PHP Snippet #09

This challenge covers the review of a snippet of code written in PHP

< 1 Hr.


The Code Review Snippet challenge provides a piece of PHP code that defines a constant key and a function to sign data using HMAC with SHA-256. The script checks whether the `$_GET["data"]` and `$_GET["signature"]` parameters are set and validates the signature by comparing the HMAC of the data with the provided signature. However, the comparison on line seven uses `==`, which can be exploited due to type juggling. If `$_GET["data"]` is an array, `hash_hmac` returns NULL, and comparing NULL with an empty string using `==` results in a valid signature.

The issue lies in the fact that the `==` operator in PHP does not check for type equivalency. This allows an attacker to bypass the signature check by setting `$_GET["signature"]` to an empty string, making the script believe the signature is valid. Additionally, the comparison method should be constant-time to prevent brute-force attacks. Understanding these nuances helps in identifying and mitigating such vulnerabilities in PHP applications.

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