CVE-2026-86431

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CVE-2026-86431

Summary: commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

Details: 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.

References: https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json, https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8, https://nvd.nist.gov/vuln/detail/CVE-2026-86431, https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass, https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea