This is obviously an attempt to trigger a redirect (for a phishing attempt),  although the fact that it causes an exception means that the attempt is failing.

The attempt is failing because the code needs to convert the "tabid' Parameter into an Integer, and as the string is not a valid integer the cast is failing and an exception is thrown.

The code that processes a normal page (eg Default.aspx?tabid=35) traps the exception, and logs an HttpException instead.  This is still logged in the EventLog BUT does not casue an asp.net yellow screen.

I will update the LinkClick code to do the same - ie redirect to the standard 404 - resource not found.