XSS in Thephpleague Commonmark
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 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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.2 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.004 (31.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 32nd percentile — 31.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8 (Exploit, vendor-advisory, Mitigation, Vendor Advisory)
- github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cad… (Patch, patch)
- www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextensio… (Patch, Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-86431?
- CVE-2026-86431 is a high-severity vulnerability in Thephpleague Commonmark, classified under Cross-site Scripting. CVSS score: 7.2/10. Published 2026-09-07.
- How severe is CVE-2026-86431?
- High severity. CVSS v3 base score is 7.2 out of 10.