GHSA-rj75-hqrm-r3gf — PostCSS: Quadratic complexity in flat selector parsing allows CPU exhaustion

Parsing flat selectors in PostCSS can process indexes with O(n²) complexity when dots and hashes occur in the selector. Parsing a 400 KB flat selector took about 34 seconds on a modern laptop and fully occupied one thread. Exposure is limited to deployments that synchronously parse untrusted, attacker-supplied selectors in a request path.