i can't actually explain why it works but it does work. I think it is result of both of the buggy regular expressions.

i am checking agains rails 4.

[29] pry(main)> x.update_attributes("client_\nsecret"=>1) (0.1ms) begin transaction (0.1ms) rollback transaction ActiveRecord::UnknownAttributeError: unknown attribute: client_ secret

But DEPRECATION WARNING: The method `sdf client_secret=', matching the attribute `client_secret' has dispatched through method_missing. This shouldn't happen, because `client_secret' is a column of the table. If this error has happened through normal usage of Active Record (rather than through your own code or external libraries), please report it as a bug. (called from block in assign_attributes at /Users/homakov/.rvm/gems/ruby-1.9.3-p194/bundler/gems/protected_attributes-369818eedeaa/lib/active_record/mass_assignment_security/attribute_assignment.rb:67)

So it's hidden in method_missing!

so there is a chain: "notprotected\nprotected" it's not found in include? so tryes to assign, then method_missing parses it and founds another attribute just below the first one.