Bringing Capslock analysis to

Jess McClintock and John Dethridge, Google Open Source Security Team

Security usability is hard — security best practices often add further toil on developers, to the point where usability and security are often considered as direct tradeoffs. But they don’t have to be. There are many fantastic analysis tools and platforms for developers who want to audit their transitive dependencies or vendor a third party package. We aim to make this process less involved by running analyses centrally and making results directly available to open source consumers without any additional steps for maintainers.

We are excited to have added Capslock results for Go packages. Capslock is a Google open source tool that identifies “capabilities” that packages have — for example, the ability to read files, or to send and receive data on the network. This will provide added visibility into the behaviors of Go packages, to assist in choosing appropriately scoped dependencies, understanding what packages are doing under the hood and noticing when dependency updates require more powerful capabilities. Our aim here, as with as a whole, is to help those choosing and using open source packages to make well informed selections based on as much security-relevant data as possible.

A difference that makes a difference?

Our analysis found that less than 2% of version updates for packages will introduce a new capability requirement. This makes sense, since the set of capabilities required by a package will usually be established by its initial behavior, and if a dependency adds new capabilities, that can be a signal for maintainers that the change is more interesting (or, perhaps, the change warrants further investigation).

We divide these results according to whether the standard library was called directly from the analyzed package or via a transitive dependency. Some capabilities, such as those involving the network, are more likely to be used directly than via a transitive dependency. Interestingly, a whopping 9% of Go packages have a transitive dependency using os/exec!

Capability Analysis for Go

Capslock was launched last year as a CLI to analyze the callpaths of Go packages and report on the privileged capabilities that are accessible. If a package unexpectedly adds a new capability in an update, or uses capabilities that aren’t required for its operation, this can be a risk indicator about the codebase. Capabilities can also be a useful signal for verifying that a package does what is expected — a well designed package should itself require minimal privileges, and allow objects and interfaces like those in io/fs to be passed in that encapsulate privileged capabilities.

Capslock results on include the list of capabilities that have been identified in each particular version of a package. Note that the analysis results are build specific, so functions that are only included when a package is built for a specific operating system might not appear in our reports. You can find more details about this analysis in the Capslock documentation.

Today we are launching Capslock capability results for Go packages on This will be our first step in making capability data more widely available to open source consumers.