GHSA-FFQ3-XPV3-J92Q

Vulnerability from github – Published: 2026-07-20 21:24 – Updated: 2026-07-20 21:24
VLAI
Summary
Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
Details

Summary

Type: Algorithmic-complexity DoS in reference-link definition handling. A markdown document with N reference-link definitions of the same key (or many distinct keys) takes O(N²) parser time. 5000 repeated [a]: u\n definitions take ~1.1 second; 10000 → ~4.5 seconds. File: src/mistune/block_parser.py (reference-link def parsing) and the surrounding ref_links env-dictionary handling. Root cause: every reference definition is parsed by scanning forward from each candidate position. The unikey normalisation runs per-def, the dictionary insert is per-def, and the lookup-by-label-then-iterate-defs path is linear in the number of stored defs. For input with N defs, the total work is O(N²).

Affected Code

src/mistune/block_parser.py — reference-definition rule fires on every line that matches [label]: url. For each one: - unikey(label) is called (linear scan of the label). - The def is appended to state.env['ref_links']. - Later inline-link resolution looks up by unikey(label) in the dict (O(1)) but the surrounding parser revisits the def list for paragraph-vs-def disambiguation.

The cumulative parse time grows as the square of the number of defs.

Why it's wrong: the parser does not amortise the def-list scan. A single forward pass with a hash-keyed dict (already in place) plus a per-line classifier should make this O(N).

Exploit Chain

  1. Application uses mistune to render attacker-supplied markdown. No plugins required.
  2. Attacker submits a 35 KB document of [a]: u\n repeated 5000 times followed by [click][a].
  3. CPU pegs for ~1.1 seconds. 10000 defs → ~4.5 s. 20000 → ~18 s. Doubling input quadruples time.

Security Impact

Attacker capability: small input → large CPU. Predictable scaling. Can be repeated. Preconditions: application uses mistune.create_markdown() (default config) on attacker-supplied markdown. Worth noting: the ref_links dictionary persists for the lifetime of the parse, so a long document with many defs builds up memory; with N defs of attacker-chosen length, the per-def normalisation cost compounds. Differential: PoC-verified against mistune@3.2.1, default config:

import mistune, time
md = mistune.create_markdown()
for n in [1000, 2000, 5000, 10000]:
    s = '[a]: u\n' * n + '[click][a]'
    t = time.time()
    md(s)
    print(f'  ref defs * {n} ({len(s)}b): {(time.time() - t) * 1000:.0f}ms')

# Output (Python 3.13, Linux, 2.5GHz CPU):
#   ref defs *  1000  ( 7012b):    46ms
#   ref defs *  2000 (14012b):   186ms
#   ref defs *  5000 (35012b):  1121ms
#   ref defs * 10000 (70012b):  4400ms

The patched build (with the surrounding parser amortised to O(N)) keeps the time linear.

Suggested Fix

Replace the per-def re-scan with a single forward pass that classifies each line into ref_def | paragraph | other once and only inserts into ref_links once per def. The dict already exists; the wasted work is in the surrounding scan loop, not in the dict operations.

A regression test asserting that md('[a]: u\n' * 50_000 + '[click][a]') completes in under 1 second would catch any regression.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-59928"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1333",
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-20T21:24:18Z",
    "nvd_published_at": "2026-07-08T17:17:28Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\n**Type:** Algorithmic-complexity DoS in reference-link definition handling. A markdown document with N reference-link definitions of the same key (or many distinct keys) takes O(N\u00b2) parser time. 5000 repeated `[a]: u\\n` definitions take ~1.1 second; 10000 \u2192 ~4.5 seconds.\n**File:** `src/mistune/block_parser.py` (reference-link def parsing) and the surrounding `ref_links` env-dictionary handling.\n**Root cause:** every reference definition is parsed by scanning forward from each candidate position. The `unikey` normalisation runs per-def, the dictionary insert is per-def, and the lookup-by-label-then-iterate-defs path is linear in the number of stored defs. For input with N defs, the total work is O(N\u00b2).\n\n## Affected Code\n\n`src/mistune/block_parser.py` \u2014 reference-definition rule fires on every line that matches `[label]: url`. For each one:\n- `unikey(label)` is called (linear scan of the label).\n- The def is appended to `state.env[\u0027ref_links\u0027]`.\n- Later inline-link resolution looks up by `unikey(label)` in the dict (O(1)) but the surrounding parser revisits the def list for paragraph-vs-def disambiguation.\n\nThe cumulative parse time grows as the square of the number of defs.\n\n**Why it\u0027s wrong:** the parser does not amortise the def-list scan. A single forward pass with a hash-keyed dict (already in place) plus a per-line classifier should make this O(N).\n\n## Exploit Chain\n\n1. Application uses mistune to render attacker-supplied markdown. No plugins required.\n2. Attacker submits a 35 KB document of `[a]: u\\n` repeated 5000 times followed by `[click][a]`.\n3. CPU pegs for ~1.1 seconds. 10000 defs \u2192 ~4.5 s. 20000 \u2192 ~18 s. Doubling input quadruples time.\n\n## Security Impact\n\n**Attacker capability:** small input \u2192 large CPU. Predictable scaling. Can be repeated.\n**Preconditions:** application uses `mistune.create_markdown()` (default config) on attacker-supplied markdown. Worth noting: the `ref_links` dictionary persists for the lifetime of the parse, so a long document with many defs builds up memory; with N defs of attacker-chosen length, the per-def normalisation cost compounds.\n**Differential:** PoC-verified against mistune@3.2.1, default config:\n\n```python\nimport mistune, time\nmd = mistune.create_markdown()\nfor n in [1000, 2000, 5000, 10000]:\n    s = \u0027[a]: u\\n\u0027 * n + \u0027[click][a]\u0027\n    t = time.time()\n    md(s)\n    print(f\u0027  ref defs * {n} ({len(s)}b): {(time.time() - t) * 1000:.0f}ms\u0027)\n\n# Output (Python 3.13, Linux, 2.5GHz CPU):\n#   ref defs *  1000  ( 7012b):    46ms\n#   ref defs *  2000 (14012b):   186ms\n#   ref defs *  5000 (35012b):  1121ms\n#   ref defs * 10000 (70012b):  4400ms\n```\n\nThe patched build (with the surrounding parser amortised to O(N)) keeps the time linear.\n\n## Suggested Fix\n\nReplace the per-def re-scan with a single forward pass that classifies each line into `ref_def | paragraph | other` once and only inserts into `ref_links` once per def. The dict already exists; the wasted work is in the surrounding scan loop, not in the dict operations.\n\nA regression test asserting that `md(\u0027[a]: u\\n\u0027 * 50_000 + \u0027[click][a]\u0027)` completes in under 1 second would catch any regression.",
  "id": "GHSA-ffq3-xpv3-j92q",
  "modified": "2026-07-20T21:24:18Z",
  "published": "2026-07-20T21:24:18Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59928"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/commit/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lepture/mistune"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/releases/tag/v3.3.0"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2216.yaml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.