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
  }
  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 back to positive, or the process restarts.