gorilla/csrf

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gorilla/csrf is a HTTP middleware library that provides cross-site request forgery (CSRF) protection. It includes:

The csrf.Protect middleware/handler provides CSRF protection on routes attached to a router or a sub-router.
A csrf.Token function that provides the token to pass into your response, whether that be a HTML form or a JSON response body.
... and a csrf.TemplateField helper that you can pass into your html/template templates to replace a {{ .csrfField }} template tag with a hidden input field.

gorilla/csrf is designed to work with any Go web framework, including:

The Gorilla toolkit
Go's built-in net/http package
Goji - see the tailored fork
Gin
Echo
... and any other router/framework that rallies around Go's http.Handler interface.

gorilla/csrf is also compatible with middleware 'helper' libraries like Alice and Negroni.

Install
Examples
Design Notes
License

Install

With a properly configured Go toolchain:

go get github.com/gorilla/csrf

gorilla/csrf is easy to use: add the middleware to your router with the below:

CSRF := csrf.Protect([]byte("32-byte-long-auth-key"))
http.ListenAndServe(":8000", CSRF(r))

...and then collect the token with csrf.Token(r) in your handlers before passing it to the template, JSON body or HTTP header (see below).

Note that the authentication key passed to csrf.Protect([]byte(key)) should:

be 32-bytes long
persist across application restarts.
kept secret from potential malicious users - do not hardcode it into the source code, especially not in open-source applications.

Generating a random key won't allow you to authenticate existing cookies and will break your CSRF validation.

gorilla/csrf inspects the HTTP headers (first) and form body (second) on subsequent POST/PUT/PATCH/DELETE/etc. requests for the token.

HTML Forms

Here's the common use-case: HTML forms you want to provide CSRF protection for, in order to protect malicious POST requests being made:

package main

import (
    "net/http"

    "github.com/gorilla/csrf"
    "github.com/gorilla/mux"
)

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/signup", ShowSignupForm)
    // All POST requests without a valid token will return HTTP 403 Forbidden.
    // We should also ensure that our mutating (non-idempotent) handler only
    // matches on POST requests. We can check that here, at the router level, or
    // within the handler itself via r.Method.
    r.HandleFunc("/signup/post", SubmitSignupForm).Methods("POST")

    // Add the middleware to your router by wrapping it.
    http.ListenAndServe(":8000",
        csrf.Protect([]byte("32-byte-long-auth-key"))(r))
    // PS: Don't forget to pass csrf.Secure(false) if you're developing locally
    // over plain HTTP (just don't leave it on in production).
}

func ShowSignupForm(w http.ResponseWriter, r *http.Request) {
    // signup_form.tmpl just needs a {{ .csrfField }} template tag for
    // csrf.TemplateField to inject the CSRF token into. Easy!
    t.ExecuteTemplate(w, "signup_form.tmpl", map[string]interface{}{
        csrf.TemplateTag: csrf.TemplateField(r),
    })
    // We could also retrieve the token directly from csrf.Token(r) and
    // set it in the request header - w.Header.Set("X-CSRF-Token", token)
    // This is useful if you're sending JSON to clients or a front-end JavaScript
    // framework.
}

func SubmitSignupForm(w http.ResponseWriter, r *http.Request) {
    // We can trust that requests making it this far have satisfied
    // our CSRF protection requirements.
}

Note that the CSRF middleware will (by necessity) consume the request body if the token is passed via POST form values. If you need to consume this in your handler, insert your own middleware earlier in the chain to capture the request body.

JavaScript Applications

This approach is useful if you're using a front-end JavaScript framework like React, Ember or Angular, and are providing a JSON API. Specifically, we need to provide a way for our front-end fetch/AJAX calls to pass the token on each fetch (AJAX/XMLHttpRequest) request. We achieve this by:

Parsing the token from the <input> field generated by the csrf.TemplateField(r) helper, or passing it back in a response header.
Sending this token back on every request
Ensuring our cookie is attached to the request so that the form/header value can be compared to the cookie value.

