This page describes a use case to explain CSRF attacks (16.1):
https://docs.spring.io/spring-security/site/docs/current/reference/html/csrf.html
But if the user is indeed logged in the bank's website, then isn't it possible for the evil website to make a GET request to get a fresh CSRF token, and craft a POST without needing the user at all?
The answer must be no, otherwise CSRF token would be useless, but I don't understand why?
The answer is "no" and the reason is Same-Origin Policy.
SOP means that a page from evil.com cannot read any response to requests that it may send to example.com. Most direct means to send a request will be blocked by the browser (SOP), but there are many workarounds. For example, evil.com could send
GET requests by imbedding an <img>, <script>, <css> and setting src="http://example.com/path" (or <a href="http://example.com/path">).
POST requests by submitting a form.
Since evil.com cannot read any of the responses, it cannot read the CSRF token.
Related
Recently I'm studying some basics of Web security and there is something I couldn't understand.
How do anti-CSRF tokens work in SPA-API communications?
As far as I understand, anti-CSRF is used in SPA-API communications as followings;
The browser sends a login request to the API.
The API servers generates a token and sends it back to the browser.
The browser stores it, and when the browser makes the next request, token with be sent together.
The API can make sure that the request came from the genuine front-end because it contains the token.
A question pops up in my mind--how can it prevent CSRF?
If the token is stored in cookie, it will automatically be sent to API whenever a request happens, like usual session cookies. And even if it's stored in other storage(like session storage or local storage), it can be accessed using JavaScript.
So once users are attracted to the attacker's site, anti-CSRF tokens are completely useless.
On the top of that, I can't understand what's the difference between anti-CSRF token and usual cookies used in authentication/authorization……
Maybe I've made a terrible misunderstanding about how anti-CSRF tokes work. Please put a finger on what's wrong about it.
One of the most common CSRF vulnerabilities exists when an attacker can submit a request to an endpoint using an authenticated user's cookies. If you're not using cookies (i.e., to authenticate a user's request) or some other automatic authentication technique (like HTTP Basic Authentication), then there's generally no need for CSRF tokens.
Example #1:
Let's say you're using a REST API that depends on an access token or bearer token for authentication. This token is usually submitted in the HTTP Authorization header (not a cookie). In this case, as long as authentication isn't automatic (e.g., using a cookie), then there's no need for a CSRF token.
Example #2:
In this case, let's say the browser does send a session cookie to a web service/API to authenticate the request. Then, yes, you would be vulnerable to CSRF if anti-CSRF controls aren't implemented. One way to prevent this is to provide an anti-CSRF token to the browser when the SPA is loaded. The browser can then send that token with the request to the endpoint. The web service will then have to validate that token when the request is received.
There are a number of ways that this validation can occur. This could be done using double-submit cookies, a cookie-to-header token, cryptographic techniques, or even a shared database.
Using Anti-CSRF Tokens:
Anti-CSRF tokens generally should not be stored in cookies. As stated in the OWASP CSRF Prevention Cheat Sheet:
A CSRF token can be included in the <meta> tag. All subsequent calls
in the page can extract the CSRF token from this <meta> tag. It can
also be stored in a JavaScript variable or anywhere on the DOM.
However, it is not recommended to store it in cookies or browser local
storage.
For example, an anti-CSRF token might get embedded in the page as:
<meta name="csrf-token" content="{{ csrf_token() }}">
Where the csrf_token() calls some server-side function that embeds the token in the tag.
It can then be read in JavaScript using:
let csrf_token = document.querySelector("meta[name='csrf-token']").getAttribute("content");
And then transmitted to the server when an API request is made (e.g., in a X-CSRF-Token header in a POST request). In addition, the token should be unique to the session.
However, even if a token were to be stored in a cookie, the cookie could be set with the HttpOnly header. This prevents the cookie from being read by JavaScript. This is more so useful in mitigating cross-site scripting (XSS) attacks.
Additional Info:
This StackExchange Security question includes a lot of good info on using CSRF protection in REST APIs.
Other good resources about CSRF in general:
https://portswigger.net/web-security/csrf
https://owasp.org/www-community/attacks/csrf
I know
this question has already been asked so many times, but after hours of searching I still don't have a clear answer to my problem.
Even projects like https://github.com/pillarjs/understanding-csrf have been abandoned and have not answered to new questions and doubts over the years like this.
PROBLEM
Let's say I have:
a back-end on back.domain.com and
a front-end on front.domain.com.
