GHSA-253c-mchw-3w2r: markdown-it linkify True Has Two Quadratic Paths, Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-253c-mchw-3w2r",
  "modified" : "2026-09-29T17:57:56Z",
  "published" : "2026-09-29T17:57:56Z",
  "aliases" : [ ],
  "summary" : "markdown-it linkify: true has two quadratic paths, so a few hundred KB of markdown blocks the event loop for tens of seconds",
  "details" : "## Summary\n\nTwo independent quadratic paths in the `linkify: true` handling. Both are in markdown-it's own code rather than in linkify-it, which stays linear on both payloads.\n\n`src/rules_core/linkify.ts` calls `arrayReplaceAt` once per linkified text token, and that rebuilds the whole `children` array each time. A paragraph of N soft-broken lines is one inline token with about 2N children, so you get N rebuilds over a 2N array. Schema-less emails are what reach it. A `http://` link gets consumed by the inline rule first and never arrives as a text token, so those stay linear.\n\n`src/rules_inline/linkify.ts` runs `state.pending.match(SCHEME_RE)` at every `://` in the source. `state.pending` only gets truncated once a link is actually produced, so an unregistered scheme leaves it growing and every `://` rescans the lot.\n\n## Proof of concept\n\nClean install of 15.0.0 from npm, `new MarkdownIt({ linkify: true }).render(payload)`, Node 24.\n\n| N | 5,000 | 10,000 | 20,000 | 40,000 |\n|---|---|---|---|---|\n| `'a@b.co\\n'.repeat(N)`, 34KB to 273KB | 1.1s | 4.7s | 19.6s | 89s |\n| `'a://'.repeat(N)`, 20KB to 156KB | 0.25s | 0.76s | 3.1s | 15.0s |\n\nDoubling the input roughly quadruples the time in both. With `linkify: false` the same inputs run in 25 to 131ms and stay flat.\n\nControls for the first one, all at N=20000: putting every email in a single text token (`'a@b.co '`) takes 0.59s, one email per paragraph (`'a@b.co\\n\\n'`) takes 0.64s, and soft-broken lines with nothing linkifiable take 42ms. So it needs many children AND many of them linkifying. For the second, replaying just the `SCHEME_RE` calls against the same growing prefixes with no markdown-it involved accounts for 11.7s of the 15s.\n\nOrdinary prose does it too. `'ping a@b.co ok\\n'.repeat(20000)` is 293KB and takes 31s.\n\n## Caveat\n\n`linkify` is off by default, so this only reaches apps that turn it on.\n\n## Impact\n\nAvailability only. A few hundred KB of fairly ordinary markdown pins one core for tens of seconds, and because it's quadratic it degrades quickly with size. Nothing is read, written or executed.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "markdown-it"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "15.0.0"
      }, {
        "fixed" : "15.0.1"
      } ]
    } ],
    "versions" : [ "15.0.0" ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "markdown-it"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "14.3.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/markdown-it/markdown-it/security/advisories/GHSA-253c-mchw-3w2r"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/markdown-it/markdown-it/commit/09fa07118dda4c953f058848f53dae88395618ca"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/markdown-it/markdown-it/commit/aaadcfa6d817b3c5f89afb49d97c7b797a6dd4fd"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/markdown-it/markdown-it/commit/ad70f6b7cff64bee10e42a774147112480ca0d49"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/markdown-it/markdown-it"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-407" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-29T17:57:56Z",
    "nvd_published_at" : null
  }
}