csrf

README

gorilla/csrf

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

With a properly configured Go toolchain:

go get github.com/gorilla/csrf

Examples

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).

gorilla/csrf inspects the form body (first) and HTTP headers (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.
    r.HandleFunc("/signup/post", SubmitSignupForm)

    // Add the middleware to your router by wrapping it.
    http.ListenAndServe(":8000",
        csrf.Protect([]byte("32-byte-long-auth-key"))(r))
}

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 your 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.

JSON Responses

This approach is useful if you're using a front-end JavaScript framework like Ember or Angular, or are providing a JSON API.

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()

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

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

Setting Options

What about providing your own error handler and changing the HTTP header the package inspects on requests? (i.e. an existing API you're porting to Go). Well, gorilla/csrf provides options for changing these as you see fit:

func main() {
    CSRF := csrf.Protect(
            []byte("a-32-byte-long-key-goes-here"),
            csrf.RequestHeader("Authenticity-Token"),
            csrf.FieldName("authenticity_token"),
            csrf.ErrorHandler(http.HandlerFunc(serverError(403))),
    )

    r := mux.NewRouter()
    r.HandleFunc("/signup", GetSignupForm)
    r.HandleFunc("/signup/post", PostSignupForm)

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

Not too bad, right?

If there's something you're confused about or a feature you would like to see added, open an issue.

Design Notes

Getting CSRF protection right is important, so here's some background:

• This library generates unique-per-request (masked) tokens as a mitigation against the BREACH attack.
• The 'base' (unmasked) token is stored in the session, which means that multiple browser tabs won't cause a user problems as their per-request token is compared with the base token.
• Operates on a "whitelist only" approach where safe (non-mutating) HTTP methods (GET, HEAD, OPTIONS, TRACE) are the only methods where token validation is not enforced.
• The design is based on the battle-tested Django and Ruby on Rails approaches.
• Cookies are authenticated and based on the securecookie library. They're also Secure (issued over HTTPS only) and are HttpOnly by default, because sane defaults are important.
• Go's crypto/rand library is used to generate the 32 byte (256 bit) tokens and the one-time-pad used for masking them.

This library does not seek to be adventurous.

License

BSD licensed. See the LICENSE file for details.

Documentation

Overview

Package csrf (gorilla/csrf) provides Cross Site Request Forgery (CSRF) prevention middleware for Go web applications & services.

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 easy to use: add the middleware to individual handlers with the below:

CSRF := csrf.Protect([]byte("32-byte-long-auth-key"))
http.HandlerFunc("/route", CSRF(YourHandler))

... and then collect the token with `csrf.Token(r)` before passing it to the template, JSON body or HTTP header (you pick!). gorilla/csrf inspects the form body (first) and HTTP headers (second) on subsequent POST/PUT/PATCH/DELETE/etc. requests for the token.

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 (
	"fmt"
	"html/template"
	"net/http"

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

var form = `
<html>
<head>
<title>Sign Up!</title>
</head>
<body>
<form method="POST" action="/signup/post" accept-charset="UTF-8">
<input type="text" name="name">
<input type="text" name="email">
<!--
    The default template tag used by the CSRF middleware .
    This will be replaced with a hidden <input> field containing the
    masked CSRF token.
-->
{{ .csrfField }}
<input type="submit" value="Sign up!">
</form>
</body>
</html>
`

var t = template.Must(template.New("signup_form.tmpl").Parse(form))

func main() {
	r := mux.NewRouter()
	r.HandleFunc("/signup", ShowSignupForm)
	// All POST requests without a valid token will return HTTP 403 Forbidden.
	r.HandleFunc("/signup/post", SubmitSignupForm)

	// Add the middleware to your router by wrapping it.
	http.ListenAndServe(":8000",
		csrf.Protect([]byte("32-byte-long-auth-key"))(r))
}

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),
	})
}

func SubmitSignupForm(w http.ResponseWriter, r *http.Request) {
	// We can trust that requests making it this far have satisfied
	// our CSRF protection requirements.
	fmt.Fprintf(w, "%v\n", r.PostForm)
}

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.

You can also send the CSRF token in the response header. This approach is useful if you're using a front-end JavaScript framework like Ember or Angular, or are providing a JSON API:

package main

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

func main() {
    r := mux.NewRouter()

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

    http.ListenAndServe(":8000",
        csrf.Protect([]byte("32-byte-long-auth-key"))(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 addition: getting CSRF protection right is important, so here's some background:

• This library generates unique-per-request (masked) tokens as a mitigation against the [BREACH attack](http://breachattack.com/).
• The 'base' (unmasked) token is stored in the session, which means that multiple browser tabs won't cause a user problems as their per-request token is compared with the base token.
• Operates on a "whitelist only" approach where safe (non-mutating) HTTP methods (GET, HEAD, OPTIONS, TRACE) are the *only* methods where token validation is not enforced.
• The design is based on the battle-tested [Django](https://docs.djangoproject.com/en/1.8/ref/csrf/) and [Ruby on Rails](http://api.rubyonrails.org/classes/ActionController/RequestForgeryProtection.html) approaches.
• Cookies are authenticated and based on the securecookie(https://github.com/gorilla/securecookie) library. They're also Secure (issued over HTTPS only) and are HttpOnly by default, because sane defaults are important.
• Go's `crypto/rand` library is used to generate the 32 byte (256 bit) tokens and the one-time-pad used for masking them.

