CVE-2026-73086: nanoid Integer Overflow Or Wraparound Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xwg4-73v4-xw9w",
  "modified" : "2026-09-01T19:23:45Z",
  "published" : "2026-09-01T19:23:45Z",
  "aliases" : [ "CVE-2026-73086" ],
  "summary" : "nanoid: Integer Overflow or Wraparound",
  "details" : "### Summary\n\nAn integer overflow in `nanoid(size)` permanently corrupts the process-wide CSPRNG pool, causing all subsequent ID generation to return the deterministic string `\"uuuuuuuuuuuuuuuuuuuuu\"`. Any application that passes user-influenced values to the `size` parameter loses all randomness guarantees for session tokens, CSRF tokens, and unique identifiers until process restart.\n\n### Details\n\n`nanoid()` at [`index.js:101`](https://github.com/ai/nanoid/blob/main/index.js#L101) coerces the `size` parameter with `size |= 0`, which converts it to a signed 32-bit integer. When `size >= 2^31` (e.g., `2147483648`), this wraps to `-2147483648`.\n\nThe negative value is passed to `fillPool()` ([`index.js:15`](https://github.com/ai/nanoid/blob/main/index.js#L15)):\n\n```javascript\nfunction fillPool(bytes) {\n  if (!pool || pool.length < bytes) {       // false: pool exists, -2B < pool.length\n    pool = Buffer.allocUnsafe(bytes * POOL_SIZE_MULTIPLIER)\n    crypto.getRandomValues(pool)\n    poolOffset = 0\n  } else if (poolOffset + bytes > pool.length) {  // false: poolOffset + (-2B) < pool.length\n    crypto.getRandomValues(pool)\n    poolOffset = 0\n  }\n  poolOffset += bytes  // poolOffset += -2147483648 → deeply negative\n}\n```\n\nNeither branch triggers, so the pool is never refreshed. `poolOffset` becomes ~-2.1 billion.\n\nSubsequent `nanoid()` calls execute:\n```javascript\nfor (let i = poolOffset - size; i < poolOffset; i++) {\n  id += scopedUrlAlphabet[pool[i] & 63]\n}\n```\n\n`pool[negative_index]` returns `undefined`. `undefined & 63` evaluates to `0`. `urlAlphabet[0]` is `'u'`. Every ID becomes `\"uuuuuuuuuuuuuuuuuuuuu\"`.\n\nThe corruption is **persistent** — it affects all subsequent calls in the process until ~100 million calls eventually wrap `poolOffset` back to positive, or the process restarts.\n\n### PoC\n\n```javascript\nimport { nanoid } from 'nanoid'\n\n// Step 1: Normal operation\nconsole.log(nanoid())  // e.g., \"V1StGXR8_Z5jdHi6B-myT\"\n\n// Step 2: Trigger overflow (e.g., from an API parameter)\ntry { nanoid(2147483648) } catch(e) {}\n\n// Step 3: All subsequent IDs are deterministic\nconsole.log(nanoid())  // \"uuuuuuuuuuuuuuuuuuuuu\"\nconsole.log(nanoid())  // \"uuuuuuuuuuuuuuuuuuuuu\"\nconsole.log(nanoid())  // \"uuuuuuuuuuuuuuuuuuuuu\"\n// ... forever, process-wide\n```\n\nRun with: `node --experimental-vm-modules poc.mjs`\n\nAttack scenario: Any API endpoint that accepts a user-controlled length/size parameter (URL shortener slug length, configurable token size, etc.) and passes it to `nanoid(userInput)`.\n\n### Impact\n\n**Complete loss of ID unpredictability and uniqueness, process-wide, from a single request.**\n\n- All session IDs, CSRF tokens, API keys, and database identifiers generated after the attack are identical and predictable\n- An attacker can predict all tokens issued to other users, enabling session hijacking and authentication bypass\n- The corruption is persistent (survives across requests) and affects all consumers of `nanoid` in the same process\n- No special privileges or preconditions required — a single unauthenticated request is sufficient\n- Affects any application that passes external input to the `size` parameter without validation",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "nanoid"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.3.12"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "nanoid"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "5.1.11"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/security/advisories/GHSA-xwg4-73v4-xw9w"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-73086"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/commit/7087969281cab8ba8ae3babf1894e819068b3bb4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/commit/821dfed7b5db7f88e92f56c60eef32c8135077c3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/commit/b0036ed60dc9facd7f1191a50dfb3076500202ac"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ai/nanoid"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/releases/tag/3.3.12"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ai/nanoid/releases/tag/5.1.11"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-190" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-01T19:23:45Z",
    "nvd_published_at" : "2026-08-11T17:19:16Z"
  }
}