Disable CSRF on JSON API Calls - go

Question

I have a website project. It uses Go and the Gorilla and it's CSRF packages to protect against CSRF. I also have a JSON API that authenticates using a JWT like token provider (internal), so a user must authenticate with that before issuing a JSON request each time. So the CSRF is not an issue on the JSON side. At least I don't think so.
Here's my code, where I am using a NewRouter for web Paths, and a Subrouter for the /api/v1/[endpoint]s. If I call a JSON endpoint that does a POST, the CSRF is engaged and I get a Forbidden - CSRF token invalid. I was under the assume, that perhaps a Sub Router would not have the middleware for the CSRF check associated with.
router := mux.NewRouter().StrictSlash(false)
router.Path("/").HandlerFunc(myApp.IndexHandler).Methods("GET")
apiRouter := router.PathPrefix("/api").Subrouter()
apiRouter.Path("/dosomething").HandlerFunc(myApp.DoSomethingAPIHandler).Methods("POST", "OPTIONS")
http.ListenAndServe(":8000",
csrf.Protect(
[]byte("my-long-key-here-redacted"),
csrf.Secure(false), // Set to false as we offload SSL elsewhere
)(router)))
Question:
How do I get my API to work with or without CSRF protection? Obviously, the web paths will need to be protected to protect form posts.

Answer 1

One option is to only use the CSRF protection on specific HTTP handlers, rather than protecting the entire router. Note that this will require you to perform a type conversion on your myApp.IndexHandler in order to satisfy the type signature for the function returned by csrf.Protect().
router := mux.NewRouter().StrictSlash(false)
// Instead of protecting your entire router, you can protect specific HTTP
// handlers.
router.Path("/").Handler(
csrf.Protect(
[]byte("my-long-key-here-redacted"),
csrf.Secure(false),
)(http.HandlerFunc(myApp.IndexHandler)),
).Methods("GET")
apiRouter := router.PathPrefix("/api").Subrouter()
apiRouter.Path("/dosomething").HandlerFunc(myApp.DoSomethingAPIHandler).Methods("POST", "OPTIONS")
http.ListenAndServe(
":8000",
router,
)
Alternatively, you can use the function returned from csrf.Protect() to create your own middleware, with logic to only add the CSRF protection on certain requests. You could use this approach to only add protection on endpoints with the prefix /api for example, as I've done in the code below.
protectionMiddleware := func(handler http.Handler) http.Handler {
protectionFn := csrf.Protect(
[]byte("my-long-key-here-redacted"),
csrf.Secure(false),
)
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// Use some kind of condition here to see if the router should use
// the CSRF protection. For the sake of this example, we'll check
// the path prefix.
if !strings.HasPrefix(r.URL.Path, "/api") {
protectionFn(handler).ServeHTTP(w, r)
return
}
handler.ServeHTTP(w, r)
})
}
router := mux.NewRouter().StrictSlash(false)
router.Path("/").HandlerFunc(myApp.IndexHandler).Methods("GET")
apiRouter := router.PathPrefix("/api").Subrouter()
apiRouter.Path("/dosomething").HandlerFunc(myApp.DoSomethingAPIHandler).Methods("POST", "OPTIONS")
http.ListenAndServe(
":8000",
protectionMiddleware(router),
)