GHSA-xwg4-73v4-xw9w: Integer Overflow

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):

javascript function fillPool(bytes) { if (!pool || pool.length < bytes) { // false: pool exists, -2B < pool.length pool = Buffer.allocUnsafe(bytes POOLSIZEMULTIPLIER) 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[negativeindex] 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., "V1StGXR8Z5jdHi6B-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