Dependencies and supply chains

Supply chain security is really, really hot right now. It’s something which folks in the “real world” of manufactured things have worried about for years – you might be surprised (and worried) how much effort aircraft operators need to pay to “counterfeit parts”, and I wish I hadn’t just searched online the words “counterfeit pharmaceutical” – but the world of software had a rude wake-up call recently with the Solarwinds hack (or crack, if you prefer). This isn’t the place to go over that: you’ll be able to find many, many articles if you search on that. In fact, many companies have been worrying about this for a while, but the change is that it’s an issue which is now out in the open, giving more leverage to those who want more transparency around what software they consume in their products or services.

When we in computing (particularly software) think about supply chains, we generally talk about “dependencies”, and I thought it might be useful to write a short article explaining the main dependency types.

What is a dependency?

A dependency on a component is one which that an application or component needs to work, and they are generally considered to come in two types:

• build-time dependencies
• run-time dependencies.

Let’s talk about those two types in turn.

Build-time dependencies

these are components which are required in order to build (typically compile and package) your application or library. For example, if I’m writing a program in Rust, I have a dependency on the compiler if I want to create an application. I’m actually likely to have many more run-time dependencies, however. How those dependencies are made visible to me will depend on the programming language and the environment that I’m building in.

Some languages, for instance, may have filesystem support built in, but others will require you to “import” one or more libraries in order to read and write files. Importing a library basically tells your build-time environment to look somewhere (local disk, online repository, etc.) for a library, and then bring it into the application, allowing its capabilities to be used. In some cases, you will be taking pre-built libraries, and in others, your build environment may insist on building them itself. Languages like Rust have clever environments which can look for new versions of a library, download it and compile it without your having to worry about it yourself (though you can if you want!).

To get back to your file system example, even if the language does come with built-in filesystem support, you may decide to import a different library – maybe you need some fancy distributed, sharded file system, for instance – from a different supplier. Other capabilities may not be provided by the language, or may be higher-level capabilities: JSON serialisation or HTTPS support, for instance. Whether that library is available in open source may have a large impact on your decision as to whether or not to use it.

Build-time dependencies, then, require you to have the pieces you need – either pre-built or in source code form – at the time that you’re building your application or library.

Run-time dependencies

Run-time dependencies, as the name implies, only come into play when you actually want to run your application. We can think of there being two types of run-time dependency:

1. service dependency – this may not be the official term, but think of an application which needs to write some data to a window on a monitor screen: in most cases, there’s already a display manager and a window manager running on the machine, so all the application needs to do is contact it, and communicate the right data over an API. Sometimes, the underlying operating system may need to start these managers first, but it’s not the application itself which is having to do that. These are local services, but remote services – accessing a database, for instance – operate in the same sort of way. As long as the application can contact and communicate with the database, it doesn’t need to do much itself. There’s still a dependency, and things can definitely go wrong, but it’s a weak coupling to an external application or service.
2. dynamic linking – this is where an application needs access to a library at run-time, but rather than having added it at build-time (“static linking”), it relies on the underlying operating system to provide a link to the library when it starts executing. This means that the application doesn’t need to be as large (it’s not “carrying” the functionality with it when it’s installed), but it does require that the version that the operating system provides is compatible with what’s expected, and does the right thing.

Conclusion

I’ve resisted the temptation to go into the impacts of these different types of dependency in terms of their impact on security. That’s a much longer article – or set of articles – but it’s worth considering where in the supply chain we consider these dependencies to live, and who controls them. Do I expect application developers to check every single language dependency, or just imported libraries? To what extent should application developers design in protections from malicious (or just poorly-written) dynamically-linked libraries? Where does the responsibility lie for service dependencies – particularly remote ones?

These are all complex questions: the supply chain is not a simple topic (partly because there is not just one supply chain, but many of them), and organisations need to think hard about how they work.