My back-end is a simply nodejs app with these rest endpoints:
POST /login:
accepts JSON body like: {"username": "myname", "password": "mypass"}
verify credentials
if OK gives 200 and create a cookie with session
if NOT gives 401
GET /players:
check session in cookie
if OK gives 200 with {"players": "[...]"}
if NOT gives 401
POST /player/1:
check session in cookie
if OK gives 200 and edit player
if NOT gives 401
My front-end app has:
/login page with a form (with username and password fields) for issue a POST request to back.domain.com/login
/players which request a GET request to back.domain.com/players
a button which issues a POST request to back.domain.com/player/1
QUESTIONS
Do I need CSRF protection in this scenario?
I think YES, I need because an attacker can issue a request to back.domain.com/player/1 from malicious.site.com and use my session cookie to edit player because I'm logged in (and I still have a session cookie) on my domain.com.
Do I need CSRF protection (e.g. an X-CSRF-Token header) when I the first time login on back.domain.com/login?
In this scenario I still don't have any session cookie in my browser.
And also I don't know where to get my CSRF token for X-CSRF-Token authorization header too.
I read on https://fractalideas.com/blog/making-react-and-django-play-well-together-single-page-app-model they are creating a dedicated endpoint on back-end for this and they explain it's not a security vulnerability.
What do you think about?
You are correct.
You DO need CSRF protection any time BOTH of the following are true:
the browser is automatically providing the authentication mechanism (most common way this is done is with a cookie)
the operation is state-changing on your backend
Of your three endpoints, only one meets both of those conditions.
GET /players/: get is not a state-changing operation. No CSRF protection needed.
POST /player/1/: authentication provided by cookie; post is state-changing. Needs CSRF protection!
POST /login/: the browser is not automatically providing the authentication information; it’s coming from data the user has intentionally typed in and submitted. No CSRF protection needed.
You’ll find other schools of thought - this other stack overflow post indicates the possibility of privacy-violation attacks, but the method described stretches credulity a bit in my opinion. And in any case you are right - if your frontend and backend are being served by totally different servers, your frontend won’t have the CSRF token before the user logs in.
Is it a good practice to save the csrf token in a cookie or is it better to use a hidden field in a form? Also is it good to regenerate csrf token every user request like what captchas are doing?
Thanks
It is best to include it in the form. The idea behind a CSRF token is that it is not passed passively (e.g. if a malicious user is able to trick the browser into accessing some URL that does something nasty). Cookies are passed passively.
The best explaination to this question can be found on OWASP website at OWASP CSRF Prevention Cheat Sheet page.
Firstly, using cookie for a CSRF token can not help much because all cookies, even the secret ones, will be submitted with every request. All authentication tokens will be submitted regardless of whether or not the end-user was tricked into submitting the request.
Secondly, the application can include hidden input parameter in the form with a common name such as "CSRFToken". The value of this token must be randomly generated such that it cannot be guessed by an attacker.
Furthermore, Challenge-Response is another defense option for CSRF. It can be implemented in following ways:
CAPTCHA
Re-Authentication (password)
One-time Token
The CSRF cookie is certainly open to attack but implementation safe as the session value will always be checked against a submitted token value either stored in the body or header of the request so I can't see a reason against. The double submit (http only cookie vs post data) or token synchronizer (session vs post data) patterns outlined on the OWASP website are good pratices and both use cookies.
Double submit as mentioned earlier moves the storage to the client so is considered stateless but either way two tokens for comparison, of which one always remains unknown to the attacker.
I had one doubt about CSRF prevention. A lot of sites say that CSRF can be prevented by using 'tokens' which are randomly generated per session.
Now my doubt is,
suppose i have a function like :
$.post("abcd.php",{'fbuid':userid,'code':'<?php echo md5($_SESSION['randcode']); ?>'}
now this md5 hash would obviously be visible to any hacker through the source code.He could simply open this page, generate a token, and keep the page open, so that the session doesn't get destroyed, and useanother tab or anything else , to start hacking,
No ?
Or is my idea of tokens incorrect ?
Thanks for your help :D
I think you are misunderstanding what needs to be done. To protect against CSRF you need to create a token and save it for that session. Then you need to append all your submits and AJAX calls with that token.
For another person to send you to a page on your website they would need to have access to the request with in the same session. It is true that one could parse the HTML and look for the token. But when they try to request a http call on your website they will have a new session created. The new session will have a new token which will not match the token that was passed.
Next you will ask what if you can copy the cookies and the session id as a result. This is not something that is protected. I can simply sit anybody's computer and copy all their cookies and I will then be logged in as them.
