PHP Snippet #04
This challenge covers the review of a snippet of code written in PHP
The Code Review Snippet challenge "jwt.php" presents a vulnerable PHP function named verify, which is designed to validate a JSON Web Token (JWT). The function splits the JWT into three parts: header, payload, and signature. It then checks if the signature is not empty and compares it with the generated signature. If the signatures do not match, the script terminates with an "Invalid Signature" message.
However, the vulnerability arises because the function only checks the signature if it is not empty. This means that if an attacker provides a JWT with an empty signature, the verification check is bypassed entirely. Consequently, this allows an attacker to bypass the security mechanism and potentially exploit the system.
To exploit this vulnerability, an attacker can craft a JWT with a valid header and payload, but omit the signature part. The function will then proceed without validating the signature, leading to a security breach. This challenge underscores the importance of thorough input validation and proper security checks in code.