CVE-2026-49851: Mistune Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-qcq2-496w-v96p",
  "modified" : "2026-07-09T23:52:28Z",
  "published" : "2026-07-09T23:52:27Z",
  "aliases" : [ "CVE-2026-49851" ],
  "summary" : " Mistune: Potential DoS via quadratic-time parsing in parse_link_text",
  "details" : "### Summary\nMistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. A relatively small input consisting of repeated [ characters causes significant parsing slowdown.\n\n### Affected component\nmistune/inline_parser.py → **parse_link_text**\n\n### Description\nWhen parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior.\nAn attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload.\n\n### Root cause\nThe vulnerability stems from a two-loop interaction:\n- The outer loop in `InlineParser.parse()` (inline_parser.py) advances \n  only 1 character at a time when parse_link() returns None\n- Each failed attempt calls `parse_link_text()` which performs an O(n) \n  scan to the end of the string looking for a closing `]`\n- With n consecutive `[` characters, this results in O(n) × O(n) = O(n²) \n  total work\n\n### PoC\nRun below python script\n```\nimport mistune\nimport time\n\nmd = mistune.create_markdown()\n\ns = \"[\" * 6400\n\nt = time.perf_counter()\nmd(s)\nprint(time.perf_counter() - t)\n```\n<img width=\"2028\" height=\"1277\" alt=\"image\" src=\"https://github.com/user-attachments/assets/15d5bc0b-35f8-4a15-85e0-cbc314a45b06\" />\n\n**Benmark poc**\nRun below code for benchmark\n```\nimport mistune\nimport time\n\nmd = mistune.create_markdown()\n\nsizes = [100,200,400,800,1600,3200,6400]\n\nfor n in sizes:\n    s = \"[\" * n\n\n    t0 = time.perf_counter()\n    md(s)\n    dt = time.perf_counter() - t0\n\n    print(f\"{n:6d} {dt:.6f}\")\n```\n<img width=\"2503\" height=\"1341\" alt=\"image\" src=\"https://github.com/user-attachments/assets/f09a7bbb-6927-4ba2-afb1-444dd913b84e\" />\n\n\n### Observed behaviour\n```\npython3 benchmark.py \n   100 0.001609\n   200 0.003207\n   400 0.012906\n   800 0.050220\n  1600 0.197307\n  3200 0.801172\n  6400 3.190393\n```\nExecution time grows superlinearly, consistent with O(n²) complex\n\n### Impact\nThis can be used as a denial-of-service attack in any application that parses user-supplied Markdown using Mistune, including:\n\n- Web applications (comments, posts, content rendering)\n- API services processing Markdown\n- Documentation rendering systems\n- A small (~6 KB) payload can block CPU for multiple seconds.\n\n### Suggested fix\nReturn the furthest scanned position from parse_link_text even on failure, so the outer loop can skip ahead instead of advancing 1 character at a time\n\n### Security Classification\nCWE-400: Uncontrolled Resource Consumption\nDenial of Service (CPU exhaustion)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mistune"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.3.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-49851"
  }, {
    "type" : "WEB",
    "url" : "https://access.redhat.com/security/cve/CVE-2026-49851"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.redhat.com/show_bug.cgi?id=2492304"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lepture/mistune"
  }, {
    "type" : "WEB",
    "url" : "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333", "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-09T23:52:27Z",
    "nvd_published_at" : "2026-06-24T18:17:18Z"
  }
}