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Introduction:
A recently disclosed Account Takeover (ATO) vulnerability on a HackerOne program has cast a stark light on a critical OAuth misconfiguration known as Account Squatting. This flaw allows an attacker to hijack a user’s identity by pre-registering an account, turning a standard authentication protocol into a powerful weapon. Understanding the mechanics and mitigation of this threat is paramount for developers and security professionals alike.
Learning Objectives:
- Understand the technical sequence of an OAuth Account Squatting attack.
- Learn to identify vulnerable OAuth implementations in web applications.
- Master the commands and configurations to test for and mitigate this vulnerability.
You Should Know:
1. Reconnaissance: Identifying Target Email Domains
Before an attack, an attacker often gathers a list of target email addresses. Tools like theHarvester are used for OSINT.
`theharvester -d target-company.com -l 500 -b google`
This command scrapes Google for 500 emails related to target-company.com. The attacker uses this list to find potential victims whose corporate emails might be used on the target platform.
2. Account Pre-Registration: The Squatting Phase
The attacker attempts to create accounts using the harvested emails. If the application does not require email verification before login, the squat is successful.
`curl -X POST ‘https://vulnerable-app.com/api/register’ -d ’[email protected]&password=AttackerP@ss123’`
This simple `curl` command mimics the registration API call. A successful response (e.g., HTTP 200) without a verification requirement indicates a critical misconfiguration.
3. OAuth Flow Exploitation: Triggering the Faulty Link
The victim later logs in via OAuth (e.g., Google). The application’s faulty logic links the verified OAuth identity to the attacker’s pre-existing, unverified account.
` Intercept the OAuth callback request with Burp Suite`
`GET /oauth/callback?code=AUTHORIZATION_CODE&state=RANDOM_STATE`
The security analyst must intercept this request to analyze how the application handles the linking of OAuth identities to local accounts.
- Testing for Vulnerability: Probing the OAuth Linking Logic
To test an application, use an intercepting proxy like Burp Suite to manipulate session states and registration flows. The key is to see if a new OAuth login associates with an existing, unverified email account.
` Step 1: Create account with victim email (no verification)`
` Step 2: Initiate OAuth login with the same email from a different browser/incognito session`
` Step 3: If logged into the attacker’s account, the vulnerability is confirmed.`
5. The Attacker’s Payload: Gaining Access
Once linked, the attacker simply logs in with the credentials they set during pre-registration, gaining full access to the victim’s resources.
` Attacker’s login for account takeover`
`curl -X POST ‘https://vulnerable-app.com/api/login’ -d ’[email protected]&password=AttackerP@ss123’`
A successful login here, after the OAuth link, confirms the complete account compromise.
6. Mitigation 1: Enforcing Strict Email Verification
The primary mitigation is to require email verification before any account can be used. This should be enforced before an account is allowed to log in via any method.
` Pseudocode for secure registration logic`
`function register(email, password) {`
` createAccount(email, password, verified=false);`
` sendVerificationEmail(email);`
` // Account cannot be logged into until ‘verified’ flag is true`
`}`
7. Mitigation 2: Correct OAuth Account Linking
The correct OAuth implementation must first check if the email from the IdP (e.g., Google) is verified and exists in the database. If it exists, it must check if it is already verified before linking.
` Pseudocode for secure OAuth linking`
`oauth_email = getVerifiedEmailFromGoogle(id_token);`
`user = findUserInDatabase(oauth_email);`
`
`if (user exists) {</h2>
<h2 style="color: yellow;"> if (user.email_verified == true) {</h2>
<h2 style="color: yellow;"> logUserIn(user); // Correct user</h2>
<h2 style="color: yellow;"> } else {</h2>
<h2 style="color: yellow;"> // This is a squatter account!</h2>
<h2 style="color: yellow;"> forceEmailVerificationWorkflow();</h2>
<h2 style="color: yellow;"> }</h2>
<h2 style="color: yellow;">} else {</h2>
<h2 style="color: yellow;"> createNewVerifiedUser(oauth_email); // Auto-verify via OAuth</h2>
<h2 style="color: yellow;">}`
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What Undercode Say:
- The Human Firewall is Not Enough: This vulnerability exists purely at the architectural level. No amount of user training can prevent it; the responsibility lies entirely with the developers.
- Verify, Then Trust: The core security principle violated here is “trust, but verify.” The application trusted an unverified email address at the time of OAuth linking, which is a catastrophic logic flaw.
This case study is a classic example of a broken authentication mechanism that slips through cracks during development. It underscores the critical need for security-minded code reviews, specifically for authentication flows. Relying on third-party OAuth does not absolve developers of implementing their own validation checks. The flaw is not in OAuth itself, but in how the application leverages the data it provides.
Prediction:
The simplicity and high impact of OAuth Account Squatting will lead to a surge in its discovery within bug bounty programs throughout 2024. As more applications rush to implement social login for user convenience, without rigorous security testing, this misconfiguration will become a low-hanging fruit for attackers. We predict a wave of automated tools designed specifically to scan for and exploit this vulnerability at scale, targeting mid-tier SaaS platforms. This will force major IdPs like Google and Microsoft to potentially introduce stricter client-side validation requirements for developers using their OAuth services.
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IT/Security Reporter URL:
Reported By: Zabitmajeed Hackerone – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