As kapep points out, you are confusing the two seperate issues of input validation and cross-site form posting. You must validate your inputs anyway, so the case of your malicious attacker using his own session token is already handled if you have sound input validation. CSRF protection is not there to protect the data, it is simply to ensure that only forms from your own application can post data back to that application. the CSRF protection is simply stopping other people being able to post data directly into your app from forms they put up on their own site.
One specific point to be aware of is that the token is visible to any javascript running on your page, so as soon as you have a cross-site scripting (XSS) vulnerability, your CSRF protection is defeated.
See Cross-site scripting and the prevention cheat sheet
You should use a per request token.
Generate a token and store it in the session.
Pass the token to the client.
Execute actions.
Destroy the token.
The token is safer and cannot be used more than one time.
I would define a stolen token as a token that is used by someone else, and not the one you have send the token to. If you send someone a token he can't really steal it from himself.
If you are concerned that a user can run a malicious script with his own token, your design seems to be broken. You can't prevent a user from sending data that you didn't indented to receive. It's your job to validate any data, the session token is just there to identify multiple requests by the same client.
It could be a security issue if you send that token over unsecured http. Then it could easily be stolen by monitoring the clients network.
I am writing an application (Django, it so happens) and I just want an idea of what actually a "CSRF token" is and how it protects the data.
Is the post data not safe if you do not use CSRF tokens?
Cross-Site Request Forgery (CSRF) in simple words
Assume you are currently logged into your online banking at www.mybank.com
Assume a money transfer from mybank.com will result in a request of (conceptually) the form http://www.mybank.com/transfer?to=<SomeAccountnumber>;amount=<SomeAmount>. (Your account number is not needed, because it is implied by your login.)
You visit www.cute-cat-pictures.org, not knowing that it is a malicious site.
If the owner of that site knows the form of the above request (easy!) and correctly guesses you are logged into mybank.com (requires some luck!), they could include on their page a request like http://www.mybank.com/transfer?to=123456;amount=10000 (where 123456 is the number of their Cayman Islands account and 10000 is an amount that you previously thought you were glad to possess).
You retrieved that www.cute-cat-pictures.org page, so your browser will make that request.
Your bank cannot recognize this origin of the request: Your web browser will send the request along with your www.mybank.com cookie and it will look perfectly legitimate. There goes your money!
This is the world without CSRF tokens.
Now for the better one with CSRF tokens:
The transfer request is extended with a third argument: http://www.mybank.com/transfer?to=123456;amount=10000;token=31415926535897932384626433832795028841971.
That token is a huge, impossible-to-guess random number that mybank.com will include on their own web page when they serve it to you. It is different each time they serve any page to anybody.
The attacker is not able to guess the token, is not able to convince your web browser to surrender it (if the browser works correctly...), and so the attacker will not be able to create a valid request, because requests with the wrong token (or no token) will be refused by www.mybank.com.
Result: You keep your 10000 monetary units.
(Your mileage may vary.)
EDIT from comment worth reading by SOFe:
It would be worthy to note that script from www.cute-cat-pictures.org normally does not have access to your anti-CSRF token from www.mybank.com because of HTTP access control. This note is important for some people who unreasonably send a header Access-Control-Allow-Origin: * for every website response without knowing what it is for, just because they can't use the API from another website.
Yes, the post data is safe. But the origin of that data is not. This way somebody can trick user with JS into logging in to your site, while browsing attacker's web page.
In order to prevent that, django will send a random key both in cookie, and form data.
Then, when users POSTs, it will check if two keys are identical. In case where user is tricked, 3rd party website cannot get your site's cookies, thus causing auth error.
The Cloud Under blog has a good explanation of CSRF tokens. (archived)
Imagine you had a website like a simplified Twitter, hosted on a.com.
Signed in users can enter some text (a tweet) into a form that’s being
sent to the server as a POST request and published when they hit the
submit button. On the server the user is identified by a cookie
containing their unique session ID, so your server knows who posted
the Tweet.
The form could be as simple as that:
<form action="http://a.com/tweet" method="POST">
<input type="text" name="tweet">
<input type="submit">
</form>
Now imagine, a bad guy copies and pastes this form to his malicious
website, let’s say b.com. The form would still work. As long
as a user is signed in to your Twitter (i.e. they’ve got a valid
session cookie for a.com), the POST request would be sent to
http://a.com/tweet and processed as usual when the user clicks the
submit button.
