What is the best way to test for command injection vulnerabilities?

1 What is command injection?

Command injection is a type of injection attack that exploits the use of user input in the construction of commands that are executed by the system. For example, if an application takes a file name from the user and passes it to a command like cat filename , an attacker can inject additional commands by using special characters like ; or & . For instance, cat filename; rm -rf / would delete all the files on the system. Command injection can affect any system that uses a command shell, such as Linux, Windows, or MacOS.

2 How does command injection work?

Command injection works by manipulating the syntax and semantics of the command interpreter. The command interpreter is the program that parses and executes commands from the user or another program. It usually has a set of rules and conventions for separating commands, arguments, and options. By using special characters or escape sequences, an attacker can break the intended structure of the command and insert their own malicious commands. For example, in Linux, the $ character can be used to expand variables or execute subcommands. An attacker can use this to inject commands that are evaluated by the shell before the main command. For example, cat filename$(rm -rf /) would also delete all the files on the system.

3 What is the best way to test for command injection?

The best way to test for command injection is to use a combination of manual and automated testing techniques. Manual testing involves using tools like Burp Suite or ZAP to intercept and modify the requests and responses between the application and the system. You can then try to inject various commands and observe the results. For example, you can try to inject commands like whoami , id , ls , or ping to see if they are executed by the system and return any output. You can also try to inject commands that cause errors or delays, such as sleep or div0 , to see if they affect the application's behavior.

Automated testing involves using tools like Nmap or Nikto to scan the application for common command injection vulnerabilities. These tools have predefined payloads and signatures that can detect command injection flaws in various parameters and functions. For example, Nmap has a script called http-shellshock that can test for the infamous Shellshock vulnerability, which allows command injection through environment variables. Automated testing can help you identify and prioritize the most critical command injection vulnerabilities in your application.

4 How to prevent command injection?

The best way to prevent command injection is to follow secure coding practices and use proper input validation and output encoding. Input validation means checking and filtering the user input before passing it to the command interpreter. You should avoid using user input directly in the command, and instead use parameterized or prepared statements, or whitelists of allowed values. Output encoding means escaping or sanitizing the output from the command interpreter before displaying it to the user. You should avoid using output that contains special characters or escape sequences, and instead use HTML or URL encoding, or hex encoding.

Another way to prevent command injection is to use secure configuration and deployment practices and limit the privileges and access of the system and the application. You should disable or restrict the use of dangerous functions and commands, such as eval , exec , or system , and use safer alternatives, such as subprocess or os.spawn . You should also use the principle of least privilege and run the application with the minimum permissions and resources needed. You should also use firewall rules and network segmentation to isolate and protect the system and the application from external attacks.

5 What are the benefits of testing for command injection?

Testing for command injection is essential for improving the security and reliability of your application and system. It can detect and fix any command injection vulnerabilities before they are exploited by attackers, preventing data loss, unauthorized access, or remote control. Moreover, testing for command injection can help you comply with security standards and regulations, such as OWASP Top 10 or PCI DSS, as well as enhance your reputation and trust among customers and users. Furthermore, it can reduce the costs and risks of security incidents and breaches. Command injection is a serious security threat that can compromise your application and system; therefore, you need to test for command injection and fix any flaws. Following the best practices and techniques described in this article can help you test for command injection effectively and efficiently.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?