Does ruby's Marshal have the same problems that python's pickle have? Could you construct a valid Marshal bitsting that when loaded would run malicious code? Is this exposing folks to a MITM attack on rubygems?
Also, Marshal doesn't allow any kind of code to be included into the stream, so there is no ability for stream to perform remote code injection.
Marshal call back into Ruby for non-builtin types, but it does so by simply calling a method on the constant and passing either the raw Marshal data or a previous created object tree. This provides enough protection that there haven't been any reported cases of it being exploited and no know issues exist with it.
Having said that, I cannot at this moment tell you how to take over a Ruby runtime with a malicious Marshal byte string.
tl;dr: Why, oh why, Marshal, and not, say, JSON?
As for why not JSON, because there is no JSON parser as part of the standard library and rubygems needs to be extremely careful about what dependencies it has.
Basically, I'm not convinced Marshal is necessarily any more risky than something like YAML would be, even though it feels scarier. But I haven't done an extensive audit or anything — I just looked over the Marshal code a while back because I was curious what it was doing.
I'd much rather be using JSON but I was told Marshal or plaintext...I'll go with Marshal. :/