Simple Tips for Bug Bounty Beginners: Finding OTP Bypass Bug

Before we begin this article, my previous article on finding open redirect bugs in websites can be found here: https://medium.com/p/cdd11c57af38

The vulnerability that will be seen today is the OTP bypass.

OTP (One-Time Password) bypass bug is a vulnerability that allows an attacker to circumvent the security provided by OTPs, typically by exploiting flaws in the implementation of the OTP system.

This can have serious implications for the security of sensitive information and transactions. Imagine a scenario where an unpatched OTP bypass bug allows an attacker to bypass the OTP verification process and gain unauthorized access to a user’s account or sensitive data. This could lead to unauthorized transactions, data breaches, and a loss of trust and reputation for the affected organization.

Attackers can exploit OTP bypass bugs to compromise user accounts, perform fraudulent transactions, and gain unauthorized access to sensitive information. This can have far-reaching consequences, including financial losses, legal implications, and damage to the organization’s reputation.

To protect against OTP bypass bugs, it is crucial for organizations to regularly patch and update their systems, conduct thorough security testing, and ensure that their OTP implementation follows best practices and security standards. This includes implementing multi-factor authentication, monitoring for unusual account activities, and staying informed about the latest security threats and mitigation strategies.

Here is a simple tip to find if a website is vulnerable to OTP bypass. The request can be intercepted using Burp Suite; generally, a mobile number parameter can be found as a part of the request.

mobile=9********1

This parameter can be tampered using two ways:

1. Modify the entire parameter: The entire parameter can be modified with another mobile number and the modified intercepted request can be forwarded. Now the OTP will go to the newly entered mobile number and OTP can be easily bypassed.

But sometimes this technique can be stopped with proper input validation.

2. Add a comma: Instead of modifying the entire parameter, a comma can be used and another mobile number can be added. Now the modified intercepted request can be forwarded. Now the OTP will go to the newly entered mobile number and OTP can be easily bypassed. Eg:

mobile=9********1,6********3

The above two methods are the easiest ways to bypass OTP.

This could occur either due to Improper Input Validation and Sanitization or Logic Flaw in OTP Dispatch.

The application fails to validate or sanitize the mobile parameter, allowing a malicious user to inject multiple phone numbers.

These methods are not just theoretical but have been tried, and tested on live websites and have also been reported.

Recommendations for Prevention:

Strict Input Validation: Enforce a single valid phone number format and reject any request with multiple values or invalid characters.Server-Side OTP Generation: Generate OTPs on the server side instead of relying on client-side values.Use time-based OTPs: Use OTPs that expire after a short duration to prevent attackers from reusing intercepted OTPs.Encrypt sensitive details: Use encryption to protect sensitive details to avoid attacks using intercepting tools.

NOTE: Make sure to test only on sites where it is allowed to test and carefully read and follow the guidelines for testing on the site.

________________

Also, check out this exciting cobalt design coffee mug from Zazzle: https://www.zazzle.com/cobalt_design_mug-256043150936677794