Saturday, August 3, 2013

In this post I am going to explain how to solve a "Crackme" challenge that I found on the Internet, in this example you can see a Login form but we don't know the Username or the Password, so what do you think could be a possible solution?, or the first step to solve it?

Well, maybe you thought that you can run a brute force tool against the login form, that is not a wrong answer but it isn't the best, so first of all I am going to write test values on the text boxes to see how the application works.

Figure 1: The Application returns an Error Message Box when the Credentials are wrong.

The next step is to verify if the application is Packed, I mean protected, this technique is used to make harder the Analyst work and also to hide the programming language in which this application was developed.

Figure 2: The Application was developed in "Microsoft Visual C# / Basic .NET" and it isn't packed!!

With this information the Analyst must be very happy, because this means that cracking the application will be much easier than expected.
Figure 3: My face when I saw the previous results. =D

What's next?, now We need a tool that allow us to see the functions (source code) that are implemented within the ".exe" file.

Figure 4: This image shows the source code of the function that is called when the Login button is pressed.

In figure 4, We can see some interesting and maybe useful information, let's pay attention to the red text (the strings), specially the ones that weren't visible during the test phase when we entered wrong values.

We can see a string that may be a username: "|Usuario|" and another one that could be a possible password:  "|J5L2C-K4B8L-D2K9S|", so let's try this combination in the text boxes to see what happens?
Figure 5: A successful message appears when using the login information found before.

Figure 6: The feeling of every malware researcher after a successful job.

Now maybe you have a better idea of how some cracks and key generators are created, well, that's all, hope you enjoyed with this post, 

I also analyzed this crackme challenge to verify if its clean, you can see the complet report here:

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