Researcher Exploits Regex Filter Flaw to Gain Remote Code Execution

Target application included a username field restricted by a frontend regex filter (/^[a-zA-Z0-9]{1,20}$/), designed to accept only alphanumeric characters.

While this initially appeared robust, the researcher discovered that the backend failed to revalidate inputs after the regex check.

This oversight allowed specially crafted payloads to bypass client-side controls and execute arbitrary commands on the server.

Key to the exploit was the backend’s trust in frontend validation. Unlike client-side JavaScript, which enforced the regex rule, the server processed raw input without additional sanitization.

The researcher emphasized that “regex is a tool, not a firewall”, noting that such misconfigurations are common in applications where security logic is fragmented across layers.

Leveraging Alternative HTTP Methods

The breakthrough came when the researcher tested alternate HTTP methods. While the frontend form used POST requests with strict regex checks, the backend API accepted PUT requests for the same endpoint without validation.

By sending ...