13 Principles for designing secure application

I am sucker for learning principles, mental models, explanatory theories of any field. Today I want to share with you, 20 essential principles for building secure software.

Famous British statistician named George Box wrote the famous line, “All models are wrong, but some are useful.”
So remember these principles aren’t rules written in stone. They are just guiding principles, useful heuristics - for making better design/implementation choices .

So here you go.

1. Zero Trust

Zero trust means to never trust anything by default, to always verify & re-verify. Every request is treated as untrusted until proven otherwise. The system continuously checks credentials and permissions, treating every request with healthy suspicion.

Even if a bad actor sneaks into one part of your app, they still face locked doors everywhere else. This keeps your sensitive data safer​.

It’s like having guards at every room in a house – a burglar might get in a window, but they can’t go much further without passing multiple security checkpoints.

For example, service A doesn’t invoke Service B’s API without presenting a valid token or key. Each service treats the other’s request as untrusted until verified (preventing a compromised service from abusing others).

2. Fail Securely

Failing securely means that when something goes wrong, the system should default to a secure state, not a wide-open state.
Think of a door that auto-locks if the power goes out. In software, if a security control crashes or an error happens, it should fail closed (deny access) rather than fail open.

Let’s face it – things will go wrong. Networks fail, code has bugs, servers go down.
But, failing securely ensures a glitch doesn’t become a breach.

For example, If a login or auth system fails open , attackers can walk right in during the confusion. But a fail-secure design would reject logins by default in that case, instead of letting users in without auth. It might cause inconvenience (legit users get blocked until it’s fixed), but that’s far better than accidentally letting the bad guys in​

3. Attack Surface Minimization

The attack surface is all the points where your system can be attacked – open ports, API endpoints, features, user inputs, you name it​. The idea is to , minimize this attack surface -as much as possible.

It’s like having all but one door to your house – fewer entry points means less ways for attackers to get in and it also make it easier to guard. In code, it might mean removing dead code, disabling unused services, or limiting the functionality exposed to users.

Every feature or open interface is a potential vulnerability. Unused functionality can become a hacker’s playground. Attackers often look for the path of least resistance. By trimming the excess, you’re cutting down opportunities for exploit.

For example, You built an e-commerce site that has a forum feature, but it’s not ready for launch – so you don’t deploy it to production. By not exposing the unfinished forum pages or APIs, you remove an entire category of potential bugs or SQL injection spots. Less code running means fewer potential vulnerabilities.

4. Least Privilege

Least Privilege means giving each user , process, service or system only the minimum permissions necessary to do its job – no more. If a module or person doesn’t need access to something, they simply don’t get it. It’s on a “need-to-know” (or need-to-use) basis.

For example, a report generation service can only read from a database table but not write to it, since it only needs read access.
Or, A microservice running in the cloud is given an IAM role with access to only one specific S3 bucket rather than all S3 buckets.

This principle dramatically limits the impact of mistakes or breaches.
By compartmentalizing privileges, you ensure that a breach of one component doesn’t automatically grant keys to the kingdom.

5. Defense In Depth

Defense in Depth is the art of having multiple layers of security controls – like an onion with many layers of protection​. The idea is that if one layer fails, the next one still stands in the attacker’s way.

It’s analogous to a castle with a moat, drawbridge, walls, archers on the walls, and guards inside. Even if the moat is crossed, the walls are there; if the wall is breached, the guards are next.

For example, A web app protects against SQL injection by using prepared statements (parameterized queries). That’s one layer. But it also has a web application firewall (WAF) in front, as a second layer, which can detect and block common injection payloads. On top of that, the database has tight permissions . So even if one layer is misconfigured, the others still mitigate the attack.

No single defense measure is foolproof. Vulnerabilities happen, human errors slip in, or an attacker finds a way to bypass one measure. With multiple layers, a single failure won’t doom the whole system

6. Separation of Code & Data

This principle insists on keeping code (instructions) and data (untrusted input) separated. In practice, that means never mixing user input directly with code execution. Treat data as data, not as part of your program.

Why?

Because, Many of the worst security bugs (SQL injection, cross-site scripting, command injection) come from the program treating malicious input as code.

For example, If an attacker’s input DROP TABLE Users is just data, it stays harmlessly as a string. But if your app naively appends it into a SQL query string

By strictly separating code and data, you avoid letting untrusted data dictate what your software does​ . It enforces a clean boundary so that no matter what input comes in, it’s handled as data, not as a script or command.

7. Trust Boundaries

A trust boundary is an imaginary line in your system where the level of trust changes. On one side of the boundary, data or users are considered untrusted, and on the other side, they might be trusted (to some extent). Crossing a trust boundary should always trigger security checks like validation, sanitization, or authentication.

It’s like the border control between countries: you show your passport and visa when you cross into a new country because the trust level changes at that border​

For example, Imagine a backend architecture where Service A calls Service B. They reside in different security zones as service B is more sensitive, then the API call crossing into B’s zone is a trust boundary. Service B should authenticate Service A’s requests (e.g., via API keys or tokens) and not assume every call is legit just because it’s from “inside the network.” Similarly, data returned from A to B might be validated.