We'll also look at applying selective CSRF protection using gorilla/mux's sub-routers, as we don't handle any POST/PUT/DELETE requests with our top-level router.

package main

import (
    "github.com/gorilla/csrf"
    "github.com/gorilla/mux"
)

func main() {
    r := mux.NewRouter()
    csrfMiddleware := csrf.Protect([]byte("32-byte-long-auth-key"))

    api := r.PathPrefix("/api").Subrouter()
    api.Use(csrfMiddleware)
    api.HandleFunc("/user/{id}", GetUser).Methods("GET")

    http.ListenAndServe(":8000", r)
}

func GetUser(w http.ResponseWriter, r *http.Request) {
    // Authenticate the request, get the id from the route params,
    // and fetch the user from the DB, etc.

    // Get the token and pass it in the CSRF header. Our JSON-speaking client
    // or JavaScript framework can now read the header and return the token in
    // in its own "X-CSRF-Token" request header on the subsequent POST.
    w.Header().Set("X-CSRF-Token", csrf.Token(r))
    b, err := json.Marshal(user)
    if err != nil {
        http.Error(w, err.Error(), 500)
        return
    }

    w.Write(b)
}

In our JavaScript application, we should read the token from the response headers and pass it in a request header for all requests. Here's what that looks like when using Axios, a popular JavaScript HTTP client library:

// You can alternatively parse the response header for the X-CSRF-Token, and
// store that instead, if you followed the steps above to write the token to a
// response header.
let csrfToken = document.getElementsByName("gorilla.csrf.Token")[0].value

// via https://github.com/axios/axios#creating-an-instance
const instance = axios.create({
  baseURL: "https://example.com/api/",
  timeout: 1000,
  headers: { "X-CSRF-Token": csrfToken }
})

// Now, any HTTP request you make will include the csrfToken from the page,
// provided you update the csrfToken variable for each render.
try {
  let resp = await instance.post(endpoint, formData)
  // Do something with resp
} catch (err) {
  // Handle the exception
}

If you plan to host your JavaScript application on another domain, you can use the Trusted Origins feature to allow the host of your JavaScript application to make requests to your Go application. Observe the example below:

package main

import (
    "github.com/gorilla/csrf"
    "github.com/gorilla/mux"
)

func main() {
    r := mux.NewRouter()
    csrfMiddleware := csrf.Protect([]byte("32-byte-long-auth-key"), csrf.TrustedOrigins([]string{"ui.domain.com"}))

    api := r.PathPrefix("/api").Subrouter()
    api.Use(csrfMiddleware)
    api.HandleFunc("/user/{id}", GetUser).Methods("GET")

    http.ListenAndServe(":8000", r)
}

func GetUser(w http.ResponseWriter, r *http.Request) {
    // Authenticate the request, get the id from the route params,
    // and fetch the user from the DB, etc.

    // Get the token and pass it in the CSRF header. Our JSON-speaking client
    // or JavaScript framework can now read the header and return the token in
    // in its own "X-CSRF-Token" request header on the subsequent POST.
    w.Header().Set("X-CSRF-Token", csrf.Token(r))
    b, err := json.Marshal(user)
    if err != nil {
        http.Error(w, err.Error(), 500)
        return
    }

    w.Write(b)
}

On the example above, you're authorizing requests from ui.domain.com to make valid CSRF requests to your application, so you can have your API server on another domain without problems.

Google App Engine

If you're using Google App Engine, (first-generation) which doesn't allow you to hook into the default http.ServeMux directly, you can still use gorilla/csrf (and gorilla/mux):

package app

// Remember: appengine has its own package main
func init() {
    r := mux.NewRouter()
    r.HandleFunc("/", IndexHandler)
    // ...

    // We pass our CSRF-protected router to the DefaultServeMux
    http.Handle("/", csrf.Protect([]byte(your-key))(r))

Csrf

Install / Use

README