Bypassing API Authentication: Exploiting Token Reuse and Expiry

API authentication vulnerabilities present a lucrative opportunity for ethical hackers and bug bounty hunters. One of the most overlooked weaknesses in many APIs is improper handling of authentication tokens, particularly around token reuse and expiry. These flaws can allow attackers to bypass authentication entirely and gain unauthorized access to protected resources.

In this guide, we’ll walk through how to identify and exploit token reuse and expiry issues in API authentication systems, uncovering vulnerabilities that can be leveraged in bug bounty programs.

Before diving into bypass techniques, it’s important to understand the basics of API authentication.

Authentication Tokens: Tokens like JWT (JSON Web Tokens) and OAuth tokens are commonly used to verify user identity in APIs. These tokens typically provide access to resources once a user is logged in.Token Expiry: Many systems set a lifespan for tokens (e.g., 30 minutes or 1 hour). After this period, the token is invalidated, and the user must authenticate again.Token Reuse: Token reuse occurs when an API allows a previously issued token to be used multiple times or across different sessions without reauthentication.

Practical Tip: Familiarize yourself with how the API you’re testing handles tokens (e.g., where they’re stored, their lifespan, etc.).

Token reuse flaws occur when a valid token can be used multiple times even after logout or across different user accounts.

Look for Persistent Authentication Tokens

Some APIs store tokens in cookies, local storage, or as HTTP headers. These tokens may be reused for subsequent requests, even after the user logs out or the session expires.

Test for session persistence: Log in, capture the token, and log out. Then, use the captured token to access endpoints without logging in again.Test across accounts: Capture a token from one account and try using it to access resources of another account.

Practical Tip: Use tools like Burp Suite to intercept API traffic and manually re-submit captured tokens.

Many APIs set an expiration date on tokens, but sometimes the expiration isn’t checked properly on the server side. This allows attackers to exploit expired tokens.

Check for Lack of Expiry Validation

Even if an API token has expired, the server may still process requests with expired tokens if proper validation is not implemented.

Test with expired tokens: Capture a token before it expires, then manipulate the token to send it past its expiry time. Send the token to endpoints and see if access is still granted.Test automatic renewal: Some APIs may attempt to auto-renew expired tokens without proper validation, making it easier for attackers to maintain access.

Practical Tip: Use a script to automate sending expired tokens to different endpoints and observe the server’s behavior.

Now that you’ve identified potential issues, here’s how to exploit them.

Token Extraction: Use tools like Burp Suite to intercept requests and capture valid tokens.Replay Tokens: Re-send the captured token in different sessions or across different user accounts to see if it grants unauthorized access.Cross-Site Token Use: Try sending the token in the headers or URL of different API requests that require authentication.Expired Token Testing: Use expired tokens in requests to identify systems that don’t properly validate expiry.Token Manipulation: Modify expiry times in JWTs or OAuth tokens and resend them to see if the API accepts them.Check for Token Renewal Vulnerabilities: Exploit APIs that do not fully check token validity during auto-renewal processes.

Bypassing API authentication through token reuse and expiry flaws is a powerful attack vector for ethical hackers. By following the steps outlined in this guide, you can identify and exploit these vulnerabilities to earn valuable bug bounty rewards. Remember to test thoroughly, understand the API’s authentication mechanisms, and always document your findings with clear, actionable evidence.

If you’ve encountered any interesting token-related vulnerabilities in your bug bounty hunts, feel free to share your experiences in the comments. Happy hunting!