CVE-2026-55586

MEDIUMPre-NVD 6.66.6
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

This medium-severity CVE scores 6.6 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.2% (6th percentile of EPSS-scored CVEs). No GitHub Security Advisory CVSS is on record for this CVE.

Triggered by: Headline CVSS from a secondary CVSS source
Sources: EPSS, a secondary CVSS source
6.6EG
EchelonGraph verdictMonitorRemediate within your standard update timelines (the SSVC decision on this page).
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.2%CVSS: 6.6Exploit: None knownExposed services: Not assessed

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

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
6.6MEDIUMmedium confidence
EG Risk
30
EG Risk 30/100

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 (value × EchelonGraph’s weight)
Severity66% × 45%
The EG Score, 6.6 of 10.
Exploitation0.2% × 40%
FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.
Automatability0% × 15%
CISA's SSVC Automatable value: no (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at every mission impact level.
Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: No fix is confirmed yet. Restrict network exposure of the affected system within your standard update timelines, and watch for a fix.
EPSS PROB
0.2%
EPSS %ILE
6th
KEV
Not listed

SSVC (CISA's decision table, applied by EchelonGraph)Track at every mission impact level.

No fix is confirmed yet. Restrict network exposure of the affected system within your standard update timelines, and watch for a fix.

Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)60 days if the affected system is publicly exposed; Fix on system upgrade if it is not.

In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

Published

August 20, 2026

Last Modified

September 18, 2026

Advisory Details (2)

Auto-updated Aug 20, 2026
No fix on record yet. 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 advisoryMentions a patch (unverified)

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 / 50× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

  1. 2026-10-13 16:34 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  2. 2026-10-12 15:55 UTCEG score recompute
  3. 2026-10-11 19:38 UTCEG score recompute
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  6. 2026-10-11 13:49 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  8. 2026-10-09 20:07 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  9. 2026-10-08 17:12 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  10. 2026-10-08 07:21 UTCEG score recompute
  11. 2026-10-07 23:48 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  13. 2026-10-06 15:18 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  15. 2026-10-05 17:43 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  17. 2026-10-04 23:22 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  19. 2026-10-03 14:26 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  20. 2026-10-03 06:38 UTCEG score recompute
  21. 2026-10-02 17:56 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  23. 2026-10-01 19:50 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  25. 2026-09-30 15:03 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  1. 2026-09-30 08:14 UTCEG score recompute
  2. 2026-09-29 15:22 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  3. 2026-09-29 01:56 UTCEG score recompute
  4. 2026-09-28 13:51 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  6. 2026-09-27 13:48 UTCEPSS rescoreEPSS 0.2% · 6th percentile
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  8. 2026-09-26 15:59 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  9. 2026-09-24 19:29 UTCEG score recompute
  10. 2026-09-24 14:04 UTCEPSS rescoreEPSS 0.2% · 6th percentile
  11. 2026-09-23 20:05 UTCEG score recompute
  12. 2026-09-23 17:54 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  13. 2026-09-22 19:45 UTCEG score recompute
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  15. 2026-09-20 20:16 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  16. 2026-09-19 19:45 UTCEG score recompute
  17. 2026-09-18 20:20 UTCEG score recompute
  18. 2026-09-18 19:28 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  19. 2026-09-17 21:19 UTCEG score recompute
  20. 2026-09-17 19:31 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
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  22. 2026-09-16 14:08 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
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  24. 2026-09-15 21:50 UTCEG score recompute
  25. 2026-09-15 03:11 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  26. 2026-09-13 23:02 UTCEG score recompute
  27. 2026-09-13 16:47 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  28. 2026-09-12 23:38 UTCEG score recompute
  29. 2026-09-12 00:13 UTCEG score recompute
  30. 2026-09-11 14:53 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  31. 2026-09-11 00:48 UTCEG score recompute
  32. 2026-09-10 09:35 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
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  38. 2026-09-06 13:47 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  39. 2026-09-05 20:34 UTCEG score recompute
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  41. 2026-09-04 05:06 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
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  44. 2026-09-01 18:30 UTCEG score recompute
  45. 2026-09-01 13:54 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
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  49. 2026-08-30 01:22 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  50. 2026-08-29 17:57 UTCEG score recompute
  51. 2026-08-28 21:41 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  52. 2026-08-27 19:01 UTCEG score recompute
  53. 2026-08-27 14:25 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  54. 2026-08-26 19:30 UTCEG score recompute
  55. 2026-08-26 14:46 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  56. 2026-08-25 20:05 UTCEG score recompute
  57. 2026-08-25 13:49 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  58. 2026-08-24 20:38 UTCEG score recompute
  59. 2026-08-23 21:14 UTCEG score recompute
  60. 2026-08-23 00:19 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  61. 2026-08-22 21:50 UTCEG score recompute
  62. 2026-08-21 23:49 UTCEPSS rescoreEPSS 0.1% · 2nd percentile
  63. 2026-08-21 22:26 UTCEG score recompute
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  65. 2026-08-20 17:33 UTCEG score recompute
  66. 2026-08-20 16:52 UTCEG score recomputeFirst assessed: 6.6 MEDIUM
  67. 2026-08-20 16:51 UTCMITRE cvelistV5first tracked

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Frequently asked(5)

What is CVE-2026-55586?
CVE-2026-55586 is a medium-severity vulnerability; its CVE record was 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 the CVE record for CVE-2026-55586 published?
The CVE record for CVE-2026-55586 was published on August 20, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of September 18, 2026; the record at its source may have been updated since.
Is CVE-2026-55586 actively exploited?
CVE-2026-55586 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.2% probability of exploitation in the next 30 days (6th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-55586?
CVE-2026-55586 has a CVSS base score of 6.6 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-55586?
No fix for CVE-2026-55586 is confirmed yet. Until one is published, restrict network exposure of the affected system — for example, keep it off the internet or limit it to trusted networks — and watch for a fix.

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