A take on supply chain security in Go

List your dependencies capabilities and monitor if dependency updates require more capabilities.

The Problem

Recently different attacks and other issues related to open-source dependencies highlighted a quite severe problem with dependencies:

Every imported package gives that package's author basically remote code execution for your software.

The Idea

A video on WASI by linclark brought me to the idea how cool it would be if we could pass permissions to our dependencies.

In Go this could look something like this:

module github.com/cugu/gocap

go 1.20

require (
	github.com/go-chi/chi       v5.0.7   (network:read)
	github.com/mattn/go-sqlite3 v1.14.10 (file:read, file:write)
	github.com/sirupsen/logrus  v1.8.1   (os:stdout)
	github.com/yuin/goldmark    v1.4.4   ()
)

chi would be able to receive network requests, go-sqlite3 would be able to read and write files and logrus could write to stdout. But also all those modules would be limited to those capabilities and, for example, the logging library logrus would not be able to interact with files, the network or execute code.

Changes of dependencies would be much less critical in many cases, as a potential attacker would have only limited attack surface besides stealing your CPU cycles.

A simpler but working approach: GoCap

Implementing the approach above would require changes to Go itself. So I came up with another, simpler approach: GoCap. GoCap can check and validate the source code of dependencies for their capabilities and is ment to be included into the testing phase of the build process. This way GoCap can at least pin the capabilities of dependencies.

GoCap provides simple capability checking for Go using a go.cap file. The go.cap files lists all package dependencies that require critical permissions like file access, execution rights or network access. Unlike the idea above GoCap works on packages not modules and capabilities are based on the imported packages of the standard library.

The go.cap file for GoCap itself looks like this:

github.com/cugu/gocap (execute, file)

github.com/alecthomas/kong (file, syscall)
github.com/pkg/errors (runtime)

Install GoCap

You can download a release or run

go install github.com/cugu/[email protected]

gocap generate

gocap generate <path> prints a valid go.cap file. It lists all dependency packages that require critical permissions like file access, execution rights or network access.

! gocap generate runs on package basis and the path argument must point to a Go package (there must be .go files) not a Go Module. Also gocap generate needs the dependencies downloaded before, e.g. run go mod download.

Example

cd ~/myProjects/gocap
go mod download
gocap generate . > go.cap

go.cap

github.com/cugu/gocap (execute, file)

github.com/alecthomas/kong (file, syscall)
github.com/pkg/errors (runtime)

gocap check

gocap check <path> compares a local go.cap file with the actual required capabilities by dependency packages. Any mismatch results in a non-zero exit code, so you can use GoCap check in your CI pipelines. See ci.yml for a working example.

Example

gocap check .

Output

github.com/alecthomas/kong
	capability 'syscall' not provided by go.cap file, add to go.cap file if you want to grant the capability
github.com/pkg/errors
	unnecessary capability 'network', please remove from go.cap file

Capabilities

Name	Description	Packages
file	Read and write access to files	os io/ioutil
network	Read and write to the network	net net/http
execute	Execute other binaries	os/exec
syscall	Perform any system call in context of the software	syscall, C
unsafe	Usage of the unsafe package in combination with a //go:linkname compiler directive can be used to load external C or assembler code	unsafe
reflect	reflect.NewAt in combination with reflect#Value.Call might be used call arbitrary functions	reflect