An unauthenticated OS command injection vulnerability exists within Xdebug versions 2.5.5 and earlier, a PHP debugging extension developed by Derick Rethans. When remote debugging is enabled, Xdebug listens on port 9000 and accepts debugger protocol commands without authentication. An attacker can send a crafted eval command over this interface to execute arbitrary PHP code, which may invoke system-level functions such as system() or passthru(). This results in full compromise of the host under the privileges of the web server user.