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There's an enormous difference between serializers that make any effort at all to be safe, and those like Ruby's YAML library, which make no effort. Python's YAML, for example, exposes a safe_load() method.

It's really criminally negligent that no such method exists in Ruby's YAML library.

Python's Pickle lib had something similar to safe_load(), that they removed because it gave a false sense of security.

If you are accepting pickled objects from a remote and using it ... you are an idiot.

Defensive programming in this case would be to not use a general purpose de-serializer for a format that is explicitly designed to allow serialising arbitrary objects....

Serializers aren't inherently unsafe. Look at Lisp's (read) method, for example, which does not evaluate anything.