It’s time to learn about another type of Code Injection which is Shell Injection!

Shell Injection also known as Command Injection is an attack which allows an attacker to execute commands on the server side via malicious user inputs. These attacks can occur when a software application takes unsafe user input and execute corresponding commands directly to give output.

So let’s take a simple example where an application shows the file content as output, based on the file name taken as its user input,

1
2
3
4
5
<?php
print(“Please specify the name of the file”);
$file=$_GET[‘filename’];
system(“cat $file”);
?>

Here if an attacker gives an input ‘ account.txt ’, corresponding file’s content will be shown.

Also an attacker can exploit this even more by giving input ‘ account.txt; rm -rf /; ’ which will delete the files in the root directory.

So as you can see, this is a serious vulnerability which needs to be taken into consideration as developers.

Now lets see how we can prevent it,

1. Verify user input
2. Never execute direct commands taken as user inputs

3. Replace or ban arguments with characters like ; , &,

   .etc

4. Limit the length of the user input data

So that’s all for shell injection, hope it will be helpful to make your application less vulnerable.