GHSA-9FRC-8383-795M Symfony's YAML Parser has a ReDoS via Catastrophic Backtracking in Parser::cleanup() Regex

Description Symfony\Component\Yaml\Parser::cleanup strips the optional %YAML directive header, leading comments, and document start/end markers before parsing. The original regexes contained overlapping quantifiers, most notably '^%YAML: \d.+.\nu', whose \d.+ and . overlap on the dot, that exhibi...