league/commonmark: Quadratic-time denial of service when parsing crafted Markdown

Impact

Affected versions of league/commonmark can have quadratic time complexity when parsing specially crafted Markdown lines. In practical terms, doubling the length of an affected line can make the parser perform roughly four times as much work. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions. The Autolink extension can also copy and validate the remaining line at every URL-like prefix.

In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine it with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines. Each trigger fits within one long line, so complex Markdown structure is unnecessary.

An attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time and allocation activity. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The core paths affect CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments. The autolink-specific path affects applications using AutolinkExtension or GithubFlavoredMarkdownExtension. Applications that process only trusted Markdown are not remotely exploitable. The impact is limited to availability: it does not disclose data, change rendered output, or bypass rendering restrictions. Settings such as html_input and allow_unsafe_links do not mitigate the issue because the expensive work occurs before rendering.

Patches

The issue is patched in 2.9.0 and later. Starting in that release, the parser records UTF-8 character-to-byte positions incrementally, converts ordered regular-expression match positions without restarting from the beginning of the line, and matches autolinks against the original line instead of copying every remaining suffix. The affected work then grows in direct proportion to the input size while preserving existing Markdown output and configuration behavior. Versions from 0.6.0 through 2.8.3 are affected. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.0 or later.

Workarounds

If you cannot upgrade immediately, reject or truncate inputs with excessively long individual lines before passing them to the converter. A total request-size limit is also useful, but a per-line limit is important because every demonstrated trigger fits on one line. Choose limits appropriate for the application and enforce them before Markdown parsing begins. Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions can further reduce exposure, but these measures are not complete substitutes for upgrading.

Disabling AutolinkExtension and avoiding GithubFlavoredMarkdownExtension removes the autolink-specific trigger, but the core multibyte parsing paths remain reachable in the standard parser. Existing nesting, delimiter, raw-HTML, and unsafe-link configuration options do not eliminate all affected paths. Applications that must continue processing untrusted Markdown should therefore enforce input limits even when autolinking is disabled.