Skip to content

AES-GCM issues in lib/gitlab/crypto_helper.rb

The Gitlab::CryptoHelper module is being used to encrypt sensitive token values within the database. The token values are encrypted utilizing AES-GCM. AES-GCM is an authenticated cipher, thus ensuring integrity of the stored ciphertexts. Within the Gitlab::CryptoHelper implementation however this cannot be guaranteed. The cipher is set up with a static key and a static nonce as follows:

    AES256_GCM_OPTIONS = {
      algorithm: 'aes-256-gcm',
      key: Settings.attr_encrypted_db_key_base_32,
      iv: Settings.attr_encrypted_db_key_base_12
    }.freeze

Both Settings.attr_encrypted_db_key_base_32 and Settings.attr_encrypted_db_key_base_12 are static values, namely the first 32 or the first 12 bytes of Gitlab::Application.secrets.db_key_base.

There are several issues with those settings:

  • Key and IV (nonce) are related to each other (IV is a substring of the key)
    • This is not advisable, even tough this is not directly exploitable the IV and key should not relate to each other
  • Gitlab::Application.secrets.db_key_base is a hexadecimal string e.g. "7ce6778d9ef49559..." this results in both IV and key are effectively half as long as intended
  • The nonce reuse allows recovery of the authentication key used within AES-GCM.
    • Just by observing two different ciphertexts and their respective authentication tags we can forge authentication tags for arbitrary ciphertexts.
  • When knowing a plain/ciphertext pair (e.g. from a project's runners token) we can deduce the keystream and thus encrypt arbitrary values up to the length of our known pair

I'd suggest to store a random IV along with the encrypted value. Additionally the key should be derived from Gitlab::Application.secrets.db_key_base with e.g. PBKDF2.