Vulnerability Development: Re: unpacking UPX or PE-packed binaries

[Blue Boar <BlueBoar () thievco com>]

Karma wrote:
> Just interested in how AV R&D companies unpack worms with complex UPX and PE
> pack protocols.
The modified UPX packing is a pretty small change usually.  Compare one 
with a standard UPX header for the same version.  There are a variety of 
unpackers out there.  Take a look at the X86emu work by Chris Eagle for an 
interesting direction that unpackers might be taking:
http://ida-x86emu.sourceforge.net/

Worst case, you've always got the option to carefully step through it with 
a debugger to the point where it's unpacked.  You may have to deal with a 
few anti-debugger tricks along the way, and once you've got a few unpacked 
segments, you'll probably have to manually put the import table back 
together, but it's doable.  I like Ollydbg for this kind of thing:
http://home.t-online.de/home/Ollydbg/

Just take care that you're running in a sandbox of some kind, in case you 
step too far, or accidentally press the "run" key.
                                                BB