GHSA-xwg4-73v4-xw9w

### 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`](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`.

The negative value is passed to `fillPool()` ([`index.js:15`](https://github.com/ai/nanoid/blob/main/index.js#L15)):

```javascript 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: ```javascript 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

```javascript 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