Vulnerability in mistune (CVE-2026-59927)
Summary
Vulnerability in mistune (CVE-2026-59927). Risk of unauthorized operations or information disclosure. Exploitable via Include. Mitigation: upgrade to 3.3.0 or later.
Original description
The description as published by the source.
Summary
Type: Uncontrolled recursion via mutual include. The Include directive checks for direct self-reference (a.md cannot include a.md), but does not detect indirect cycles. Two markdown files that include each other (a.md → includes b.md → includes a.md) cause unbounded recursion until Python's stack limit fires RecursionError. The exception propagates out of the renderer and crashes the calling code.
File: src/mistune/directives/include.py, lines 33-37 (the self-include check is the only cycle-detection logic).
Root cause: the include logic only compares os.path.abspath(dest) == os.path.abspath(source_file). There is no per-render set of "files already included" that would catch transitive cycles. When a.md includes b.md, the recursive block.parse(new_state) call uses dest (b.md) as the new __file__, which then includes a.md (passing the self-check, because the immediate parent file is b.md, not a.md), which then includes b.md, and so on. Each recursion level adds Python frames; the default stack limit of 1000 frames trips after ~7-10 cycle iterations and Python raises RecursionError. Since the directive does not catch the exception, it propagates out of Markdown.parse() and surfaces in the calling code, crashing the request.
Affected Code
File: src/mistune/directives/include.py, lines 28-54.
relpath = self.parse_title(m)
dest = os.path.join(os.path.dirname(source_file), relpath)
dest = os.path…
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-2026-59927 has been found in mistune.
Attackers can target a specific entry point like Include over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade mistune to 3.3.0 or later.
If unsure, ask your IT team or search "mistune CVE-2026-59927" on the vendor's site.
CVE-2026-59927 (mistune) — CWE-674 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Include / a.md / b.md / RecursionError
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-674) exists in mistune. Attackers reach the vulnerable code path via Include without authentication.
Check the vendor advisory for the affected versions of mistune. Attack surface: Include / a.md / b.md / RecursionError.
Severity: Medium (CVSS 5.3/10). 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
-
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-2026-59927 mistune
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com