Cross-Site Scripting (XSS) in mistune (CVE-2026-59926)
Summary
Cross-site scripting in mistune (CVE-2026-59926). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 3.3.0 or later.
Original description
The description as published by the source.
In src/mistune/directives/admonition.py, the render_admonition() function concatenates the :class: option directly into the HTML class attribute without escaping (lines 63-68).
This allows attribute injection and XSS even when HTMLRenderer(escape=True) is used.
The directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.
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-2026-59926 has been found in mistune.
Risk of unauthorized operations or information disclosure. CVSS score: 6.1/10.
What to do: upgrade mistune to 3.3.0 or later.
If unsure, ask your IT team or search "mistune CVE-2026-59926" on the vendor's site.
CVE-2026-59926 (mistune) — CWE-79 / CVSS v3 6.1
Attack vector: remote (network-reachable) / unauthenticated
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.
Cross-site scripting (XSS) (CWE-79) exists in mistune.
Check the vendor advisory for the affected versions of mistune.
Severity: Medium (CVSS 6.1/10). 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-2026-59926 mistune
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com