Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

Summary Type: Algorithmic-complexity DoS in reference-link definition handling. A markdown document with N reference-link definitions of the same key or many distinct keys takes ON² parser time. 5000 repeated a: u\n definitions take 1.1 second; 10000 → 4.5 seconds. File: src/mistune/blockparser.p...