Calling YAML::load on attacker-controlled content in a Ruby app of any complexity is very bad news. As Ben and 'judofyr said: this is remote code execution.
Is this because Yaml doesn't whitelist the classes for the objects that may be instantiated? They are allocated and then instance_variable_set'd so I'd be Very interested to learn how this poses a risk.
If I implied that I doubted them, then I failed to communicate my point effectively -- I am very curious about how to turn a class allocate + instance_variable_set into remote code exec. I see how you can create the arel objects for sqli, but not arbitrary ruby.
If you wait a couple of weeks you are much more likely to get an answer. Since all rails apps were vulnerable, most people who know how to execute arbitrary code are keeping silent for now.
In much the same way that letting attackers control the parameters to fork() would be a bad idea for a C program or letting attackers control the parameters to Runtime.exec() would be a bad idea for a Java program.
This is a Rails vulnerability, not a Ruby vulnerability.
