cmark-gfm is an extended version of the C reference implementation of CommonMark, a rationalized version of Markdown syntax with a spec. Three polynomial time complexity issues in cmark-gfm may lead to unbounded resource exhaustion and subsequent denial of service. These vulnerabilities have been patched in 0.29.0.gfm.12.
https://github.com/github/cmark-gfm/security/advisories/GHSA-w4qg-3vf7-m9x5
https://github.com/github/cmark-gfm/releases/tag/0.29.0.gfm.12