Vulnerability in mistune (CVE-2026-59925)
Summary
Vulnerability in mistune (CVE-2026-59925). Risk of unauthorized operations or information disclosure. Exploitable via EMPHASIS_END_RE. Mitigation: upgrade to 3.3.0 or later.
Original description
The description as published by the source.
Summary
Type: Algorithmic-complexity DoS in core emphasis parsing. A long sequence of well-formed **x** (strong) or ***x*** (strong-emphasis combined) pairs causes O(N²) parser work. Distinct from the bracket-bomb DoS ([ repetition) and from the formatting-plugin DoS (~~/==/^^); this one fires on default-config mistune with no plugins required.
File: src/mistune/inline_parser.py lines 41-48 (the EMPHASIS_END_RE family) and the surrounding emphasis dispatch.
Root cause: for every opening run of *s the parser scans forward using one of EMPHASIS_END_RE['*'] / ['**'] / ['***'] to find the matching close. Each scan is bounded per call, but the parser invokes the scan from every potential start position. For input shaped **x** repeated N times, every ** is treated as a potential start, each scan can cover up to the end of input. Total work is O(N²). The triple-emphasis variant ***x*** is slightly worse due to the extra alternation between *, **, and *** close patterns. Reproducible against default mistune with no plugins.
Affected Code
File: src/mistune/inline_parser.py, lines 41-48.
EMPHASIS_END_RE = {
"*": re.compile(r"(?:" + PREVENT_BACKSLASH + r"\\\*|[^\s*])\*(?!\*)"),
"_": re.compile(r"(?:" + PREVENT_BACKSLASH + r"\\_|[^\s_])_(?!_)\b"),
"**": re.compile(r"(?:" + PREVENT_BACKSLASH + r"\\\*|[^\s*])\*\*(?!\*)"),
"__": re.compile(r"(?:" + PREVENT_BACKSLASH + r"\\_|[^\s_])__(?!_)\b"),
"***…
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-2026-59925 has been found in mistune.
Attackers can target a specific entry point like EMPHASIS_END_RE 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-2026-59925" on the vendor's site.
CVE-2026-59925 (mistune) — CWE-1333 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: EMPHASIS_END_RE / commonmark.js
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 EMPHASIS_END_RE without authentication.
Check the vendor advisory for the affected versions of mistune. Attack surface: EMPHASIS_END_RE / commonmark.js.
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-2026-59925 mistune
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com