GHSA-3q6v-r5mr-hxv8: league/commonmark: Quadratic-time denial of service in the GitHub Flavored Markdown Table extension block-start scan

league/commonmark’s GitHub Flavored Markdown Table extension registers TableStartParser as a block-start parser. While a paragraph is the active block, the core block parser calls TableStartParser::tryStart() on every non-blank line. Its first action fetches the entire growing paragraph buffer via getParagraphContent() and runs strpos($paragraph, '|') 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 GithubFlavoredMarkdownConverter (or any environment that enables TableExtension) drives seconds to tens of seconds of single-core CPU that grows quadratically with body size, enough to exhaust worker processes and deny service.