Capslock: What is your code really capable of?

Jess McClintock and John Dethridge, Google Open Source Security Team, and Damien Miller, Enterprise Infrastructure Protection Team

When you import a third party library, do you review every line of code? Most software packages depend on external libraries, trusting that those packages aren’t doing anything unexpected. If that trust is violated, the consequences can be huge—regardless of whether the package is malicious, or well-intended but using overly broad permissions, such as with Log4j in 2021. Supply chain security is a growing issue, and we hope that greater transparency into package capabilities will help make secure coding easier for everyone.

Avoiding bad dependencies can be hard without appropriate information on what the dependency’s code actually does, and reviewing every line of that code is an immense task.  Every dependency also brings its own dependencies, compounding the need for review across an expanding web of transitive dependencies. But what if there was an easy way to know the capabilities–the privileged operations accessed by the code–of your dependencies? 

Capslock is a capability analysis CLI tool that informs users of privileged operations (like network access and arbitrary code execution) in a given package and its dependencies. Last month we published the alpha version of Capslock for the Go language, which can analyze and report on the capabilities that are used beneath the surface of open source software. 

This CLI tool will provide deeper insights into the behavior of dependencies by reporting code paths that access privileged operations in the standard libraries. In upcoming versions we will add support for open source maintainers to prescribe and sandbox the capabilities required for their packages, highlighting to users what capabilities are present and alerting them if they change.

Capabilities vs Vulnerabilities

Vulnerability management is an important part of your supply chain security, but it doesn’t give you a full picture of whether your dependencies are safe to use. Adding capability analysis into your security posture, gives you a better idea of the types of behavior you can expect from your dependencies, identifies potential weak points, and allows you to make a more informed choice about using a given dependency. 

Capslock is motivated by the belief that the principle of least privilege—the idea that access should be limited to the minimal set that is feasible and practical—should be a first-class design concept for secure and usable software. Applied to software development, this means that a package should be allowed access only to the capabilities that it requires as part of its core behaviors. For example, you wouldn’t expect a data analysis package to need access to the network or a logging library to include remote code execution capabilities. 

Capslock is initially rolling out for Go, a language with a strong security commitment and fantastic tooling for finding known vulnerabilities in package dependencies. When Capslock is used alongside Go’s vulnerability management tools, developers can use the additional, complementary signals to inform how they interpret vulnerabilities in their dependencies. 

These capability signals can be used to

• Find code with the highest levels of access to prioritize audits, code reviews and vulnerability patches

• Compare potential dependencies, or look for alternative packages when an existing dependency is no longer appropriate

• Surface unwanted capability usage in packages to uncover new vulnerabilities or identify supply chain attacks in progress

• Monitor for unexpected emerging capabilities due to package version or dependency changes, and even integrate capability monitoring into CI/CD pipelines 

• Filter vulnerability data to respond to the most relevant cases, such as finding packages with network access during a network-specific vulnerability alert  

Using Capslock

We are looking forward to adding new features in future releases, such as better support for declaring the expected capabilities of a package, and extending to other programming languages. We are working to apply Capslock at scale and make capability information for open source packages broadly available in various community tools like deps.dev. 

You can try Capslock now, and we hope you find it useful for auditing your external dependencies and making informed decisions on your code’s capabilities.

We’ll be at Gophercon in San Diego on Sept 27th, 2023—come and chat with us!