league/commonmark XSS: on* event-handler filter in AttributesExtension bypassed with a U+000C form feed

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-86431: 0.39% estimated exploitation activity in the next 30 days; percentile 32.97 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The AttributesExtension documents a security guarantee: Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them. — docs/2.x/extensions/attributes.md Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FFonclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler. The same prefix defeats the allowunsafelinks check, letting a javascript: URI through on href / src even when allowunsafelinks is false. This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes. Throughout this report <FF denotes a literal U+000C…

Published by the source

1 Sep 2026, 20:18 UTC

Last modified by the source

8 Sep 2026, 12:38 UTC

Source last checked

26 Oct 2026, 14:51 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-f8fg-pg57-v4j8

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-f8fg-pg57-v4j8

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# Patch Next advisory brief: CVE-2026-86431

Advisory: league/commonmark XSS: `on*` event-handler filter in `AttributesExtension` bypassed with a U+000C form feed
IDs: CVE-2026-86431, GHSA-f8fg-pg57-v4j8
Record: https://patchnext.com/advisories/cve-2026-86431 (revision 2, source checked 26 Oct 2026, 14:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: league/commonmark 2.9.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist league/commonmark: affected >= 2.7.0, < 2.9.1; fixed in 2.9.1

## Suggested inspection (read-only)
- Packagist league/commonmark: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on league/commonmark.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-f8fg-pg57-v4j8 (OSV record GHSA-f8fg-pg57-v4j8 lists affected ranges: Packagist league/commonmark >=2.7.0 <2.9.1 (fixed))
- Verified: https://github.com/advisories/GHSA-f8fg-pg57-v4j8 (GitHub-reviewed advisory GHSA-f8fg-pg57-v4j8 lists affected ranges: Packagist league/commonmark >=2.7.0 <2.9.1)

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