CVE-2026-55586 has a CVSS v3 base score of 6.6 (NVD).
How do I remediate CVE-2026-55586?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-55586, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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