The table extension in GitHub Flavored Markdown before version 0.29.0.gfm.1 takes O(n * n) time to parse certain inputs. An attacker could craft a markdown table which would take an unreasonably long time to process, causing a denial of service. This issue does not affect the upstream cmark project. The issue has been fixed in version 0.29.0.gfm.1.
https://github.com/github/cmark-gfm/security/advisories/GHSA-7gc6-9qr5-hc85
https://github.com/github/cmark-gfm/commit/85d895289c5ab67f988ca659493a64abb5fec7b4