GHSA-ffq3-xpv3-j92q: 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 O(N²) parser time. 5000 repeated [a]: u\n definitions take ~1.1 second; 10000 → ~4.5 seconds. File: src/mistune/block_parser.py (reference-link def parsing) and the surrounding ref_links env-dictionary handling. Root cause: every reference definition is parsed by scanning forward from each candidate position. The unikey normalisation runs per-def, the dictionary insert is per-def, and the lookup-by-label-then-iterate-defs path is linear in the number of stored defs. For input with N defs, the total work is O(N²). Affected Code src/mistune/block_parser.py — reference-definition rule fires on every line that matches [label]: url. For each one: - unikey(label) is called (linear scan of the label). - The def is appended to state.env['ref_links']. - Later inline-link resolution looks up by unikey(label) in the dict (O(1)) but the surrounding parser revisits the def list for paragraph-vs-def disambiguation. The cumulative parse time grows as the square of the number of defs. Why it's wrong: the parser does not amortise the def-list scan. A single forward pass with a hash-keyed dict (already in place) plus a per-line classifier should make this O(N). Exploit Chain 1. Application uses mistune to render attacker-supplied markdown. No plugins required. 2. Attacker submits a 35 KB document of [a]: u\n repeated 5000 times followed by [click][a]. 3. CPU pegs for ~1.1 seconds. 10000 defs → ~4.5 s. 20000 → ~18 s. Doubling input quadruples time. Security Impact Attacker capability: small input → large CPU. Predictable scaling. Can be repeated. Preconditions: application uses mistune.create_markdown() (default config) on attacker-supplied markdown. Worth noting: the ref_links dictionary persists for the lifetime of the parse, so a long document with many defs builds up memory; with N defs of attac