If you handle boundaries right, an untrusted entity never gets to act as if it’s trusted without proving itself. This confines potential threats to the untrusted side unless they legitimately pass the security gateway​

8. Separation of Duties

Separation of Duties (SoD) is a principle usually applied to processes and people: it means splitting responsibilities among multiple individuals or systems so that no one entity has full control.

For example, consider an app that has an “admin” role that could do everything. That’s a violation of single responsibility in a sense – one role has too many powers.

But by splitting responsibilities, you create more granular roles: e.g., one role for user management, another for content moderation, another for financial oversight. Each admin now has a narrower scope. This limits the damage if one admin account is compromised

The goal here is to prevent fraud and mistakes. If one person has total control, they could abuse it or just make an error without anyone noticing. By dividing duties, you introduce checks and balances. In security terms, it reduces the risk that a malicious insider or compromised account can wreak havoc alone.

9. Sandboxing

Sandboxing is running code or programs in a restricted, isolated environment where they can’t harm the rest of the system
In computing, a sandbox might be a virtual machine, container, or an interpreted environment with strict limits. The sandbox closely monitors and controls what the code inside can do – e.g., no network access, limited memory, no access to your main files, etc.

For example, On Android and iOS, every app is sandboxed. Each app has access only to its own data and a limited set of phone features . If you install a rogue app, the mobile OS prevents it from directly reading data from other apps or the system without explicit permission.

It’s why, for example, your note-taking app can’t arbitrarily read your SMS or see your banking app’s data unless you allowed some integration – the sandbox walls isolate their data.

When you run third-party or untrusted code (or even your own code that parses untrusted data), there’s a risk it could be malicious or just buggy. Sandboxing ensures that even if the code misbehaves, the damage is contained to the sandbox and doesn’t spread to your actual system.

10. Single Responsibility

The Single Responsibility Principle (SRP) originally comes from software design (SOLID principles) and says that a module or class should have one and only one reason to change – basically one job.

In a security context, having a single responsibility often means each component is focused and simple, making it easier to secure and less likely to have unintended side effects.

For example, you have a dedicated authentication service whose sole job is to handle logins and JWT tokens. It doesn’t also send emails or process payments.

Because of this single responsibility, you can harden that auth service (focus on secure password storage, etc.) and know that it won’t unexpectedly do file operations or other unrelated tasks.
Or, If there’s a vulnerability in the payment processing code, it won’t affect the auth service and effect everything -because they’re separate.

11. Trusted Computing Base Minimization

The Trusted Computing Base (TCB) is the set of all components (hardware, software, etc.) that you must trust for the system’s security. TCB minimization means keep the core that you rely on for security as small, simple & auditable as possible

For example, If you design a secure messaging app, you might decide that only the encryption library and your core messaging logic are the TCB, and things like the user interface or emoji parser are not trusted for security. By doing that, even if the UI code has a bug, it might not compromise the encryption of messages. Also, you’d choose a well-vetted encryption library as part of your TCB, instead of writing your own – because the TCB must be rock solid.

Every extra component you trust implicitly is another thing that can betray you if it has a bug or backdoor. By minimizing the TCB, you reduce the attack surface for catastrophic failure. you have fewer things to perfectly get right.

12. Fail Fast

Fail Fast means – if something is wrong, crash or stop immediately rather than tolerating it or muddling through. In a security context, it implies that upon detecting an anomaly or invalid state, the system should rapidly error out (and ideally alert) instead of trying to continue in a flawed state.

For example, Suppose an API expects a number but got a string. A fail-fast approach is to immediately return an error (or throw an exception) when you detect this. Don’t try to coerce it or continue; stop the process. This not only prevents weird behavior down the line (which could be exploitable), but it also signals to developers and users that something is wrong with the input handling. Early failure = early fix.

Security issues often get worse the longer they go undetected. If your system “fails slow” (keeps going despite errors), you might be unknowingly running with a security hole or corrupted state for a while. Failing fast makes bugs and breaches visible early- this helps stop the escalation.

13. Application Partitioning

Application Partitioning is all about splitting your application into isolated parts so each part can be secured (and fail) separately. It’s like having watertight compartments in a ship – if one compartment floods, the whole ship doesn’t sink.

In practice, this might mean separating the user-facing components from critical backend services, or running certain modules on different servers.

By partitioning an app, you limit how far an attacker can get. If the front-end is compromised, the attacker still can’t directly access the database if it’s partitioned off on a different server with its own safeguards​. Such isolation limits the blast radius of attacks

For example, an application might have an admin panel that is deployed on a separate subdomain or port, with its own server and stricter firewall rules. Regular users can’t even reach the admin app partition. This way, a vulnerability in the user-facing site doesn’t expose the admin functionality.

These 20 principles won’t guarantee perfect security. But like good armor, they stack your defenses, reduce risks, and help you sleep better knowing your software can take a punch.