So far this is not a big issue as long as the user is made aware about
what the form exactly does, but what if our bad guy tweaks the form
like this:
<form action="https://example.com/tweet" method="POST">
<input type="hidden" name="tweet" value="Buy great products at http://b.com/#iambad">
<input type="submit" value="Click to win!">
</form>
Now, if one of your users ends up on the bad guy’s website and hits
the “Click to win!” button, the form is submitted to
your website, the user is correctly identified by the session ID in
the cookie and the hidden Tweet gets published.
If our bad guy was even worse, he would make the innocent user submit
this form as soon they open his web page using JavaScript, maybe even
completely hidden away in an invisible iframe. This basically is
cross-site request forgery.
A form can easily be submitted from everywhere to everywhere.
Generally that’s a common feature, but there are many more cases where
it’s important to only allow a form being submitted from the domain
where it belongs to.
Things are even worse if your web application doesn’t distinguish
between POST and GET requests (e.g. in PHP by using $_REQUEST instead
of $_POST). Don’t do that! Data altering requests could be submitted
as easy as <img src="http://a.com/tweet?tweet=This+is+really+bad">,
embedded in a malicious website or even an email.
How do I make sure a form can only be submitted from my own website?
This is where the CSRF token comes in. A CSRF token is a random,
hard-to-guess string. On a page with a form you want to protect, the
server would generate a random string, the CSRF token, add it to the
form as a hidden field and also remember it somehow, either by storing
it in the session or by setting a cookie containing the value. Now the
form would look like this:
<form action="https://example.com/tweet" method="POST">
<input type="hidden" name="csrf-token" value="nc98P987bcpncYhoadjoiydc9ajDlcn">
<input type="text" name="tweet">
<input type="submit">
</form>
When the user submits the form, the server simply has to compare the
value of the posted field csrf-token (the name doesn’t
matter) with the CSRF token remembered by the server. If both strings
are equal, the server may continue to process the form. Otherwise the
server should immediately stop processing the form and respond with an
error.
Why does this work? There are several reasons why the bad guy from our
example above is unable to obtain the CSRF token:
Copying the static source code from our page to a different website
would be useless, because the value of the hidden field changes with
each user. Without the bad guy’s website knowing the current user’s
CSRF token your server would always reject the POST request.
Because the bad guy’s malicious page is loaded by your user’s browser
from a different domain (b.com instead of a.com), the bad guy has no
chance to code a JavaScript, that loads the content and therefore our
user’s current CSRF token from your website. That is because web
browsers don’t allow cross-domain AJAX requests by default.
The bad guy is also unable to access the cookie set by your server,
because the domains wouldn’t match.
When should I protect against cross-site request forgery? If you can
ensure that you don’t mix up GET, POST and other request methods as
described above, a good start would be to protect all POST requests by
default.
You don’t have to protect PUT and DELETE requests, because as
explained above, a standard HTML form cannot be submitted by a browser
using those methods.
JavaScript on the other hand can indeed make other types of requests,
e.g. using jQuery’s $.ajax() function, but remember, for AJAX requests
to work the domains must match (as long as you don’t explicitly
configure your web server otherwise).
This means, often you do not even have to add a CSRF token to AJAX
requests, even if they are POST requests, but you will have to make
sure that you only bypass the CSRF check in your web application if
the POST request is actually an AJAX request. You can do that by
looking for the presence of a header like X-Requested-With, which AJAX
requests usually include. You could also set another custom header and
check for its presence on the server side. That’s safe, because a
browser would not add custom headers to a regular HTML form submission
(see above), so no chance for Mr Bad Guy to simulate this behaviour
with a form.
If you’re in doubt about AJAX requests, because for some reason you
cannot check for a header like X-Requested-With, simply pass the
generated CSRF token to your JavaScript and add the token to the AJAX
request. There are several ways of doing this; either add it to the
payload just like a regular HTML form would, or add a custom header to
the AJAX request. As long as your server knows where to look for it in
an incoming request and is able to compare it to the original value it
remembers from the session or cookie, you’re sorted.
The site generates a unique token when it makes the form page. This token is required to post/get data back to the server.
Since the token is generated by your site and provided only when the page with the form is generated, some other site can't mimic your forms -- they won't have the token and therefore can't post to your site.
The root of it all is to make sure that the requests are coming from the actual users of the site. A csrf token is generated for the forms and Must be tied to the user's sessions. It is used to send requests to the server, in which the token validates them. This is one way of protecting against csrf, another would be checking the referrer header.