Because skipping it turns security into a gamble — and against real attackers, an unmodelled product is one you're statistically sure to lose.

The SDL (Security Development Lifecycle) bakes security into every stage of building software instead of bolting it on at the end. The guiding maxim: "the earlier the better, but never too late, and never ignore it" — catching a flaw in design costs a fraction of catching it after release.

It grew out of Microsoft's 2002 "Trustworthy Computing" initiative: after a run of high-profile worms and vulnerabilities, Bill Gates sent a company-wide memo making security the top priority over new features across all Microsoft products. The SDL is the concrete process that came out of that mandate.