CVE-2026-71488

league/commonmark from 2.0.0 before 2.10.2 contains a quadratic-time denial of service vulnerability in the GitHub Flavored Markdown Table extension's TableStartParser::tryStart() block-start scan. Unauthenticated attackers can submit a large paragraph of pipe-free lines not starting with letters, forcing repeated full-buffer strpos scans that exhaust PHP worker CPU.

League CommonMark from 1.3.0 before 2.10.2 contains a cross-site scripting vulnerability allowing users posting Markdown to bypass the DisallowedRawHtml extension by ending raw HTML with a bare disallowed tag name. Attackers can place a lone <script or <iframe line followed by a block supplying attributes like src or onload, executing stored scripts in viewers' browsers under default GFM settings.

league/commonmark versions before 2.6.0 contain polynomial time complexity vulnerabilities in Markdown parsing that allow attackers to cause denial of service. Attackers can submit carefully crafted Markdown inputs designed to trigger worst-case performance, and sending multiple requests in parallel exhausts CPU resources and PHP-FPM processes.

Denial of service vulnerability in league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) in UniqueSlugNormalizer::normalize(). On slug collision, the numeric-suffix search restarts from 1, resulting in O(K^2) time complexity for K headings collapsing to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered.

Cross-site scripting vulnerability in AttributesExtension of league/commonmark versions >= 2.7.0 and < 2.9.1. Prefixing an attribute name with a single U+000C form feed byte bypasses the AttributesHelper::filterAttributes() 'on*' filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false.

Multiple denial of service vulnerabilities in league/commonmark before 2.9.1 in fenced code block detection, reference link label lookup, and emphasis delimiter processing that perform super-linear work on crafted input. Attackers can submit specially crafted Markdown with long backtick runs, nested brackets, or delimiter sequences to consume disproportionate CPU time and prevent legitimate requests from completing.

The league/commonmark library in versions >= 1.5.0 and < 2.9.1 contains quadratic parsing complexity in its SmartPunctExtension and AttributesExtension. When either extension is explicitly registered on the Environment (they are not enabled by default and are excluded from the standard CommonMark and GitHub-Flavored Markdown converters), an unauthenticated attacker can submit small, specially crafted Markdown documents — such as text alternating with unpaired quotes, contiguous runs of block-level attribute blocks, or repeated class attributes — to trigger disproportionate CPU consumption and cause a denial of service.

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.