Vulnerability in mistune (CVE-2026-59930)
Summary
Vulnerability in mistune (CVE-2026-59930). Risk of unauthorized operations or information disclosure. Exploitable via toc. Mitigation: upgrade to 3.3.0 or later.
Original description
The description as published by the source.
Summary
Type: Predictable identifier generation. The toc plugin and TableOfContents directive both default to generating heading IDs of the form toc_1, toc_2, toc_3, ... with no input-derived component. An attacker who can place a heading anywhere in the document can predict which toc_N ID it will receive, and can inject HTML elsewhere (in a non-heading context) that uses the same id="toc_N" to either (a) shadow the legitimate heading anchor, breaking same-page navigation, or (b) collide with CSS or JavaScript that targets #toc_N selectors, redirecting click handlers and styling to attacker-chosen content.
File: src/mistune/toc.py line 36-37 (heading_id = lambda token, index: "toc_" + str(index + 1)); src/mistune/directives/toc.py line 33 (same default).
Root cause: the default heading_id callback ignores the heading's text content and uses only the headings's order in the document. Two documents rendered together (or one document with attacker-influenced headings spliced into trusted content) produce overlapping toc_N IDs. Because id attribute uniqueness is required by HTML, browsers behaviour on duplicate IDs is undefined; document.getElementById('toc_1') returns the first match, getElementsByTagName + querySelector semantics differ across paths, and CSS rules targeting #toc_1 apply to whichever element matches first in tree order.
Affected Code
File: src/mistune/toc.py, lines 33-39.
def add_toc_hook(md…
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-2026-59930 has been found in mistune.
Attackers can target a specific entry point like toc over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 4.3/10.
What to do: upgrade mistune to 3.3.0 or later.
If unsure, ask your IT team or search "mistune CVE-2026-59930" on the vendor's site.
CVE-2026-59930 (mistune) — CWE-1284 / CVSS v3 4.3
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: toc / TableOfContents / toc_1 / toc_2
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-1284) exists in mistune. Attackers reach the vulnerable code path via toc without authentication.
Check the vendor advisory for the affected versions of mistune. Attack surface: toc / TableOfContents / toc_1 / toc_2.
Severity: Medium (CVSS 4.3/10). 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
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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-2026-59930 mistune
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com