vulnerability / NVI-00016041 / Revision 1
Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
GitHub lists this reviewed advisory for mistune. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
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7.5 high
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As of · GitHub Advisory Database
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
mistune
source listed< 3.3.0
## 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 de
03 / Response
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04 / Limits
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05 / Provenance
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- GHSA-ffq3-xpv3-j92q ↗
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- Revision 1 ·
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