Command Line on a Linux Server (en)

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CLI is a text-based user interface that allows users to interact directly with the operating system. In Linux, the CLI is very powerful and flexible, enabling users to execute a variety of commands to manage the system, run applications, and much more.

Why is the Command Line Vulnerable to Injection Attacks?

  • Direct User Input: Many applications accept user input directly and then use that input in system commands. If the input is not properly sanitized, an attacker can inject additional commands that will be executed by the system.
  • Powerful System Commands: Commands such as `system()`, `exec()`, and `eval()` in programming languages allow direct execution of system commands. If not used carefully, these commands can become an entry point for attackers.

Commands Commonly Abused for Command Injection:

  • mfifo:
    • Typically used to create a named pipe, which can be used for inter-process communication.
    • Abuse: An attacker can create a named pipe with an unexpected name and then redirect the output of harmful commands into it.
  • nc (netcat):
    • Versatile, can be used for various network tasks such as port scanning, file transfer, and reverse shells.
    • Abuse: An attacker can use nc to open a reverse shell connection to the target system, allowing them to execute commands interactively.
  • Other Shell Commands:
    • `bash`, `sh`, `ksh`: These shells allow direct command execution and can be abused to run harmful commands.
    • `find`, `grep`, `sed`, `awk`: These commands are often used in combination with pipe operators (|) to build complex commands. An attacker can inject additional commands into these pipes.

Example of a Command Injection Attack:

Suppose a web application has a search form that sends queries directly to a database. If the query is not sanitized, an attacker can inject a command like:

'; DROP TABLE users; --

This command would delete the users table from the database.

Preventing Command Injection:

  • Sanitize Input: Always sanitize all user input before using it in system commands. Use appropriate escaping functions to prevent special characters from being misinterpreted as part of the command.
  • Parameterize Queries: Use parameterized queries to prevent injection attacks on databases.
  • Limit Access Rights: Limit regular users' access to harmful commands.
  • Use Safe Functions: Use functions specifically designed to safely execute system commands, such as `escapeshellarg()` and `escapeshellcmd()` in PHP.
  • Input Validation: Perform input validation to ensure that the input matches the expected format.
  • Least Privilege Principle: Run processes with the least amount of privilege necessary.
  • Regularly Update Systems: Patch systems regularly to fix security vulnerabilities.

Conclusion

Command injection is one of the most common and dangerous types of attacks. By understanding how these attacks work and implementing appropriate preventive measures, you can protect your systems from them.

Important: Remember that this is just an overview. For more comprehensive protection, it is recommended to learn more about web application and operating system security.

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