Vulnerability in mistune (CVE-2026-59928)
Summary
Vulnerability in mistune (CVE-2026-59928). Risk of unauthorized operations or information disclosure. Exploitable via ref_links. Mitigation: upgrade to 3.3.0 or later.
Original description
The description as published by the source.
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
- Application uses mistune to render attacker-supplied markdown. No plugins required.
- Atta…
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-59928 has been found in mistune.
Attackers can target a specific entry point like ref_links over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade mistune to 3.3.0 or later.
If unsure, ask your IT team or search "mistune CVE-2026-59928" on the vendor's site.
CVE-2026-59928 (mistune) — CWE-1333 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: ref_links / unikey
Patched: 3.3.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-1333) exists in mistune. Attackers reach the vulnerable code path via ref_links without authentication.
Check the vendor advisory for the affected versions of mistune. Attack surface: ref_links / unikey.
Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.
Update to 3.3.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'mistune' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'mistune' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
mistuneを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade mistune to 3.3.0ステージング環境で 3.3.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Ubuntu UBUNTU-CVE-2026-59928 mistune
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com