You need to consider and deal with security threats and risks at many different points in your design.
Adam Shostack’s new book on Threat Modeling explores how to do this in detail, with lots of exercises and examples on how to look for and plug security holes in software design, and how to think about design risks.
But some important basic ideas in secure design will take you far:
Know your Tools
When deciding on the language(s) and technology stack for the system, make sure that you understand the security constraints and risks that your choices will dictate. If you’re using a new language, take time to learn about how to write code properly and safely in that language. If you’re programming in Java, C or C++ or Perl check out CERT’s secure coding guidelines for those languages. If you're writing code on iOS, read Apple's Secure Coding Guide. For .NET, review OWASP's .NET Security project.
And make sure that you (or ops) understand how to lock down or harden the O/S and to safely configure your container and database (or NoSQL data) manager.
Tiering and Trust
Tiering or layering, and trust in design are closely tied together. You must understand and verify trust assumptions at the boundaries of each layer in the architecture and between systems and between components in design, in order to decide what security controls need to be enforced at these boundaries: authentication, access control, data validation and encoding, encryption, logging.
Understand when data or control crosses a trust boundary: to/from code that is outside of your direct control. This could be an outside system, or a browser or mobile client or other type of client, or another layer of the architecture or another component or service.
Thinking about trust is much simpler and more concrete than thinking about threats. And easier to test and verify. Just ask some simple questions:
Where is the data coming from? How can you be sure? Can you trust this data – has it been validated and safely encoded? Can you trust the code on the other side to protect the integrity and confidentiality of data that you pass to it? Do you know what happens if an exception or error occurs – could you lose data or data integrity, or leak data, does the code fail open or fail closed?
Before you make changes to the design, make sure that you understand these assumptions and make sure that the assumptions are correct.
The Application Attack Surface
Finally, it’s important to understand and manage the system’s Attack Surface: all of the ways that attackers can get in, or get data out, of the system, all of the resources that are exposed to attackers. APIs, files, sockets, forms, fields, URLs, parameters, cookies. And the security plumbing that protects these parts of the system.
Your goal should be to try to keep the Attack Surface as small as possible. But this is much easier said than done: each new feature and new integration point expands the Attack Surface. Try to understand the risks that you are introducing, and how serious they are. Are you creating a brand new network-facing API or designing a new business workflow that deals with money or confidential data, or changing your access control model, or swapping out an important part of your platform architecture? Or are you just adding yet another CRUD admin form, or just one more field to an existing form or file. In each case you are changing the Attack Surface, but the risks will be much different, and so will the way that you need to manage these risks.
For small, well-understood changes the risks are usually negligible – just keep coding. If the risks are high enough you’ll need to do some abuse case analysis or threat modeling, or make time for a code review or pen testing.
And of course, once a feature or option or interface is no longer needed, remove it and delete the code. This will reduce the system’s Attack Surface, as well as simplifying your maintenance and testing work.
That’s it. We’re done.
The 10 things you can do as a developer to make your app secure: from thinking about security in architectural layering and technology choices, including security in requirements, taking advantage of other people’s code by using frameworks and libraries carefully, making sure that you implement basic security controls and features like Authentication and Access Control properly, protecting data privacy, logging with security in mind, and dealing with input data and stopping injection attacks, especially SQL injection.This isn’t an exhaustive list. But understanding and dealing with these important issues in application security – including security when you think about requirements and design and coding and testing, knowing more about your tools and using them properly – is work that all developers can do, and will take you a long, long way towards making your system secure and reliable.