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Permalink CVE-2026-86431
6.9 MEDIUM
  • CVSS version (CVSS): 4.0
  • Attack Vector (AV): Network (N)
  • Attack Complexity (AC): Low (L)
  • Attack Requirement (AT): None (N)
  • Privileges Required (PR): None (N)
  • User Interaction (UI): None (N)
  • Vulnerable System Impact Confidentiality (VC): Low (L)
  • Vulnerable System Impact Integrity (VI): Low (L)
  • Vulnerable System Impact Availability (VA): None (N)
  • Subsequent System Impact Confidentiality (SC): Low (L)
  • Subsequent System Impact Integrity (SI): Low (L)
  • Subsequent System Impact Availability (SA): None (N)
  • Modified Attack Vector (MAV): Network (N)
  • Modified Attack Complexity (MAC): Low (L)
  • Modified Attack Requirement (MAT): None (N)
  • Modified Privileges Required (MPR): None (N)
  • Modified User Interaction (MUI): None (N)
  • Modified Vulnerable System Impact Confidentiality (MVC): Low (L)
  • Modified Vulnerable System Impact Integrity (MVI): Low (L)
  • Modified Vulnerable System Impact Availability (MVA): None (N)
  • Modified Subsequent System Impact Confidentiality (MSC): Low (L)
  • Modified Subsequent System Impact Integrity (MSI): Low (L)
  • Modified Subsequent System Impact Availability (MSA): Negligible (N)
  • Safety (S): Not Defined (X)
  • Automatable (AU): Not Defined (X)
  • Recovery (R): Not Defined (X)
  • Value Density (V): Not Defined (X)
  • Vulnerability Response Effort (RE): Not Defined (X)
  • Provider Urgency (U): Not Defined (X)
  • Confidentiality Req. (CR): Not Defined (X)
  • Integrity Req. (IR): Not Defined (X)
  • Availability Req. (AR): Not Defined (X)
  • Exploit Maturity (E): Not Defined (X)
updated 1 week, 6 days ago by @LeSuisse Activity log
  • Created suggestion 1 week, 6 days ago
  • @LeSuisse dismissed (not in Nixpkgs) 1 week, 6 days ago
commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

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.

Affected products

commonmark
  • ==2.9.1
  • <2.9.1

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