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Database

Remote command execution

Need

Prevention of unauthorized remote command execution

Context

• Usage of C# 7.0 for modern language features and enhancements

• Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core

• Usage of System.Diagnostics for logging and performance monitoring in .NET development

Description

1. Non compliant code

public class CommandController : Controller
{
    [HttpGet]
    public IActionResult ExecuteCommand(string cmd)
    {
        var process = new Process()
        {
            {...

The above code represents a controller in an ASP.NET Core application that takes an input string `cmd` from the user and executes it as a bash command. The vulnerability in this code is the `ExecuteCommand` method that takes a user-supplied string and directly uses it as an argument for a new process start info. This is a clear example of a Remote Command Execution vulnerability. In this scenario, an attacker could potentially execute arbitrary commands on the server by simply passing them through the `cmd` parameter. For example, they could pass something like `; rm -rf /` to delete all files in the root directory or `; cat /etc/passwd` to read sensitive information. This vulnerability exists because the application does not sanitize or validate the `cmd` input in any way before using it. This means that special characters like `;` are not neutralized and can be used to chain multiple commands together.

2. Steps

• Validate and sanitize user input before using it to build OS commands.

• Use parameterized queries or prepared statements to prevent command injection.

• Avoid using user input directly in command arguments or parameters.

• Implement input validation to restrict the type and format of user input.

• Implement a whitelist approach to only allow specific characters or patterns in user input.

• Implement proper error handling to prevent sensitive information leakage.

• Regularly update and patch the underlying operating system and dependencies to mitigate potential vulnerabilities.

3. Secure code example

public class CommandController : Controller
{
    private readonly Regex _regex = new Regex(@"^[a-zA-Z0-9\\s]*$");

    [HttpGet]
    public IActionResult ExecuteCommand(string cmd)
    {
        {...

The updated code includes several measures to mitigate the risk of remote command execution: 1. **Input Validation**: The code checks if the command is null or empty, and if it matches a regular expression that only allows alphanumeric characters and spaces. This prevents the injection of special characters that could modify the OS command. 2. **Error Handling**: The code includes a try-catch block to handle any exceptions that occur during the execution of the command. This prevents the leakage of sensitive information in case of an error. 3. **Response Codes**: The code returns appropriate HTTP response codes based on the result of the command execution. A 400 (Bad Request) status is returned for invalid commands, and a 500 (Internal Server Error) status is returned in case of an error. 4. **Whitelisting**: The regular expression used for input validation acts as a whitelist, only allowing specific characters in the command. Remember to regularly update and patch the underlying operating system and dependencies to mitigate potential vulnerabilities.

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