This library does not seek to be adventurous.

Index

• Variables
• func CookieName(name string) func(*csrf)
• func Domain(domain string) func(*csrf)
• func ErrorHandler(h http.Handler) func(*csrf)
• func FailureReason(r *http.Request) error
• func FieldName(name string) func(*csrf)
• func HttpOnly(h bool) func(*csrf)
• func MaxAge(age int) func(*csrf)
• func Path(p string) func(*csrf)
• func Protect(authKey []byte, opts ...func(*csrf)) func(http.Handler) http.Handler
• func RequestHeader(header string) func(*csrf)
• func Secure(s bool) func(*csrf)
• func TemplateField(r *http.Request) template.HTML
• func Token(r *http.Request) string

Variables

var (
	// ErrNoReferer is returned when a HTTPS request provides an empty Referer
	// header.
	ErrNoReferer = errors.New("referer not supplied")
	// ErrBadReferer is returned when the scheme & host in the URL do not match
	// the supplied Referer header.
	ErrBadReferer = errors.New("referer invalid")
	// ErrNoToken is returned if no CSRF token is supplied in the request.
	ErrNoToken = errors.New("CSRF token not found in request")
	// ErrBadToken is returned if the CSRF token in the request does not match
	// the token in the session, or is otherwise malformed.
	ErrBadToken = errors.New("CSRF token invalid")
)

var TemplateTag = "csrfField"

TemplateTag provides a default template tag - e.g. {{ .csrfField }} - for use with the TemplateField function.

Functions

func CookieName

func CookieName(name string) func(*csrf)

CookieName changes the name of the CSRF cookie issued to clients.

Note that cookie names should not contain whitespace, commas, semicolons, backslashes or control characters as per RFC6265.

func Domain

func Domain(domain string) func(*csrf)

Domain sets the cookie domain. Defaults to the current domain of the request only (recommended).

This should be a hostname and not a URL. If set, the domain is treated as being prefixed with a '.' - e.g. "example.com" becomes ".example.com" and matches "www.example.com" and "secure.example.com".

func ErrorHandler

func ErrorHandler(h http.Handler) func(*csrf)

ErrorHandler allows you to change the handler called when CSRF request processing encounters an invalid token or request. A typical use would be to provide a handler that returns a static HTML file with a HTTP 403 status. By default a HTTP 404 status and a plain text CSRF failure reason are served.

Note that a custom error handler can also access the csrf.Failure(r) function to retrieve the CSRF validation reason from the request context.

func FailureReason

func FailureReason(r *http.Request) error

FailureReason makes CSRF validation errors available in the request context. This is useful when you want to log the cause of the error or report it to client.

func FieldName

func FieldName(name string) func(*csrf)

FieldName allows you to change the name value of the hidden <input> field generated by csrf.FormField. The default is {{ .csrfToken }}

func HttpOnly

func HttpOnly(h bool) func(*csrf)

HttpOnly sets the 'HttpOnly' flag on the cookie. Defaults to true (recommended).

func MaxAge

func MaxAge(age int) func(*csrf)

MaxAge sets the maximum age (in seconds) of a CSRF token's underlying cookie. Defaults to 12 hours.

func Path

func Path(p string) func(*csrf)

Path sets the cookie path. Defaults to the path the cookie was issued from (recommended).

This instructs clients to only respond with cookie for that path and its subpaths - i.e. a cookie issued from "/register" would be included in requests to "/register/step2" and "/register/submit".

func Protect

func Protect(authKey []byte, opts ...func(*csrf)) func(http.Handler) http.Handler

Protect is HTTP middleware that provides Cross-Site Request Forgery protection.

It securely generates a masked (unique-per-request) token that can be embedded in the HTTP response (e.g. form field or HTTP header). The original (unmasked) token is stored in the session, which is inaccessible by an attacker (provided you are using HTTPS). Subsequent requests are expected to include this token, which is compared against the session token. Requests that do not provide a matching token are served with a HTTP 403 'Forbidden' error response.

Example:

package main

import (
	"github.com/elithrar/protect"
	"github.com/gorilla/mux"
)

func main() {
  r := mux.NewRouter()

  mux.HandlerFunc("/signup", GetSignupForm)
  // POST requests without a valid token will return a HTTP 403 Forbidden.
  mux.HandlerFunc("/signup/post", PostSignupForm)

  // Add the middleware to your router.
  http.ListenAndServe(":8000",
		  csrf.Protect([]byte("32-byte-long-auth-key"))(r))
}

func GetSignupForm(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 your sending JSON to clients or a front-end JavaScript
	// framework.
}

func RequestHeader

func RequestHeader(header string) func(*csrf)

RequestHeader allows you to change the request header the CSRF middleware inspects. The default is X-CSRF-Token.

func Secure

func Secure(s bool) func(*csrf)

Secure sets the 'Secure' flag on the cookie. Defaults to true (recommended).

func TemplateField

func TemplateField(r *http.Request) template.HTML

TemplateField is a template helper for html/template that provides an <input> field populated with a CSRF token.

Example:

// The following tag in our form.tmpl template:
{{ .csrfField }}

// ... becomes:
<input type="hidden" name="gorilla.csrf.Token" value="<token>">

func Token

func Token(r *http.Request) string

Token returns a masked CSRF token ready for passing into HTML template or a JSON response body. An empty token will be returned if the middleware has not been applied (which will fail subsequent validation).