Inappropriate coding practices - Eval function
Need
Secure input validation and sanitization
Context
• Usage of C# for building robust and scalable applications
• Usage of Microsoft.CSharp for dynamic compilation and execution of C# code
• Usage of System.CodeDom.Compiler for dynamic code compilation
Description
1. Non compliant code
public IActionResult UnsafeEval(string command)
{
var result = Eval(command);
return Ok(result);
}
private object Eval(string command)
return Microsoft.CSharp.CSharpCodeProvider.CreateProvider("CSharp")...The above code is an example of a vulnerable C# backend application that uses the ASP.NET Core framework. The `UnsafeEval` method takes a string parameter `command`, which is then passed to the `Eval` method. The `Eval` method uses the `Microsoft.CSharp.CSharpCodeProvider.CreateProvider("CSharp")` method to compile and execute the command. The vulnerability lies in the `Eval` method, specifically in the way it handles the `command` parameter. The `command` parameter is directly concatenated into the C# code string that is compiled and executed. This code is not validated or sanitized before it is executed. This means that an attacker could inject malicious code into the `command` parameter. For example, if an attacker sends a request with a `command` parameter value of `System.IO.File.Delete("importantFile.txt")`, the server would execute this command and delete the "importantFile.txt" file. This is an example of an injection vulnerability, where untrusted input is allowed to be part of a command or query that is then executed by the server. In this case, the injection happens in the C# code that is compiled and executed by the `Eval` method. This vulnerability could lead to serious security issues, such as data loss, data corruption, or even full system compromise.
2. Steps
• Avoid using the eval function altogether.
• Validate and sanitize all user input before using it in any code execution.
• Use parameterized queries or prepared statements to prevent SQL injection.
• Implement a strict input validation mechanism to ensure that only expected input is accepted.
• Implement a whitelist approach for input validation, allowing only specific characters or patterns.
• Implement output encoding to prevent cross-site scripting (XSS) attacks.
• Implement proper exception handling to prevent sensitive information leakage.
• Regularly update and patch the application and its dependencies to fix any known vulnerabilities.
3. Secure code example
public IActionResult SafeEval(string command)
{
if (IsValid(command))
{
var result = Eval(command);
return Ok(result);
}
{...The original code was vulnerable because it used the `Eval` function with unvalidated user input, which could lead to code injection attacks. The `Eval` function is dangerous because it executes the string it is passed as code. If an attacker can control the string that is passed to `Eval`, they can execute arbitrary code. The fixed code adds an `IsValid` function that validates the `command` before it is passed to `Eval`. The validation function checks if the command contains only alphanumeric characters. This is a very basic validation and might not cover all possible cases. Depending on the context of your application, you might need to implement a more complex validation logic. Please note that using `Eval` is generally discouraged because of its potential security risks. If possible, consider refactoring your code to avoid using `Eval`. If you must use `Eval`, make sure to validate and sanitize all user input thoroughly.