Cross-Site Scripting Vulnerability in league/commonmark Affected by thephpleague
CVE-2026-86431

6.9MEDIUM

Key Information:

Vendor

ThePHPleague

Status
Commonmark
Vendor
CVE Published:
7 September 2026

What is CVE-2026-86431?

The league/commonmark component versions 2.7.0 to 2.9.0 are susceptible to a cross-site scripting vulnerability within the AttributesExtension. By using a U+000C form feed byte to prefix attribute names, attackers can bypass the intended attribute filtering mechanisms. This allows malicious scripts to be injected into the rendered HTML, executing JavaScript when a user views the output. This issue requires the processing of untrusted Markdown while having the AttributesExtension enabled. The vulnerability is addressed in version 2.9.1.

Affected Version(s)

commonmark 2.7.0 < 2.9.1

commonmark 2.9.1

References

https://github.com/thephpleague/commonmark/security/advis...
vendor-advisory
https://github.com/thephpleague/commonmark/commit/4320725...
patch
https://www.vulncheck.com/advisories/commonmark-before-2....
third-party-advisory

CVSS V4

Score:
6.9
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
Low
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
None
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

StarPlatinu
.