Command Injection Attack Detailed Explanation

Command Injection Attack Detailed Explanation

Description
Command injection attack is a common security vulnerability that occurs when an application directly passes user input to the system shell for execution. Attackers construct malicious input to execute arbitrary commands on the system, potentially leading to data leakage, system damage, or complete server compromise. This vulnerability is commonly found in web applications, network devices, or management interfaces where functionality requires invoking system commands.

Progressive Explanation of Key Points

  1. Basic Principle of Command Injection

    • When an application needs to execute system commands (such as ping, ls, dir, etc.), it may directly concatenate user-provided data into the command string.
    • Example: A website provides a ping function. After the user inputs an IP address, the backend code might execute ping <user input>.
    • If the user input is 8.8.8.8; cat /etc/passwd, the concatenated command becomes ping 8.8.8.8; cat /etc/passwd.
    • The semicolon (;) is a command separator in Unix-like systems, causing the system to execute both commands sequentially.

  2. Common Scenarios for Command Injection

    • Network tools: ping, traceroute, nslookup, etc.
    • File operations: file managers, log viewers
    • System administration: process management, service control
    • Data backup: packaging, compression functions

  3. Command Separators and Connectors

    • Semicolon (;) : Executes multiple commands sequentially.
    • Ampersand (&) : Executes a command in the background.
    • Pipe (|) : Uses the output of the previous command as input for the next.
    • Logical Operators: && (executes the next command if the previous one succeeds), || (executes the next command if the previous one fails).
    • Backticks (`) and $() : Command substitution, executes the command inside.

  4. Attack Step Demonstration

    • Step 1: Identify Injection Points
      Look for functionalities that accept input and execute system commands, such as ping tests, file upload processing.

    • Step 2: Test Command Separators
      Add simple commands after normal input, e.g., 127.0.0.1; whoami or 127.0.0.1 && id.

    • Step 3: Confirm Successful Injection
      Observe if the response includes command execution results, or use time delays to judge (e.g., ; sleep 5).

    • Step 4: Exploit the Injection Point

      • Information gathering: ; uname -a (system info), ; cat /etc/passwd (user list).
      • Establish persistent access: Download and execute a reverse shell script.
      • Data exfiltration: Transfer sensitive files to the attacker's server over the network.

  5. Defensive Measures

    • Input Validation: Whitelist validation, only allowing expected character formats (e.g., IP addresses only allowing digits and dots).
    • Parameterized Calls: Use secure functions provided by the programming language (e.g., Python's subprocess.run(["/bin/ping", "-c", "4", user_input])).
    • Avoid Direct Shell Calls: Avoid functions like system(), exec(); use safer APIs.
    • Principle of Least Privilege: The user running the application should have the minimum necessary permissions.
    • Output Encoding: Appropriately encode command execution results before displaying them to the user.

  6. Advanced Detection Techniques

    • Blind Command Injection: When command execution results are not directly displayed, use time delays, DNS exfiltration, etc., for detection.
    • Bypassing Filters: Use encoding, variable substitution, wildcards, etc., to bypass security filtering mechanisms.

By understanding the principles, attack methods, and defensive measures of command injection, this type of high-risk vulnerability can be effectively prevented, enhancing application security.