Cloak PBKDF2 field ignores the configured iteration count and runs only :size rounds

Summary

Use of Password Hash With Insufficient Computational Effort vulnerability in danielberkompas cloak_ecto and danielberkompas cloak allows an attacker who holds the hashed values and the configured secret to brute-force low-entropy plaintexts much faster than configured.

The dump/1 callback that Cloak.Ecto.PBKDF2 (Cloak.Fields.PBKDF2 in cloak before the Ecto code moved to cloak_ecto) injects into a field module calls :pbkdf2.pbkdf2/4 with config[:size] in the iteration-count position. The :iterations setting is validated but never used. With the cloak_ecto defaults (iterations: 600_000, size: 32) each hash runs 32 PBKDF2 rounds instead of 600,000, so offline guessing of values such as email addresses costs about 18,750 times less than configured.

This issue affects cloak_ecto: from 1.0.0-alpha.0 onward; cloak: from 0.7.0 before 1.0.0-alpha.0.

Proof of concept

1. Define a field module with use Cloak.Ecto.PBKDF2 and configure only a :secret, so that the defaults iterations: 600_000 and size: 32 apply.
2. Call dump/1 on a value.
3. Compare the result with :pbkdf2.pbkdf2({:hmac, :sha256}, value, secret, 32) and with the same call at 600,000 rounds. It matches the 32-round hash, not the 600,000-round hash, and returns in microseconds instead of about one second.

Impact

An attacker who obtains a database dump and the PBKDF2 secret can recover hashed values with low entropy, such as email addresses, by offline guessing at a cost far below what the configuration promises.

Workarounds

Override dump/1 in the field module that uses Cloak.Ecto.PBKDF2, so that it calls :pbkdf2.pbkdf2/5 with the configured :iterations and :size. Then recompute all stored hashes, because the existing values no longer match.