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BREW-adr-viewer-CVE-2026-59928 — Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

Affected: Homebrew: adr-viewer

## 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. Att…

EUVD-2026-334045

fkie_cve-2026-59928

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

GHSA-ffq3-xpv3-j92q — Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

Affected: PyPI: mistune

## Summary

## Affected Code

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. Att…

msrc_CVE-2026-59928 — Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

msrc_CVE-2026-59928

openSUSE-SU-2026:21339-1 — Security update for python-mistune

Security update for python-mistune

PYSEC-2026-2216

Affected: PyPI: mistune

SUSE-SU-2026:22655-1 — Security update for python-mistune

Security update for python-mistune

UBUNTU-CVE-2026-59928

Affected: Ubuntu:16.04:LTS: mistune, Ubuntu:18.04:LTS: mistune, Ubuntu:20.04:LTS: mistune, Ubuntu:22.04:LTS: mistune, Ubuntu:24.04:LTS: mistune, Ubuntu:25.10: mistune, Ubuntu:26.04:LTS: mistune