league/commonmark security advisory

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

CVE-2026-71478 is a medium-severity XSS vulnerability in the PHP package league/commonmark (CVSS 6.1), published 6 August 2026. The advisory does not name a fixed version. It can be exploited without an account.

medium 6.1 published 2026-08-06 · updated 2026-10-24

Affected versions: >= 1.5.0, <= 2.8.3
Fixed in: not stated
Newest release: 2.10.7
Severity: Medium · CVSS 6.1
Type: Cross-site scripting (XSS)
CWE-79, CWE-86, CWE-692
Exploited in the wild: Not on the CISA KEV list
checked 2026-11-04

Is your league/commonmark version affected?

Run composer show league/commonmark in your project, then enter the version. We check it against this and every other known league/commonmark advisory.

Who can exploit CVE-2026-71478

Access: No account. Anyone who can send a request to an app that uses the affected code.
Interaction: A user also has to do something, such as open a link or a page the attacker prepared.

CVSS vector (scored by GitHub): CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Your app is only exposed if it uses the affected part of league/commonmark. The advisory below says which part that is. If you are not sure, upgrade anyway: it costs less than finding out.

How to fix CVE-2026-71478 in league/commonmark

Find the installed version: composer show league/commonmark. If you never required it yourself, composer why league/commonmark shows which package pulls it in.
Update it: composer update league/commonmark --with-all-dependencies.
If Composer will not go that high, your composer.json constraint is holding it back. Widen it (for example "league/commonmark": "^2.10.7" if you can take that version), or upgrade the package that requires it.
Run composer audit. It reads the same advisory data and should no longer list CVE-2026-71478.
Commit composer.lock and deploy it. On the server, run composer install --no-dev from the lock file, not composer update.

If you cannot upgrade yet

General hardening for XSS issues while the upgrade waits. None of it replaces the fix.

Escape output in your own templates and add a Content-Security-Policy that blocks inline scripts.
Limit who can save the content the advisory names (for example rich text, SVG or form fields).
If your app does not use the affected feature, note that in your audit, but still plan the upgrade.

The CVE-2026-71478 advisory

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.

Description from the CVE record, published by GitHub (the CVE Numbering Authority), via the NVD entry for CVE-2026-71478. Advisory title: “league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes”.

References

GitHub advisory GHSA-29pj-957v-52mc github.com
Fix commit 493a5aa github.com
Release 2.9.0 github.com

Other league/commonmark advisories

medium League/commonmark: DisallowedRawHtml bypassed when a disallowed tag name ends the raw-HTML literal 2026-09-30
high League/commonmark: Quadratic-time denial of service in the GitHub Flavored Markdown Table extension block-start scan 2026-09-30
high League/commonmark: Denial of service via distinctly-named attributes in the Attributes extension 2026-09-01
high League/commonmark: Denial of service in the SmartPunct and Attributes extensions 2026-09-01
high League/commonmark XSS: on* event-handler filter in AttributesExtension bypassed with a U+000C form feed 2026-09-01
high League/commonmark: Denial of service via crafted code fences, reference links, and emphasis delimiters 2026-09-01

Need help patching or upgrading? Laskyr does PHP and Laravel security fixes and upgrades. Tell us what you run and we reply with a plan and a price.

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Where this data comes from

Affected and fixed versions: the Packagist security advisories API (GitHub Advisory Database and FriendsOfPHP), the same data composer audit uses. Synced 2026-11-03.
Description, score, weakness and references: NIST National Vulnerability Database (NVD), checked 2026-11-04. This product uses data from the NVD API but is not endorsed or certified by the NVD.
Exploited-in-the-wild status: the CISA Known Exploited Vulnerabilities catalog, checked 2026-11-04.
Compiled by Daniel Lasek at Laskyr from the sources above. The summary, “who can exploit it”, “how to fix it” and hardening sections are our plain-English reading of the record and are generated from it. The quoted description is the record's own text. Spot an error? Tell us.