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HIGH

CVE-2026-86431

CVSS v3

7.2

HIGH

EPSS Score

—

exploit probability

CISA KEV

No

known exploited

Exploitation

—

SSVC status

Description

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 de

Technical details

CVSS v3 Vector
3.1
Published
2026-09-07
Last Modified
2026-09-07

Frequently asked questions

What is CVE-2026-86431?

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 de

Is CVE-2026-86431 actively exploited?

Active exploitation of CVE-2026-86431 has not been confirmed. The EPSS score is N/A%, indicating the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-86431?

CVE-2026-86431 has a CVSS v3 base score of 7.2 (HIGH severity), with vector string 3.1.

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