How to Perform Command Injection Attacks

Introduction: Command injection is an attack in which the goal is execution of arbitrary commands on the host operating system via a vulnerable application. Command injection attacks are possible when an application passes unsafe user supplied data (forms, cookies, HTTP headers etc.) to a system shell. In this attack, the attacker-supplied operating system commands are usually executed with the privileges of the vulnerable application. Command injection attacks are possible largely due to insufficient input validation.
How to Perform Command Injection Attacks
In this artick, we will talk about the varieties of command injections and how they can be executed. There are a variety of ways to inject shell commands. Assume for a moment that you have found the page, which takes as an argument a filename as input and executes the shell command "cat" against that file. For example, a semicolon was used to separate out one command form another, to indicate that after the cat command completed, another function should be called in the same line. There are a number of ways to string shell commands together to create new commands.
Here are the common operators you can use, as well as examples of how they might be used in an attack:

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Redirection Operators

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Examples: <, >>, >
These operators redirect either input or output somewhere else on the server. < will make whatever comes after it standard input. Replacing the filename with < filename will not change the output, but could be used to avoid some filters. > redirects command output, and can be used to modify files on the server, or create new ones altogether. Combined with the cat command, it could easily be used to add unix users to the system, or deface the website. Finally, >> appends text to a file and is not much different from the original output modifier, but again can be used to avoid some simplistic detection schemes.

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Pipes

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Examples: |
Pipes allow the user to chain multiple commands. It will redirect the output of one command into the next. So you can run unlimited commands by chaining them with multiple pipes, such as cat file1 | grep "string".

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Inline commands

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Examples: ;, $
This is the original example. Putting a semicolon asks the command line to execute everything before the semicolon, then execute everything else as if on a fresh command line.

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Logical Operators

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Examples: $, &&, ||
These operators perform some logical operation against the data before and after them on the command line.
Common Injection Patterns & Results
Here are the expected results from a number of common injection patterns (appending the below to a given input string, assuming all quotes are correctly paired:

`shell_command` - executes the command
$(shell_command) - executes the command
| shell_command - executes the command and returns the output of the command
|| shell_command - executes the command and returns the output of the command
; shell_command - executes the command and returns the output of the command
&& shell_command executes the command and returns the output of the command
> target_file - overwrites the target file with the output of the previous command
>> target_file - appends the target file with the output of the previous command
< target_file - send contents of target_file to the previous command
- operator - Add additional operations to target command

These examples are only scratching the surface of possible command injection vectors. The full breadth of attack possibilities is dependent upon the underlying function calls. For instance, if an underlying function is using a shell program such as awk, many more attack possibilities arise than laid out here.
Finally, command injection can be more subtle than finding applications which directly call underlying operating system functions. If it is possible to inject code, say PHP code, then you can also perform command injections. Assume you find an application with a PUT vulnerability on a site which is PHP enabled. An attacker could simply upload a PHP file with a single line to have full access to a shell:

<?php
echo
shell_exec('cat '.$_GET[
'command']);
?>

Thus, it should be noted that many types of attacks, including SQL Injection, have shell injection as an end primary goal to gaining control of the server.