The league/commonmark (thephpleague/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. Fixed in 2.9.1.

The league/commonmark PHP library (thephpleague/commonmark) in versions >= 1.5.0 and < 2.9.1 is vulnerable to a denial-of-service attack due to quadratic parsing complexity in its SmartPunctExtension and AttributesExtension. These extensions are not enabled by default but when explicitly registered, an unauthenticated attacker can craft small Markdown documents with patterns such as unpaired quotes, repeated block-level attribute blocks, or repeated class attributes to trigger disproportionate CPU consumption. The vulnerability does not affect standard CommonMark or GitHub-Flavored Markdown converters out of the box. Applications that explicitly enable either of the two affected extensions are at risk. The issue has been fixed in version 2.9.1 of the library.