The built-in Webcam light is on. It shouldn't be on. I'm not using any recording, video-conferencing or photo applications. Why is it on? Is someone watching me? It's at times like this I get the eerie feeling that I don't actually, fully and completely own my machine. Turns out it was a driver problem, all fixed now. But I still can't shake that feeling.

Sounds like a silly question. After all, as security professionals we must be able keep our machines secure, right? Or if something infects our machine, surely we would notice. When a non-expert asks us is there is a virus on his machine, we should be able to answer with absolute authority, after perhaps running a few tests. But when I get that question nowadays I'm not so sure. I do run the tests, loading up three to four different antivirus engines, a few anti-Trojan or antispyware tools and wait for a couple of hours. Invariably, they come back clean. So I have my answer: If the machine is compromised it has a stealthy, undiscovered, zero-day and well-hiddenTrojan or infection.

Gone are the days of loud infections that mangled your operating system, popped up gambling sites and hijacked your browser. Those pests are still around and usually easy to deal with. But today we have to contend with a far stealthier set of foes that can hide in the deepest recesses of the operating system.

If we look at biological comparisons we've gradually moved down the lethality scale -- from ebola-like boot sector infections that would corrupt your MBR and taunt you with a message, to more subtle flu-like pop-ups that come and go every season without killing too many machines. And finally we have arrived at the subtlest of infections: like gut bacteria or parasitic organisms, these infections use your resources, steal your vital information but without making your systems too sick. Your machine's ailment is so subtle that it's hard to tell: Am I infected or is my registry too big? Is it a rogue process or a system process? Your antivirus engine is less subtle about its CPU impact than the threat it is protecting you from.

Nowadays, I have no idea if my machines are infected or compromised. Given enough time and resources I could attempt to find out. I could freeze the system and audit it on a file-by-file, byte-by-byte basis. I could compare it to a pristine operating system and forensically analyze every trace of every application. It would take weeks and even then I might not be able to state definitively: This machine is clean. So that leaves me wondering if I can live symbiotically with an infected machine. I try not to let paranoia set in. I cover the lens of the Webcam with cardboard. For banking I use a pristine virtual machine, snapshot at the moment of first boot and reverting to a newborn state each time. Even if I still own my machine, I no longer enjoy the naïve certainty of ownership.