A backdoor shell can be a PHP, ASP, JSP, etc. piece of code which can be uploaded on a site to gain or retain access and some privileges on a website. Once uploaded, it allows the attacker to execute commands through the shell_exec () function, upload/delete/modify/download files from the web server, and many more. For defacers, it allows them to navigate easily to the directory of the public_html or /var/www and modify the index of the page.
In this write-up, we will be talking about PHP backdoor shells, how they work, how to detect them and remove them. Below is a simple PHP code that is very popular and is scattered all over the web (http://stackoverflow.com/questions/3115559/exploitable-php-functions; http://shipcodex.blogspot.com/2012/01/simple-php-backdoor-shell.html). This code allows an attacker to execute *nix commands. For the full write up at
InfoSec Institute, check here: