> It's because of the internal mechanism (a "Merkel Damgard Construction") intrinsic to hash functions like MD5, SHA1, SHA256 and friends. Even if MD5 were cryptographically secure, this vulnerability would still present itself if used in the way I described.

Thanks for pointing that out, didn't know that.

Just for fun, if they had written message+secret instead of secret+message it would have been ok (although bad practice)?

calculated_mac = OpenSSL::Digest::MD5.hexdigest(message+secret)