CSRF (Cross-site request forgery) attack example and prevention in PHP

First, GET request shouldn't be used to alter states on the server, so for your vote service I would recommend POST/PUT. This is only a guideline, but a clever one.

So to your question, CSRF is a client issue so it doesn't matter what kind of server language you use (PHP in your case). The standard fix is the same and goes like this: Have a random value in the URI/POST-data and the same value in the Cookie header. If those matches you could be sure there is no CSRF. There are a lot of info about how this could be done here on StackOverflow eg. this one.
Good luck!


There are 3 players in a CSRF attack

  1. the victim website (your voting website in your example) [knows his logged in users cookies]
  2. your client's browser (while he is logged in) [knows his cookies]
  3. an attacker website [Doesn't know the logged-in users cookies]

CSRF attacks depend on 2 facts

  1. browsers send cookies automatically with every request
  2. we depend on cookies to identify our logged-in users (e.g: setcookie("sessionID", "0123456789ABCDEF", time()+3600); )

If an attacker could by away or another make a logged-in user requests this

// http://victim.website/vote/30

for example by putting the link on the attacker website or send it in an email, the logged in client browser will send the identifying cookies(sessionID) along with this request, which will make the victim website think that his logged-in user really wants to vote!

But if the victim's website more clever and verifies the requests of his logged-in users with additional GET or POST parameter (not cookies), the attacker now is in a problem because GET and POST parameters are not sent automatically by browsers, and he has to guess it.

// http://victim.website/vote/30?csrfSecret=0123456789ABCDEF

The attacker doesn't know the csrfSecret parameter which is a secret between the victim website and his client (just like the session token), so the attacker has no way to build the URL that he wants forge a request by.

Similarly, if the vote is done by POST requests, the attacker will not be able to make the form on his website(or third party website) because he doesn't know the secret between the victim's website and his users.

<form method="post" action="http://victim.website/vote" >
    <input type="hidden" name="vote" value="30">
    <input type="hidden" name="csrfSecret" value="????? I don't know it :(">
</form>

OWASP has a CSRFGuard for PHP, and ESAPI for PHP that I wrote a long time ago for XMB -> UltimaBB -> GaiaBB.

http://code.google.com/p/gaiabb-olpc/source/search?q=function+get_new_token&origq=function+get_new_token&btnG=Search+Trunk

It seems some others have cleaned up that code and allowed for stronger tokens:

https://www.owasp.org/index.php/PHP_CSRF_Guard

thanks, Andrew


This could become an example of CSRF if :

  • that link is fetched (via an <img> tag, for example) : forgery
  • from another site : cross-site


For example, if I could inject this <img> tag in the HTML source-code of stackoverflow (and I can, as stackoverflow allows one to use <img> tags in his posts) :

<img src="http://mysite.com/vote/30" />

You would just have voted for that item ;-)


The solution that is generally used is to place a token, that has a limited life-time, in the URL, and, when the URL is fetched, check that this token is still valid.

The basic idea would be :

  • When generating the page :
    • generate a unique token
    • store it in the user's session
    • and place it in the links of the page -- which would look like this : http://mysite.com/vote/30?token=AZERTYUHQNWGST
  • When the voting page is called :
    • Check if the token is present in the URL
    • Check if it's present in the user's session
    • If not => do not register the vote

The idea there is :

  • Tokens don't have a long life-time, and are hard to guess
  • Which means your attacker :
    • has only a window of a few minutes during which his injection will be valid
    • will have to be good at guessing ^^
    • will have to generate a different page for each user.


Also, note that the shorter the user's session remains active after he has left your site, the less risks there are that it's still valid when he visits the bad website.

But here, you have to choose between security and user-friendly...


Another idea (that's not perfectly secure, but helps against guys would don't know how to force a POST request), would be to only accept POST requests when people are voting :

  • The browser is sending GET requests for injected tags
  • As this URL is modifying some data, anyway, it should not work with GET, but only with POST

But note that this is not perfectly safe : it's (probably ? ) possible to force/forge a POST request, with some bit of Javascript.

Tags:

Php

Csrf

Owasp

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