Microsoft Visual Basic 6.0 was developed in 1998 and more than a decade later, many malware created in Visual Basic are still running rampant. One of those is W32.Changeup, a polymorphic worm that comprises 25 percent of all malware written in Visual Basic.

In order to develop a better understanding of Changeup, I set out to analyze it in great depth. To do so, I had to manually decompile it as it could not be decompiled using decompiler tools. It’s also worth noting that Visual Basic programs require special knowledge to analyze, due to their flexible source code syntax. In particular, spotting variants and arrays is the key to precise analysis.
Once I completed analysis of the worm, I wrote a white paper that details my findings. In...