Building security into the SDLC means placing a specific, testable control in every phase, from requirements through operations, rather than saving all of it for a pre-release scan. Security in SDLC phases works when each stage has an owner and a check that fails loudly, because a flaw caught in design costs a fraction of the same flaw caught in production.
Teams say they "do security" and mean a scanner runs before release. That is one control in one phase. Below is what a control looks like at each stage, and why not implementing security in SDLC phases early is more expensive than it appears.
Requirements: threat model before you build
Security starts before a line of code. In the requirements phase, the control is a lightweight threat model: what data does this feature touch, who is the attacker, what are the trust boundaries, and what would abuse look like? Writing down "this endpoint accepts file uploads from unauthenticated users" during requirements forces the auth and validation questions early, when changing the answer is free.
Skip this and you inherit design decisions that no amount of later scanning can fix. A missing authorization boundary is an architecture problem, not a bug a tool flags.
Design: pick secure defaults and abuse cases
In design, the control is choosing secure architecture and writing abuse cases alongside use cases. Decide how secrets are stored, how sessions work, how services authenticate to each other, and where input crosses trust boundaries. Document the "unhappy paths": what happens when the attacker sends the input you did not expect.
This is also where you decide dependencies and frameworks. Choosing a framework with safe defaults (parameterized queries, output encoding, CSRF protection) prevents whole vulnerability classes from ever being possible in your code.
Development: secure coding and dependency hygiene
During development, controls run continuously. Two matter most:
- Secure coding practices: parameterized queries against injection, output encoding against XSS, least-privilege everywhere, and never rolling your own crypto.
- Dependency management: most modern code is mostly other people's code. Pin versions in a lockfile and scan them, because a vulnerable transitive dependency is your vulnerability the moment you ship it.
A pre-commit secret scanner belongs here too, so a leaked API key never reaches history in the first place. Catching a hardcoded credential at commit time is trivial; rotating one that reached a public repo is an incident.
Testing: SAST, SCA, and DAST as gates
The testing phase is where automated security testing becomes a build gate rather than an afterthought:
- SAST reads your source for insecure patterns before the app runs.
- SCA inventories your dependencies and flags known-vulnerable versions. An SCA tool does this against your full transitive tree, not just direct dependencies.
- DAST exercises the running application the way an attacker would, catching issues that only appear at runtime. A DAST scanner is the complement to static analysis, not a replacement for it.
The point is to make these gates block a merge or a release, not just produce a report someone reads later.
Deployment: harden the configuration
A perfectly coded app deployed with a wide-open security group is still compromised. In deployment, controls cover infrastructure-as-code scanning, hardened container images, least-privilege IAM, and secrets injected at runtime rather than baked into images. This phase is where "shift left" meets reality: the misconfiguration, not the code bug, causes a large share of real breaches.
Operations: monitor, patch, and respond
Security does not end at release. In operations, the controls are continuous: log and monitor for anomalous behavior, patch dependencies as new advisories land, and have an incident-response plan you have actually rehearsed. A dependency that was clean at release becomes vulnerable the day a new CVE is published against it, so the scan that ran once in CI has to keep running against what is live.
The cost of skipping phases
Not implementing security in SDLC phases early does not make the work disappear; it defers and multiplies it. An authorization flaw found in design is a whiteboard edit. The same flaw found in a penetration test before release is a sprint of rework. Found in production after a breach, it is customer notifications, regulatory exposure, and reputational damage. The industry rule of thumb (that defects get roughly an order of magnitude more expensive to fix at each later phase) is directional, but the direction is not in dispute. Teams that formalize these gates can walk through them in a structured way with resources like our security academy.
FAQ
What are the main phases of the SDLC for security?
Requirements, design, development, testing, deployment, and operations. Each needs its own control: threat modeling in requirements, secure architecture in design, secure coding and dependency scanning in development, automated testing gates in testing, hardened configuration in deployment, and monitoring plus patching in operations.
What happens if you do not implement security in SDLC phases?
The work does not vanish; it moves later and gets more expensive. A flaw caught in design is cheap to fix, the same flaw in production can mean rework, breach response, and regulatory cost. Skipping early phases also lets architecture-level problems set in that scanning cannot fix.
Is a security scan before release enough?
No. A single pre-release scan is one control in one phase. It misses design flaws, misconfigurations, and issues that only appear at runtime, and it does nothing after release when new CVEs land. Security has to be distributed across every phase.
What is the difference between SAST, SCA, and DAST?
SAST analyzes your source code statically for insecure patterns. SCA inventories your dependencies and flags known-vulnerable versions. DAST tests the running application from the outside like an attacker. They catch different classes of issue and are complementary, not interchangeable.