Monday, November 26, 2007

Digital Criminalistics

Once upon a time a forensics investigator would arrive on a scene and seize a computer. The scene would be photographed, evidence would be collected, bagged & tagged and so on. A few floppies would be collected, maybe a CD but not much more.

Fast forward about 10 years....

A forensics investigator arrives on scene to discover a wireless linksys router with a 4 port switch on the back. 3 wires connect to a triplex data drop in the wall. One room with a computer became a house with a computer in each room. A 3.5" floppy is now obselete, and data is every where. Did you remember the cell phone? The PDA? The camera, and the media cards? The DVR or media center computer? It just keeps going...

Even a few years ago as Incident Responders we knew what to look for. Botnets were rampant. They were easily detected and a vast majority of them followed the same standard. These attacks adapted and became more complex. Fast Flux networks are in full swing.

Digital Forensics is quickly becoming as complex as real world criminalistics. In the sea of agile data, we must be as agile. Our methods need to be fluid and adaptable. Simply pulling the plug is no longer the best practice. Digital Forensics doesn't begin and end on the disk. It begins at point A and ends at Point Z. There are many data points in between. We need to know what these points are, and how to collect relevant data from them. We must also understand the forces at work and how they influence the data to be collected.

After a scene has been contained..note I said contained. Let me digress....Containment is: the action of keeping something harmful under control or within limits. While pulling the plug is a method of keeping something harmful under control, we must understand that it is the most extreme method in use today. Pulling the plug is what I consider to be a knee-jerk reaction used due to lack of understanding of data, the forces at play, and the influences those forces have.

Getting back on track..after a scene has been contained, the evidence preserved and collected the reconstruction begins. This is where things can get difficult, because we as an industry don't have a scientific background for our field. Consider if you will that the source code of Windows is closed. This leads us to seeking empirical truth, or that which we can observe. This is actually converse to what we are seeking to accomplish. Simply because we can observe something, doesn't mean it's the only explanation. We can only ever hope to be obtain a level of certainty in our conclusions.

In search of science we begin with induction: "The timestamp was modified". After finding a binary capable of modifying timestamps we move quickly to say "This binary is responsible for modifying timestamps on the system". Is this correct? Maybe. It requires additional work to be a more complete conclusion.

Next we have deductive reasoning: "The timestamp was modified by this binary". After experimenting we deduce that "The timestamps could have been modified by this binary". This is better. We are not absolute in our conclusion.

Neither of these is wrong, and neither is right, however both are incomplete. Why? We're only looking at one source, and we can't make a dogmatic statement based on a sole source of evidence. As digital criminalists, we must be able to say that given all data points, and after careful evaluation the most likely explanation for the modification of the timestamps is this binary. There are many potential methods of timestamp modification, but given the data I am reasonably certain that any other explanation would be less believable and unlikely.

We must understand that our conclusion is not simply the correct one. It is the correct one, because other possible explanations have been ruled out as being plausible given the dataset. To truly achieve this we must have corroboration. Multiple sources of data must be in support of one another to have an accurate conclusion.

As the complexity increases, we must become more certain of our conclusions. We must truly understand what is the cause, and what is the effect. If we understand causality we can move towards science.