package csrf

import "github.com/gorilla/csrf"

Installation | Overview | API | Files

Installation

$ go get github.com/gorilla/csrf

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.

Note that the authentication key passed to `csrf.Protect([]byte(key))` should be 32-bytes long and persist across application restarts. Generating a random key won't allow you to authenticate existing cookies and will break your CSRF validation.

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))
        // 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),
        })
    }

    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.

API

Package Files

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.

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 Protect

func Protect(authKey []byte, opts ...Option) 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 (
    "html/template"

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

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

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

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

    // Add the middleware to your router.
    http.ListenAndServe(":8000",
    // Note that the authentication key provided should be 32 bytes
    // long and persist across application restarts.
          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 you're sending JSON to clients or a front-end JavaScript
    // framework.
}

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).

func UnsafeSkipCheck

func UnsafeSkipCheck(r *http.Request) *http.Request

UnsafeSkipCheck will skip the CSRF check for any requests. This must be called before the CSRF middleware.

Note: You should not set this without otherwise securing the request from CSRF attacks. The primary use-case for this function is to turn off CSRF checks for non-browser clients using authorization tokens against your API.

type Option

type Option func(*csrf)

Option describes a functional option for configuring the CSRF handler.

func CookieName

func CookieName(name string) Option

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) Option

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) Option

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 403 status and a plain text CSRF failure reason are served.

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

func FieldName

func FieldName(name string) Option

FieldName allows you to change the name attribute of the hidden <input> field inspected by this package. The default is 'gorilla.csrf.Token'.

func HttpOnly

func HttpOnly(h bool) Option

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

func MaxAge

func MaxAge(age int) Option

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

func Path

func Path(p string) Option

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 RequestHeader

func RequestHeader(header string) Option

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

func Secure

func Secure(s bool) Option

Secure sets the 'Secure' flag on the cookie. Defaults to true (recommended). Set this to 'false' in your development environment otherwise the cookie won't be sent over an insecure channel. Setting this via the presence of a 'DEV' environmental variable is a good way of making sure this won't make it to a production environment.