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Command Injection Attacks: Types

OS Command Injection Attacks, often referred to as Command Injection, are a type of security vulnerability that occurs in web applications. These attacks occur when an application allows a user to input data, such as through a web form or URL parameter, and doesn't properly validate or sanitize the input. An attacker can then manipulate this input to execute arbitrary operating system commands on the server where the web application is hosted.

Types of OS Command Injection Attacks:

  • Blind OS Command Injection: In a blind OS command injection, the application doesn't display the output of the executed commands directly to the attacker. Instead, the attacker has to infer the success or failure of the command based on the application's behavior. For example, if an application allows a user to ping an IP address and doesn't display the results, an attacker might inject a command that pings an IP under their control and then observe any changes in the application's behavior.
  • Classic OS Command Injection: In this type, the application directly executes commands with the user's input. If a user provides input without proper validation and sanitation, an attacker can inject arbitrary commands. The output of these commands may be visible to the attacker.

Example of OS Command Injection Attack:

Let's consider a simple example of a web application that allows a user to ping a server to check its availability. The user is supposed to input an IP address or domain name, and the application will return the results of the ping operation. However, if the application doesn't validate or sanitize the input correctly, an attacker can exploit this vulnerability.

Suppose the application constructs a command to ping the given IP address as follows (in a Python-like pseudocode):

user_input = get_user_input() # User input not properly sanitized
command = "ping " + user_input
result = execute_command(command)

If the user provides the following input:; ls

The command executed by the application becomes:

ping; ls

The semicolon (
;) is used to separate multiple commands in many operating systems. In this case, the attacker has injected a second command ls, which lists the files in the server's current directory. The application executes both commands, and the result of ls is returned, potentially disclosing sensitive server information to the attacker.

To prevent OS Command Injection Attacks, it's essential to validate and sanitize all user inputs, use parameterized queries when interacting with databases, and restrict the application's use of system commands. Additionally, the principle of least privilege should be followed, ensuring that the web application runs with minimal access rights on the server to limit potential damage in case of a successful attack.