CVE-2026-48988: markdown-it Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6v5v-wf23-fmfq",
  "modified" : "2026-06-15T20:41:06Z",
  "published" : "2026-06-15T20:41:06Z",
  "aliases" : [ "CVE-2026-48988" ],
  "summary" : "markdown-it: Quadratic complexity DoS in smartquotes rule via replaceAt string operations",
  "details" : "### Summary\n\nA quadratic time complexity vulnerability exists in markdown-it's smartquotes rule (enabled via the `typographer: true` option). An attacker can craft a markdown input consisting of consecutive quotation marks that causes the parser to consume excessive CPU time, leading to denial of service.\n\n### Details\n\nThe vulnerability is in the `replaceAt()` helper function used by the smartquotes rule in `lib/rules_core/smartquotes.mjs`:\n\n```javascript\nfunction replaceAt (str, index, ch) {\n  return str.slice(0, index) + ch + str.slice(index + 1)\n}\n```\n\nWhen markdown-it processes a text token containing many quotation marks (either `\"` or `'`) with `typographer: true`, the smartquotes rule iterates through each quote character and calls `replaceAt()` to substitute it with a typographic (curly) quote. Each call to `replaceAt()` creates three new string slices and concatenates them, which is an O(n) operation where n is the length of the string.\n\nSince this is called once per quote character in the token, and there are n quote characters, the total time complexity becomes O(n^2).\n\nThe root cause is that the smartquotes rule modifies `token.content` in place using string slicing rather than building the result incrementally. The `process_inlines()` function (line 14) processes each quote in the text token, and for matching quote pairs, calls `replaceAt()` on both the opening and closing token's content (lines 151-152). When the entire input is a single text token of quote characters, this results in quadratic behavior.\n\n### PoC\n\n```javascript\nconst md = require('markdown-it');\nconst instance = md({ typographer: true });\n\n// 160,000 consecutive double-quote characters\nconst payload = '\"'.repeat(160000);\n\nconsole.time('render');\ninstance.render(payload);\nconsole.timeEnd('render');\n// Output: render: ~21000ms (21 seconds)\n\n// Compare with typographer disabled:\nconst safe = md({ typographer: false });\nconsole.time('render-safe');\nsafe.render(payload);\nconsole.timeEnd('render-safe');\n// Output: render-safe: ~8ms\n```\n\nMeasured timing on a modern system:\n- 10,000 quotes: ~19ms\n- 20,000 quotes: ~51ms\n- 40,000 quotes: ~212ms\n- 80,000 quotes: ~5,430ms\n- 160,000 quotes: ~21,198ms\n\nThe scaling is clearly superlinear (quadratic), with the 80K->160K step showing a ~3.9x increase for a 2x input increase, consistent with O(n^2).\n\n### Impact\n\nApplications that render user-supplied markdown with `typographer: true` are vulnerable to denial of service. An attacker can submit a relatively small payload (160KB of quote characters) that causes the server to spend over 21 seconds processing a single request. Repeated submissions can exhaust server CPU resources and prevent legitimate users from being served.\n\nThe impact is mitigated by the fact that the `typographer` option defaults to `false` and must be explicitly enabled. However, the typographer feature is commonly enabled in production applications that want smart typography, and the markdown-it documentation prominently suggests enabling it.\n\nA suggested fix would be to replace the `replaceAt()` approach with an array-based or StringBuilder-style approach that collects all replacements and applies them in a single pass, reducing the time complexity to O(n).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "markdown-it"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "14.2.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 14.1.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/markdown-it/markdown-it/security/advisories/GHSA-6v5v-wf23-fmfq"
  }, {
    "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" : "2026-06-15T20:41:06Z",
    "nvd_published_at" : null
  }
}