Although the Internet was originally designed as a system to withstand attacks by hostile agents, it developed into a cooperative environment of relatively trusted entities. Alas, those days are long gone. Spam mail, denial of service (DoS) attacks, phishing attempts, and so on are indicative that anyone using the Internet does so at their own risk.
Applications have to be built to work correctly in hostile situations. “Correctly” no longer means just getting the functional aspects of the program correct, but also means ensuring privacy and integrity of data transferred, access only to legitimate users, ...
