Optimize for Auditability

by alex_gaynor

When we write code, we optimize for many different things. We optimize for writability: how easy it is to write the code in the first place? We optimize for maintainability: how easy it is to make ongoing changes? We optimize for readability: how easy it is to understand what the code does?

However, we rarely optimize for auditability: how easy it is to tell if the code has a security vulnerability? By ignoring this aspect of software design we increase the burden on people reviewing code for vulnerabilities, which reduces the overall security of our software.

Some might ask, why optimize for auditability? After all, isn't it the same as readability? And if we know how to make bugs obvious, why not just fix them instead? Both of these have the same answer: it's very easy to write code whose intended purpose is clear to any engineer, but which has vulnerabilities that only a security expert will recognize. Auditability means designing languages, APIs, and patterns such that places in the code which are deserving of more stringent review are clearly delineated. This allows an auditor to focus their time as much as possible.

I have explored auditability as a goal in two different security contexts: cryptography and memory unsafety. In both contexts, I've found that code which was written with auditability in mind has allowed me to perform audits significantly more quickly and to have much higher confidence that the audit found everything it should have.

Cryptography

When we built the pyca/cryptography Python library, auditability was a core design criteria for our API. We were responding to very negative experiences we had with other libraries, where low-level APIs had defaults which were often dangerous and always hindered review. An example of this is a symmetric block cipher API with a default mode, or providing a default initialization vector when using CBC mode. While the danger of insecure defaults (such as ECB mode, or an all-zero IV) is clear, even places with acceptable defaults stymied reviews because they made it more difficult to answer questions like "Which cryptographic algorithms does this project use?"

As a result, we decided that we'd have two styles of APIs: low-level ones, where users were required to explicitly specify every parameter and algorithm, and high-level ones which should have no choices at all, and which clearly documented how they were built. The goal was that auditors could easily do things like:

  • Find uses of high-level recipes and know they were implemented securely, and not requiring significant review.
  • Search for known insecure algorithms such as MD5, ECB, or PKCS1v15 encryption very quickly.
  • Assess whether an encrypt() function you were auditing was secure easily, by putting the cryptographic algorithms front and center.

Our API design strategy works. In auditing numerous applications and libraries using pyca/cryptography, I've found that I've been able to very easily identify uses of poor algorithms, as well as limit the scope of code I needed to consider when trying to answer higher level questions about protocols and algorithm compositions.

Memory unsafety

Frequent readers of me will know I'm a big fan of Rust, and more broadly of moving away from memory unsafe languages like C and C++ to memory safe languages, be they Swift, Rust, or Go. One of the most common reactions I get when I state my belief that Rust could produce an order of magnitude fewer vulnerabilities is that Rust has unsafe. And since unsafe allows memory corruption vulnerabilities, that breaks the security of the entire system, therefore Rust is really no better than C or C++. Leaving aside the somewhat tortured logic here, ensuring that unsafe does not provide an unending stream of vulnerabilities is an important task.

I've recently had the opportunity to audit a few medium-sized Rust code bases in the thousands of lines of code range. They made significant use of unsafe, primarily for interacting with C code and unsafe system APIs. In each codebase I found one memory corruption vulnerability, one of which was unexploitable and the other was probably exploitable. In my experience this is fewer vulnerabilities than I would have identified in similar codebases written in C/C++, but the far more interesting element was how easy it was to perform the audit.

For C/C++ codebases like this, I'd start my audit by identifying all the entrypoints where untrusted data is introduced into the system, for example socket reads, public API functions, or RPC handlers. Then for each of these, depending on size, I'd try to fuzz it with something like libFuzzer and manually review the code to look for vulnerabilities. For these Rust audits, I took a dramatically different approach. I was only interested in memory corruption vulnerabilities, so I simply grepped for "unsafe" and reviewed each instance for vulnerabilities. In some cases this required reviewing callers, callees, or other code within the module, but many sites could be resolved just by examining the unsafe block itself. As a result, I was able to perform these audits to a high level of confidence in just a few hours.

Requiring code which uses dangerous features that can cause memory corruption to be within an unsafe block is a powerful form of optimizing for auditability. With C/C++, code is guilty until proven innocent; any code could have memory unsafety vulnerabilities until you've given it a look. Rust provides a sharp contrast by making code innocent until proven guilty, unless it's within an unsafe block (or in the same module and responsible for maintaining invariants that an unsafe block depends on), Rust code can be trusted to be memory safe. This dramatically reduces the scope of code you need to audit.

Conclusion

In both of these domains, optimizing for auditability lived up to my hopes. Code which was optimized for auditability took less time to review, and when I completed the reviews I was more confident I'd found everything there was to Find. This dramatic improvement in the ability to identify security issues means less bugs to become critical incidents.

Optimizing for auditability pairs well with APIs which are designed to be easy to use securely. It should be a component of more teams' strategies for building software that’s secure.


Hi, I'm Alex. I'm a software engineer at Mozilla, working on Firefox security. Before that I was a software engineer with the U.S. Digital Service. I'm an avid open source contributor and live in Washington, DC.