CVE-2026-86431: league/commonmark XSS: `on*` event-handler filter in `AttributesExtension` bypassed with a U+000C form feed

high Affects league/commonmark. Feed 2026.10.29.4, updated 2026-10-29; published 2026-09-01; advisory modified 2026-09-08.

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

Package: league/commonmark Composer
Affected versions: >=2.7.0 and <2.9.1
Fixed in: 2.9.1
Severity: high CVSS 7.2
Exploitation: not on CISA KEV · EPSS 0% (32.9th percentile)
Published: 2026-09-01
Source: osv.dev

References

CWE-79 CWE-86

NVD record league/commonmark advisories

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CVE-2026-86429 high league/commonmark: Denial of service in the SmartPunct and Attributes extensions same package: league/commonmark
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CVE-2026-86428 high league/commonmark: Denial of service via distinctly-named attributes in the Attributes ext same package: league/commonmark