Description

nanoid: Integer Overflow or Wraparound

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...
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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) {}
...
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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

Mitigation

Aliases

References