league/commonmark security advisory

CVE-2026-71488: League/commonmark: Quadratic-time denial of service when parsing crafted Markdown

CVE-2026-71488 is a high-severity denial of service vulnerability in the PHP package league/commonmark (CVSS 7.5), published 6 August 2026. It is fixed in 2.9.0; earlier versions are affected. It can be exploited without an account.

high 7.5 published 2026-08-06 · updated 2026-10-24

Affected versions: >= 0.6.0, < 2.9.0
Fixed in: 2.9.0
Newest release: 2.10.7
Severity: High · CVSS 7.5
Type: Denial of service
CWE-407, CWE-1050
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-2026-71488

Access: No account. Anyone who can send a request to an app that uses the affected code.

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

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-2026-71488 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. You need 2.9.0 or newer.
If Composer will not go that high, your composer.json constraint is holding it back. Widen it (for example "league/commonmark": "^2.9.0" 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-2026-71488.
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 denial of service issues while the upgrade waits. None of it replaces the fix.

Rate-limit the affected endpoint and cap request body, header and upload sizes at the web server.
If your app does not use the affected feature, note that in your audit, but still plan the upgrade.

The CVE-2026-71488 advisory

league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 0.6.0 until 2.9.0, specially crafted Markdown lines can cause the parser to have quadratic time complexity when converting, because several parsing paths repeatedly rescan growing portions of a line to translate between character positions and byte positions, and the Autolink extension can also copy and validate the remaining line at every URL-like prefix, allowing an attacker who can submit Markdown for conversion to consume disproportionate CPU time with a comparatively small request. 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-2026-71488. Advisory title: “league/commonmark: Quadratic-time denial of service when parsing crafted Markdown”.

References

GitHub advisory GHSA-2q4p-g7hv-5rgv github.com
Fix commit a6ef6cd github.com
Fix commit a70979e github.com
Fix commit c97b02e 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

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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.