Bypass Vulnerability in league/commonmark PHP Library for Enhanced Markdown Handling
CVE-2026-71478

6.1MEDIUM

Key Information:

Vendor

ThePHPleague

Status
Commonmark
Vendor
CVE Published:
6 August 2026

What is CVE-2026-71478?

The league/commonmark PHP library, which is utilized for parsing and rendering CommonMark Markdown, has a vulnerability that permits the bypassing of its AttributesExtension's href and src unsafe-link filter. This occurs when control bytes—such as tab, carriage return, line feed, or leading C0 control characters—are embedded within a javascript: URL. Browsers may discard these control bytes before parsing the scheme, allowing scripts to execute even when precautions are in place. This flaw affects versions 1.5.0 through 2.9.0 and has been resolved in version 2.9.0.

Affected Version(s)

commonmark >= 1.5.0, <= 2.8.3

References

https://github.com/thephpleague/commonmark/security/advis...
x_refsource_CONFIRM
https://github.com/thephpleague/commonmark/commit/493a5aa...
x_refsource_MISC
https://github.com/thephpleague/commonmark/releases/tag/2...
x_refsource_MISC

CVSS V3.1

Score:
6.1
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
Low
Availability:
Low
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
Required
Scope:
Changed

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.