Cross-Site Request Forgery

By OWASP's definition "Cross-Site Request Forgery (CSRF) is an attack that forces an end user to execute unwanted actions on a web application in which they're currently authenticated.". (source)

CSRF attacks do not target data theft but state-changing requests. With a little of social engineering (such as sharing a link via email or chat) the attacker may trick users to execute unwanted web application actions such as changing account's recovery email.

Attack scenario

Let's say that foo.com uses HTTP GET requests to set the account's recovery email

GET https://foo.com/account/recover?email=me@somehost.com

A simple attack scenario may look like

  1. Victim is authenticated at https://foo.com
  2. Attacker sends a chat message to the Victim with a link
    https://foo.com/account/recover?email=me@attacker.com
    
  3. Victim's account recovery email address is changed to me@attacker.com given Attacker full control over it.

The Problem

Changing the HTTP verb from GET to POST (or any other) won't solve the issue as secret cookies, URL rewriting or HTTPS won't do it either.

The attack is possible because server does not distinguish between requests made during a legit user session workflow (navigation) and "malicious" ones.

The Solution

in theory

As said before CSRF targets state-changing requests, what for Web Applications most of the time means POST requests issued by form submission.

In this scenario, when first requesting the page which renders the form, the server computes a nonce (an arbitrary number intended to be used once). This token is then included into the form as a field (most of the time this field is hidden but it is not mandatory).

Then, when the form is submitted the hidden field is sent along with other user input and. The server should then validated whether the token is part the request data and it is valid.

Such nonce/token should obey to the following requirements:

  • Unique per user session
  • Large random value
  • Generated by a cryptographically secure random number generator

Note: Although HTTP GET requests are not expected to change state (said to be idempotent), due to bad programming practices they can in fact modify resources and because of that they should be also targeted by CSRF attacks.

When dealing with APIs, PUT and DELETE are other two common targets of CSRF attacks.

in practice

Doing all this by hand is not a good idea as it is error prone.

Most Web Application Frameworks already offer it out-of-the-box and you're advised to enable it or, if you're not using a Framework to adopt one.

The following example is part of the Gorilla web toolkit for go programming language. You can find gorilla/csrf on GitHub

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))
    // 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.
}

OWASP has a detailed Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet which you're recommended to read.

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