CVE-2026-76098: Mistune Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6m44-fpc8-c3rq",
  "modified" : "2026-09-02T15:14:55Z",
  "published" : "2026-09-02T15:14:55Z",
  "aliases" : [ "CVE-2026-76098" ],
  "summary" : "Mistune: Denial of Service — RecursionError via Excessive Emphasis Markers in Markdown",
  "details" : "## Summary\nMistune v3.3.2 is vulnerable to a Denial of Service (DoS) attack via uncontrolled recursion in the HTML rendering of deeply-nested emphasis tokens. By submitting Markdown containing approximately 1,000 consecutive asterisk characters, an attacker causes the Python process to crash with RecursionError.\n\n## Details\nThe InlineParser's _process_emphasis_delimiters() creates deeply nested <strong> tokens from consecutive emphasis markers (every 2 asterisks add one nesting level). With 1,000 consecutive asterisks, approximately 500 levels of nesting are produced. The HTMLRenderer.render_token() method (src/mistune/renderers/html.py:40-57) renders these tokens recursively: when a token has children, line 48 calls self.render_tokens(token['children'], state), entering child rendering. Each nesting level produces ~2 stack frames, so 500 levels ≈ 1,000 frames, exceeding Python's default recursion limit (sys.getrecursionlimit() = 1000). The emphasis() and strong() methods (lines 80-84) wrap recursively rendered child content in <em> and <strong> tags, perpetuating the recursion. This vulnerability affects all mistune APIs including markdown() and html().\n\nCore vulnerable code path:\n\n```python\n# src/mistune/renderers/html.py:40-57\ndef render_token(self, token: Dict[str, Any], state: BlockState) -> str:\n    func = self._get_method(token[\"type\"])\n    attrs = token.get(\"attrs\")\n    if \"raw\" in token:\n        text = token[\"raw\"]\n    elif \"children\" in token:\n        text = self.render_tokens(token[\"children\"], state)\n    else:\n        if attrs:\n            return func(**attrs)\n        else:\n            return func()\n    if attrs:\n        return func(text, **attrs)\n    else:\n        return func(text)\n```\n\nThe recursive call to render_tokens() on line 48 processes nested child tokens. With 500 levels of nested emphasis/strong tokens, this recursion exceeds Python's default recursion limit of 1000, causing a RecursionError.\n\n```python\n# src/mistune/renderers/html.py:80-84\ndef emphasis(self, text: str) -> str:\n    return \"<em>\" + text + \"</em>\"\n\ndef strong(self, text: str) -> str:\n    return \"<strong>\" + text + \"</strong>\"\n```\n\nThe emphasis() and strong() methods wrap their text content (which is itself the recursively-rendered output of nested child tokens) in HTML tags, creating the chain of recursion: each call to strong() includes rendered children that themselves call strong(), etc.\n\n## POC\n\n``` wiki\nfrom mistune import html\n\npayload = '*' * 1000\ntry:\n    result = html(payload)\n    print('No crash - recursion handled')\nexcept RecursionError as e:\n    print(f'[VULN] RecursionError: {e} - Process would crash!')\nexcept Exception as e:\n    print(f'Error: {type(e).__name__}: {e}')\n```\n\n<img width=\"1139\" height=\"664\" alt=\"1\" src=\"https://github.com/user-attachments/assets/d2095b9f-0a6a-4bef-8013-cbc6946cf8f1\" />\n\n\n\n## Impact\nCVSS 3.1: 7.5 (High) — AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. An attacker can crash any server process using mistune with approximately 2KB of Markdown input. In web applications, this can be triggered through user-generated content such as forum posts or comments, causing denial of service for all concurrent users sharing the same Python process. Both mistune.markdown() and mistune.html() are affected.\n\n## Remediation\n1. Add a maximum nesting depth limit in the emphasis parsing stage, similar to BlockParser's max_nested_level mechanism (currently at DEFAULT_MAX_NESTED_LEVEL = 20). When the limit is exceeded, treat excess emphasis markers as literal text. 2. Alternatively, refactor HTMLRenderer to use an explicit stack-based iterative approach instead of recursive calls for rendering nested tokens. 3. As a defense-in-depth measure, document the recursion risk and recommend that applications deploying mistune set a higher recursion limit or implement request-level timeouts.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mistune"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.3.0"
      }, {
        "fixed" : "3.3.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/security/advisories/GHSA-6m44-fpc8-c3rq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-76098"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/commit/0938fb781d0aded99de801b340ec1f8debeae5b2"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lepture/mistune"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/releases/tag/v3.3.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-674" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:14:55Z",
    "nvd_published_at" : "2026-08-24T20:17:19Z"
  }
}