OAuth Misconfiguration: Preemptive Account Registration Exploitation

1 min readJun 20, 2024



The OAuth misconfiguration allows an attacker to preemptively register an account using a victim’s email. When the victim later tries to register with OAuth, the system confirms the attacker’s account instead. This grants the attacker access to the victim’s account using the credentials provided during the initial registration. This oversight compromises account integrity and highlights the importance of robust email verification and account existence checks in OAuth implementations to prevent unauthorized access.

1.Attacker Registers First:

  • The attacker registers an account using a victim’s email address preemptively, before the victim attempts to use OAuth for registration.

2. Victim Completes OAuth Registration:

  • When the victim later attempts to register using OAuth with their email address, the system recognizes that an account already exists for that email due to the attacker’s registration.

3. Account Confirmation:

  • As a result, the account that the attacker registered earlier is now confirmed by the victim completing the OAuth registration process.

4. Attacker’s Access:

  • The attacker can now log in using the email address and password they provided during their initial registration, gaining access to the account intended for the victim.

Key Points Clarified:

  • Preemptive Registration: The attacker exploits the system’s lack of early account existence verification to register first.
  • Confirmation Issue: The victim’s completion of OAuth inadvertently confirms the account registered by the attacker.
  • Access Granted: The attacker gains access to the account they registered, compromising the victim’s intended account.

This situation highlights a critical flaw where improper account verification during OAuth registration can lead to unauthorized access and compromise user security.