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OS Command Injection

Need​

Prevention of OS command injection attacks

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of System for managing and interacting with the operating system
  • Usage of System.Diagnostics for performance monitoring and debugging in .NET development

Description​

Non compliant code​

using System;
using System.Diagnostics;

public class CommandInjection
{
    public void ExecuteCommand(string cmd)
    {
        var processInfo = new ProcessStartInfo("cmd.exe", "/c " + cmd)
        {
            CreateNoWindow = true,
            UseShellExecute = false
        };

        var process = Process.Start(processInfo);
        process.WaitForExit();
    }
}

The above code represents a severe vulnerability of OS Command Injection in a C# backend application that uses ASP.NET Core.

In this code, the ExecuteCommand method takes a string cmd as an input parameter and passes it directly to a new process that executes commands in the system shell (cmd.exe). This is done using the ProcessStartInfo class, which is part of the System.Diagnostics namespace.

The vulnerability lies in the fact that the input parameter cmd is used directly without any form of sanitization or validation. This means that an attacker can pass malicious commands through this parameter, which will then be executed directly on the system shell.

For example, an attacker could pass the string ; rm -rf / (on Unix-based systems) or ; del /F /S /Q * (on Windows systems) to delete all files on the server. They could also pass commands to escalate privileges, obtain or modify sensitive information stored in the machine, or perform other harmful actions.

This vulnerability can highly increase the attack vectors and can have severe impacts on the security and integrity of the system.

Steps​

  • Validate and sanitize user input: Before executing any command, validate and sanitize the user input to ensure it does not contain any malicious characters or commands.
  • Use parameterized queries or commands: Instead of concatenating user input directly into the command, use parameterized queries or commands to separate the command from the user input. This helps prevent command injection attacks.
  • Implement input validation: Implement input validation to ensure that only expected and valid input is accepted. Reject any input that does not meet the defined criteria.
  • Limit privileges: Ensure that the process executing the command has the minimum necessary privileges to perform the required task. Avoid running commands with elevated privileges unless absolutely necessary.
  • Use a secure execution environment: Consider using a secure execution environment or sandbox to isolate the execution of potentially dangerous commands. This can help mitigate the impact of command injection vulnerabilities.
  • Regularly update and patch dependencies: Keep all dependencies, including the ASP.NET Core framework, up to date with the latest security patches to minimize the risk of known vulnerabilities being exploited.
  • Implement logging and monitoring: Implement logging and monitoring mechanisms to detect and track any suspicious or malicious activities, including command injection attempts.
  • Educate developers: Provide training and education to developers on secure coding practices, including the risks and mitigation strategies for command injection vulnerabilities.

Compliant code​

using System;
using System.Diagnostics;
using System.Text.RegularExpressions;

public class CommandInjection
{
    public void ExecuteCommand(string cmd)
    {
        // Validate and sanitize user input
        if (!IsValidCommand(cmd))
        {
            throw new ArgumentException("Invalid command");
        }

        var processInfo = new ProcessStartInfo("cmd.exe", "/c " + cmd)
        {
            CreateNoWindow = true,
            UseShellExecute = false
        };

        // Limit privileges
        processInfo.RedirectStandardOutput = true;
        processInfo.RedirectStandardError = true;
        processInfo.UseShellExecute = false;

        var process = new Process
        {
            StartInfo = processInfo,
            EnableRaisingEvents = true
        };

        process.Start();
        process.WaitForExit();
    }

    private bool IsValidCommand(string cmd)
    {
        // Implement input validation
        var regex = new Regex(@"^[a-zA-Z0-9\\s\\.\\-\\_]*$");
        return regex.IsMatch(cmd);
    }
}

The updated code includes a method IsValidCommand that validates the user input before executing the command. This method uses a regular expression to ensure that the command only contains alphanumeric characters, spaces, dots, hyphens, and underscores. Any command that does not match this pattern is considered invalid and an ArgumentException is thrown.

The ProcessStartInfo object is configured to redirect the standard output and error streams, and to not use the shell to execute the command. This helps to limit the privileges of the process executing the command.

The Process object is configured to raise events, which can be used to implement logging and monitoring mechanisms to detect and track any suspicious or malicious activities, including command injection attempts.

This code should be used in a secure execution environment or sandbox to further mitigate the impact of command injection vulnerabilities. It is also important to regularly update and patch all dependencies, including the ASP.NET Core framework, to minimize the risk of known vulnerabilities being exploited.

Finally, developers should be educated on secure coding practices, including the risks and mitigation strategies for command injection vulnerabilities.

References​

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