CVE-2026-86431

HIGHPre-NVD 7.27.2—
EchelonGraph scoreHIGH confidence

Score 7.2 from GitHub Security Advisory published 2026-09-07. a secondary CVSS source baseline 7.2; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 3 sources updated this week
7.2EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 7.2Exploit: None knownExposed: 0

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

league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.

CVSS v3
7.2
EG Score
7.2(high)
EG Risk
47(Track)
EG Risk 47/100SSVC: 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
Severity72% × 45%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
13%
KEV
Not listed

Published

September 7, 2026

Last Modified

September 9, 2026

Advisory Details (3)

Auto-updated Sep 7, 2026
🔬 Proof of concept available. Patch available. Sources: github, github_commit.
generic

commonmark before 2.9.1 XSS via AttributesExtension form feed bypass | Advisories | VulnCheck

https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass
github_commit Patch Available

commit 43207253ea5f (thephpleague/commonmark)

Patch available: thephpleague/commonmark 2.7.0 (contains commit 43207253ea5f)

https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea
github Patch Available🟡 PoC Available

XSS: `on*` event-handler filter in `AttributesExtension` bypassed with a U+000C form feed · Advisory · thephpleague/commonmark · GitHub

https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8

Vendor Advisories for CVE-2026-86431(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(4 across 4 ecosystems)
Debian:12(1)
PackageVulnerable rangeFixed inDependents
php-league-commonmark2.10.0-1 ... 2.9.0-1 (23 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
php-league-commonmark2.10.0-1 ... 2.9.0-1 (11 versions)——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
php-league-commonmark2.7.0-1 ... 2.9.0-1 (9 versions)2.10.0-1—
Packagist(1)
PackageVulnerable rangeFixed inDependents
league/commonmark2.7.0 ... 2.9.0 (7 versions)2.9.1—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 26× in last 7d / 49× 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. 2026-09-18 21:22 UTCEG score recompute
  2. 2026-09-18 21:22 UTCGHSA enrichment
  3. 2026-09-18 19:28 UTCEPSS rescore
  4. 2026-09-18 08:22 UTCGHSA enrichment
  5. 2026-09-17 20:21 UTCEG score recompute
  6. 2026-09-17 20:21 UTCGHSA enrichment
  7. 2026-09-17 19:32 UTCEPSS rescore
  8. 2026-09-17 06:45 UTCGHSA enrichment
  9. 2026-09-16 18:29 UTCEG score recompute
  10. 2026-09-16 18:29 UTCGHSA enrichment
  11. 2026-09-16 14:09 UTCEPSS rescore
  12. 2026-09-16 06:28 UTCEG score recompute
  13. 2026-09-16 06:27 UTCGHSA enrichment
  14. 2026-09-16 05:15 UTCEPSS rescore
  15. 2026-09-15 18:26 UTCGHSA enrichment
  16. 2026-09-15 06:23 UTCEG score recompute
  17. 2026-09-15 06:23 UTCGHSA enrichment
  18. 2026-09-15 03:12 UTCEPSS rescore
  19. 2026-09-15 03:12 UTCEPSS rescore
  20. 2026-09-14 18:21 UTCGHSA enrichment
  21. 2026-09-14 05:35 UTCGHSA enrichment
  22. 2026-09-13 17:33 UTCEG score recompute
  23. 2026-09-13 17:33 UTCGHSA enrichment
  24. 2026-09-13 05:31 UTCGHSA enrichment
  25. 2026-09-12 17:29 UTCEG score recompute
Show 24 more
  1. 2026-09-12 17:29 UTCGHSA enrichment
  2. 2026-09-12 05:27 UTCGHSA enrichment
  3. 2026-09-11 17:25 UTCEG score recompute
  4. 2026-09-11 17:25 UTCGHSA enrichment
  5. 2026-09-11 14:53 UTCEPSS rescore
  6. 2026-09-11 14:53 UTCEPSS rescore
  7. 2026-09-11 04:59 UTCGHSA enrichment
  8. 2026-09-10 16:57 UTCEG score recompute
  9. 2026-09-10 16:57 UTCGHSA enrichment
  10. 2026-09-10 04:55 UTCGHSA enrichment
  11. 2026-09-09 16:52 UTCGHSA enrichment
  12. 2026-09-09 08:15 UTCEG score recompute
  13. 2026-09-09 08:15 UTCGHSA enrichment
  14. 2026-09-08 22:01 UTCEPSS rescore
  15. 2026-09-08 20:13 UTCGHSA enrichment
  16. 2026-09-08 14:50 UTCEG score recompute
  17. 2026-09-08 14:49 UTCGHSA enrichment
  18. 2026-09-08 13:48 UTCEG score recompute
  19. 2026-09-08 13:48 UTCGHSA enrichment
  20. 2026-09-08 01:30 UTCEG score recompute
  21. 2026-09-08 01:29 UTCGHSA enrichment
  22. 2026-09-07 13:28 UTCEG score recompute
  23. 2026-09-07 12:57 UTCEG score recompute
  24. 2026-09-07 12:56 UTCMITRE cvelistV5first tracked

Frequently asked(5)

What is CVE-2026-86431?
CVE-2026-86431 is a high vulnerability published on September 7, 2026. league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*'…
When was CVE-2026-86431 disclosed?
CVE-2026-86431 was first published in the National Vulnerability Database on September 7, 2026, with the most recent update on September 9, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-86431 actively exploited?
CVE-2026-86431 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 86.7% of all scored CVEs.
What is the CVSS score of CVE-2026-86431?
CVE-2026-86431 has a CVSS v3 base score of 7.2 (NVD).
How do I remediate CVE-2026-86431?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-86431, 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

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-86431

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