There was an RSCS mod flying around a while back which helps a little with
insuring that a file came from where it says it did. (I can't forward a copy of
the mod, as I never got my copy. If anyone else has it, I still want it.)
 
The jist of the mod is that it verifies that the sending site, in the RSCS
header, is possible. In other words, a file coming in on physical link X
has to have a header from one of the sites reachable on link X.