EUVD-2026-72389
GHSA-m24x-mf52-x8gc
Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
AC:H because AttributesExtension must be explicitly enabled; UI:R because victim must render the HTML; S:C for XSS cross-browser-context impact.
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
Lifecycle Timeline
2DescriptionCVE.org
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.
AnalysisAI
Cross-site scripting in league/commonmark 2.7.0-2.9.0 allows unauthenticated attackers to inject arbitrary JavaScript into rendered HTML output by prefixing an attribute name with a U+000C form feed byte, bypassing the AttributesExtension's event-handler filter and the allow_unsafe_links safeguard. The bypass exploits a mismatch between what PHP's built-in trim() considers whitespace (excluding U+000C) and what PCRE \s matches (including U+000C), leaving the byte embedded in the attribute name where PHP's string comparison logic fails to recognize it but browsers' HTML parsers strip it and treat the remainder as a genuine event handler. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires two non-default application-level conditions: (1) The AttributesExtension (League\CommonMark\Extension\Attributes\AttributesExtension) must be explicitly registered with the CommonMark Environment - it is an optional extension and is not loaded by default, so default CommonMark installations are not affected. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 vector provided (AV:N/AC:L/AT:N/PR:N/UI:N - score 6.9) rates this moderate in severity, though the UI:N metric appears to understate the requirement that a victim must visit or render the output page for the injected script to execute. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to league/commonmark 2.9.1, which is the vendor-confirmed fixed release per the GHSA advisory and CHANGELOG. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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