So someone I know got hit with some malware recently, and i'm trying to understand exactly what happened, as best I can. Heres the sequence.

non admin User downloaded a zip file, and scanned it with MSE. While MSE didnt alert (strangely) it did in fact detect a trojan in the zip file and it was supposedly removed.

After the scan the user preceded to unzip the supposedly now clean zip file. This triggered some kind of malware that killed MSE, User immediatly took remediatory action.

Now, it appears to me that one of several possibilities existed... here are my thoughts:

original trojan was easily detected, was this intentional, perhaps luring the target into a false sense of security?

traditionally trojans drop files, or provide access through which further comprimise can occur. There have been no indicators of further comprimise, what am I missing?

What caused MSE to be killed? I know that commands can be executed to kill AV, but they generally require Admin or SYSTEM privlidges, user was un-privlidged. I thought maybe some sort of client side exploit to gain privlidges. I'm just not sure.

Finally, the user immediatly pulled the network cable, scanned the computer in safe mode, and regular mode with up to date scanners (3) and nothing else was detected, but can we trust this? outside of MSE which was originally killed, the other software installed at the time did not stop or detect an attack. In addition the user ran a networksniffer, and did not detect any malicious traffic after the comprimise...

Thoughts?

I know it's not too hard to encode and obfuscate malware. Using Metasploit makes it quite easy: http://oldmanlab.blogspot.ca/2011/11/metasploit-shellcode-obfuscation-to.html

That being said, it doesn't answer everything. I am curious to see what the others think of that...

I'd doubt that any malware author would deliberately design something to be detected. A detection of anything would surely only increase user suspicion of what's being installed vs an installation with no AV alert?

Even with encoding/obfuscation techniques you're not guaranteed to bypass AV, overtime these become known to the AV vendors. Hence, why one may have been detected and not the other.

It's certainly possible this was an 0 day on MSE. If it is, maybe the vulnerability the attacker used requires MSE to pick up some kind of virus, to trigger his 0 day exploit. Since MSE runs as SYSTEM, if you pwned it, you' would be able to kill the service and ultimately control the box.

What's the name of the virus it successfully caught?

Is there any evidence that something malicious actually occurred when the file was extracted? Maybe MSE just crashed. Maybe your acquaintance has found the 0-day

User's will also bend the truth to avoid embarrassment. I'd get a copy and do analysis in a safe environment before jumping to grand conclusions. That'll get you a lot farther than just speculating.