Javascript Apache Flex Exploit

Understanding the Danger of Executing JavaScript from Flex Applications

In the world of application development, security is a paramount concern, especially when it involves the execution of code that can potentially manipulate or harm user environments. Recently, a question emerged regarding the execution of raw JavaScript from a Flex application, specifically about how it can be done and the risks involved. The initial approach involved using a JavaScript function to evaluate strings of code. In this post, we’ll explore this topic, consider the potential dangers, and suggest a more secure method for executing JavaScript in Flex applications.

The Challenge: Executing Custom JavaScript

When developing a Flex application, the ability to generate and execute JavaScript dynamically can be a powerful feature. For instance, developers might want to call JavaScript functions or execute commands directly from their Flex code. The traditional method that has come up in conversations is using a method like this:

function doScript(js){ eval(js); }

This function allows for any JavaScript code to be executed. To execute it from Flex, one might use:

ExternalInterface.call("doScript", "alert('foo')");

However, this raises an important question: Does this pose a security risk? The assumption is that since both Flex and JavaScript run client-side, the risk is minimal. However, the truth is more complex.

The Risks of Using eval()

Using eval() in JavaScript can lead to various vulnerabilities and issues such as:

  • Code Injection: If the input to eval() is influenced by user input, it can lead to arbitrary code execution, effectively allowing attackers to run malicious scripts.
  • XSS (Cross-Site Scripting): This vulnerability can occur if an attacker injects harmful scripts into web pages, and the eval() function unwittingly executes them.

Because of these risks, it’s generally recommended to avoid using eval() wherever possible unless absolutely necessary and controlled.

A Better Solution: Direct Execution without a Function

The great news is, there is a more efficient way to execute JavaScript from Flex without compromising security. Instead of relying on a custom function with potential pitfalls, you can execute JavaScript directly using the ExternalInterface. Here’s how:

Instead of this approach:

ExternalInterface.call("doScript", "alert('foo')");

You can simply do this:

ExternalInterface.call("alert('hello')");

Key Benefits of This Approach:

  • Increased Security: By avoiding eval(), you significantly reduce the risk of code injection attacks.
  • Simplicity: Directly calling the JavaScript function eliminates unnecessary complexity and enhances maintainability.
  • Clarity: This method clearly states what you aim to achieve without going through an intermediate function, making your code easier to read.

Conclusion

In conclusion, while the ability to execute JavaScript from a Flex application can add functional depth, it’s crucial to consider the security implications involved in such actions. By avoiding the use of eval() and taking direct calls to JavaScript functions, developers can both enhance the security of their applications and maintain a clean, efficient codebase.

When in doubt, always opt for the simplest and most direct methods that adhere to security best practices. This way, your applications can thrive without exposing users to unnecessary risks.