open relay
termsecurityops culture
A mail server that will forward messages from anyone to anyone - the default in the early internet and a liability ever since.
It was not a defect originally: forwarding for strangers was cooperative behaviour on a network of colleagues. Spam turned it into free infrastructure for whoever found it, and the response invented the reputation list, which is the mechanism that later spread far beyond mail. It is worth remembering as the clearest case of a design that was correct for the network it was built on and became a vulnerability when the population changed.