Why should security be considered from the beginning of software development?
Because fixing a flaw gets dramatically more expensive the later you find it — "the earlier, the cheaper."
Several factors compound to make late security fixes extremely expensive:
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Cost escalation: A flaw caught in the requirements phase is cheap to fix; the same flaw found in production can cost orders of magnitude more (commonly cited figures reach ~100x or higher) because it may require redesign, re-coding, and re-testing.
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80/20 maintenance rule: ~80% of a system's lifetime cost is maintenance, and security patches land in that expensive 80%.
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Perimeter limitations: Firewalls filter access but can't fix software bugs — the flaw still has to be fixed in the code.
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Most vulnerabilities are introduced early: A missing security requirement or a weak design choice propagates into hundreds of lines of insecure code, so addressing it up front prevents the whole cascade.
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Zero Trust reality: Modern systems (cloud, remote work, BYOD) can't rely on a network boundary for protection, so each component must be secure on its own.
Principle: Security by design beats security by patching.
See: OWASP Proactive Controls — top 10 things developers should do from the start