csrf

README

goji/csrf

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

• The csrf.Protect middleware/handler that can be used with goji.Use to provide 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.

This library is designed to work with the Goji micro-framework, which is a simple web framework for Go that is broadly compatible with other parts of the Go ecosystem. It makes use of Goji's web.C request context, which doesn't rely on a global map, and is therefore safe to attach to your top-level router (if you so wish).

The library also assumes HTTPS by default: sending cookies over vanilla HTTP is risky and you're likely to get hurt.

Examples

goji/csrf is easy to use: add the middleware to your stack with the below:

goji.Use(csrf.Protect([]byte("32-byte-long-auth-key")))

... and then collect the token with csrf.Token(c, r) before passing it to the template, JSON body or HTTP header (you pick!). goji/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 (
    "html/template"
    "net/http"

    "github.com/goji/csrf"
    "github.com/zenazn/goji"
)

func main() {
    // Add the middleware to your router.
    goji.Use(csrf.Protect([]byte("32-byte-long-auth-key")))
    goji.Get("/signup", ShowSignupForm)
    // POST requests without a valid token will return a HTTP 403 Forbidden.
    goji.Post("/signup/post", SubmitSignupForm)

    goji.Serve()
}

func ShowSignupForm(c web.C, 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(c, r),
    })
    // We could also retrieve the token directly from csrf.Token(c, 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(c web.C, w http.ResponseWriter, r *http.Request) {
    // We can trust that requests making it this far have satisfied
    // our CSRF protection requirements.
}

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 Goji's sub-routers, as we don't handle any POST/PUT/DELETE requests with our top-level router.

package main

import (
    "github.com/goji/csrf"
    "github.com/zenazn/goji/graceful"
    "github.com/zenazn/goji/web"
)

func main() {
    r := web.New()
    // Our top-level router doesn't need CSRF protection: it's simple.
    r.Get("/", ShowIndex)

    api := web.New()
    r.Handle("/api/*", s)
    // ... but our /api/* routes do, so we add it to the sub-router only.
    s.Use(csrf.Protect([]byte("32-byte-long-auth-key")))

    s.Get("/api/user/:id", GetUser)
    s.Post("/api/user", PostUser)

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

func GetUser(c web.C, 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(c, r))
    b, err := json.Marshal(user)
    if err != nil {
        http.Error(...)
        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, goji/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"),
            // Note that csrf.ErrorHandler takes a Goji web.Handler type, else 
            // your error handler can't retrieve the error reason from the context.
            // The signature `func UnauthHandler(c web.C, w http.ResponseWriter, r *http.Request)`
            // is a web.Handler, and the simplest to use if you'd like to serve
            // "pretty" error pages (who doesn't?).
            csrf.ErrorHandler(web.HandlerFunc(serverError(403))),
        )

    goji.Use(CSRF)
    goji.Get("/signup", GetSignupForm)
    goji.Post("/signup", PostSignupForm)

    goji.Serve()
}

Not too bad, right?

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

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 (goji/csrf) provides Cross Site Request Forgery protection middleware for the Goji microframework (https://goji.io).

Index

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

func Domain(domain string) func(*csrf) error

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 web.Handler) func(*csrf) error

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(c, r) function to retrieve the CSRF validation reason from Goji's request context.

func FailureReason

func FailureReason(c web.C, r *http.Request) error

FailureReason makes CSRF validation errors available in Goji's 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) error

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) error

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

func MaxAge

func MaxAge(age int) func(*csrf) error

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) error

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) error) func(*web.C, 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/goji/csrf"
    "github.com/zenazn/goji"
)

func main() {
    // Add the middleware to your router.
    goji.Use(csrf.Protect([]byte("32-byte-long-auth-key")))
    goji.Get("/signup", GetSignupForm)
    // POST requests without a valid token will return a HTTP 403 Forbidden.
    goji.Post("/signup/post", PostSignupForm)

    goji.Serve()
}

func GetSignupForm(c web.C, 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(c, r),
    })
    // We could also retrieve the token directly from csrf.Token(c, 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) error

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) error

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

func TemplateField

func TemplateField(c web.C, 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="goji.csrf.Token" value="<token>">

func Token

func Token(c web.C, 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).