league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying service.

league/commonmark versions >= 2.0.0 and < 2.8.4 contain a denial of service vulnerability in the UniqueSlugNormalizer::normalize() function. The flaw causes an O(K^2) time complexity when K headings collapse to the same base slug due to restarting the numeric-suffix search from 1 on every collision. The vulnerability is triggered when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can craft a small Markdown document with many headings sharing the same base slug (e.g., empty ATX headings, identical text, or punctuation-only headings) to consume excessive CPU. The issue is patched in version 2.9.0. This is a remotely exploitable, low-complexity attack requiring no authentication or special privileges.