It can write a copy of itself into the working memory of another computer by once again exploiting a side-channel (wi-fi receiver DMA on a smartphone laying nearby, say.) Once it has Internet access, it can just yank some credit-card numbers and start ordering parts and subcontracting work to arbitrary addresses...
...but this is all assuming high-level side-channels. There are things you can do using the acoustics available in spinning hard disks that can produce any sort of resonance you like. I would imagine that, much in the same way a modern-day scientist could go go back to 300AD and still be able to construct a crystal radio just out of some spare bits of metal laying about (it's not a hard problem, just a theory you need to know to even think of approaching the problem that way), a sufficiently-advanced intelligence might be able to just do nanoscale assembly of whatever it likes using coalesced heat radiating off your RAM chips.
