I would think that anaerobic bacteria could survive in melting snow gullies or hydrothermal areas (if any exist). Kind of unlikely for them to get blown to these restricted areas I suppose. More dangerous is that the bacteria would sit around encysted for thousands of years until the climate goes through one of its cyclical changes that might make conditions more favorable in the northern polar regions. There's some cause for concern.

My money on the under-estimated risk of forward contamination is for the 1995 Galileo Jupiter probe, as well as for any future Jupiter atmospheric probes that aren't carefully decontaminated. I still haven't been able to find out what level of protection was done for that probe.