Vulnerability · Published Aug 6, 2026 · Assigned by GitHub_M
CVE-2026-71478league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes
Medium 6.1
In plain English
A medium-severity cross-site scripting in commonmark from thephpleague. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 29% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 1.5.0 until 2.9.0, the AttributesExtension's href and src unsafe-link filter can be bypassed by embedding control bytes, such as a tab, carriage return, line feed, or leading C0 control character, in a javascript: URL that browsers discard before parsing the scheme, causing the browser to still execute the script even when the unsafe-link filter is enabled. This issue is fixed in 2.9.0.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | php-league-commonmark | no fix listed | DEBIAN-CVE-2026-71478 |
| Debian:13 | php-league-commonmark | no fix listed | DEBIAN-CVE-2026-71478 |
| Debian:14 | php-league-commonmark | 2.9.0-1 | DEBIAN-CVE-2026-71478 |
| Packagist | league/commonmark | 2.9.0 | GHSA-29pj-957v-52mc |
| Ubuntu:22.04:LTS | commonmark | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:24.04:LTS | commonmark | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:26.04:LTS | commonmark | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:26.10 | commonmark | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:22.04:LTS | markdown | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:24.04:LTS | markdown | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:26.04:LTS | markdown | no fix listed | UBUNTU-CVE-2026-71478 |
| Ubuntu:26.10 | markdown | no fix listed | UBUNTU-CVE-2026-71478 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| thephpleague | commonmark | >= 1.5.0, <= 2.8.3 |
Weakness: CWE-79, CWE-86, CWE-692 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N