Findings

CVE-2026-105830

league/commonmark: Quadratic-time denial of service in the GitHub Flavored Markdown Table extension block-start scan

Published Sep 30, 2026 · updated Oct 9, 2026

## Summary 's GitHub Flavored Markdown Table extension registers as a block-start parser. While a paragraph is the active block, the core block parser calls on every non-blank line. Its first action fetches the entire growing paragraph buffer via and runs across all of it. For a paragraph of M pipe-free lines, line k rescans about k lines of buffer, so total work is 1+2+...+M, which is O(M^2). An unauthenticated user who submits a single large paragraph of pipe-free lines that do not begin with a letter (see Attack Chain) to any service that converts untrusted Markdown with (or any environment that enables ) drives seconds to tens of seconds of single-core CPU that grows quadratically with body size, enough to exhaust worker processes and deny service. ## Root Cause The GFM table detector performs a per-line "quick check" against the whole accumulated paragraph rather than only the portion that could form a table header. A paragraph never closes while non-blank lines keep arriving, so its buffer grows without bound, and the quick check rescans the entire buffer on each line. Only the paragraph's last line can ever be a table header (it is extracted separately via / ), so scanning the full multi-line buffer for a pipe on every line is unnecessary work and creates quadratic time complexity. There is no input-size cap and the default nesting limit is not reached, so nothing bounds the scan. ## Impact An unauthenticated, remote attacker with no user interaction can cause denial of service against any application that renders untrusted Markdown with the GitHub Flavored Markdown converter or any configuration that enables the Table extension (the default GFM bundle enables it). A single request carrying a large paragraph of pipe-free, blank-line-free lines that do not begin with a letter consumes CPU proportional to the square of the input size: measured runs show roughly 4x CPU growth per input doubling. A body of a few megabytes ties up a worker for seconds to tens of s