To achieve a reverse shell or system command execution:

Immediate mitigation steps (prioritize)

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Run this on your web servers:

Check your access logs for suspicious patterns. Look for POST requests to any path containing phpunit/src/Util/PHP/eval-stdin.php or eval-stdin.php .

Update PHPUnit to a secure version. The maintainers patched this vulnerability in versions and 5.6.3 . 2. Restrict Web Access to the Vendor Directory

Many developers discover this vulnerability when they deploy complete project directories without pruning development dependencies — a common mistake that attackers actively exploit.

This article explains how the vulnerability works, how attackers exploit it, and how to protect your server. The Root Cause

Update your development dependencies to pull in patched versions of PHPUnit: composer update phpunit/phpunit --dev Use code with caution.

location ~ /vendor/ deny all; return 403;

: An HTTP status code of 200 OK indicates the file is exposed and active.

If the vendor directory is publicly accessible via the web server, an attacker can send an HTTP POST request containing malicious PHP code directly to this file, forcing the server to execute it. How the Attack Works

And she never trusted a Composer require-dev package in production again.