Your computer seems to be running slower than normal. The router shows that your computer is transmitting data out to the Internet without you knowing it. Friends are complaining about you sending them e-mails you never composed. Determined to see if you have a Trojan running on your computer, you take a look at your process list to see if there is anything out of the ordinary. Much to your dismay, you notice a program running that you have never seen before and didn’t explicitly start. You have been backdoored by malware.

There are many questions you should be asking in these situations. What does the program do? Does it use network resources? Can outside hackers now access my computer? Am I being used as a zombie for DDoS attacks? This chapter will focus on methods and tools you can use to determine what these programs do and how they do them, without having the source code. In the past, reverse engineering was something of a black art. Typically it involved some type of decompilation using a tool such as IDA or GDB to extract the assembly out of the binary, and the best you could hope for was to have that assembly converted into a low-level C code that you could use to understand what was going on. These tools have evolved, however, and you no longer need a PhD in Computer Science to be able to reverse engineer binaries. That being said, however, a brief primer will go miles in helping you understand when to use certain tools and when to use others.