Skip to main content

Top 10 OAuth 2 Implementation Vulnerabilities

Some time ago I posted a blogpost abut  Top 5 OAuth 2 Implementation Vulnerabilities.
This week I have extended the list while presenting Top X OAuth 2 Hacks at OWASP Switzerland.

This blog post (like the presentation) is just a collection of interesting attack OAuth related.

#10 The Postman Always Rings Twice 

I have introduced this 'attack' in last year post . This is for provider implementer, it is not extremely severe but, hey, is better to follow the spec. Specifically

The client MUST NOT use the authorization code  more than once.  If an authorization code is used more than once, the authorization server MUST deny the request and SHOULD revoke (when possible) all tokens previously issued based on that authorization code.

It turned out that even Facebook and Google did it wrong... :)

#9 Match Point

To all OAuth Providers be sure to follow section 4.1.3 of the spec in particular

...if the "redirect_uri" parameter was included in the initial authorization request as described in Section 4.1.1, and if included ensure that their values are identical.

Should you fail to do it, this in combination with Lassie Come Home below is game over (even for implementer that support only the Authorization Code Grant flow).

#8 Open redirect in rfc6749 

If you want to implent OAuth Authorization Server and  follow verbatim the OAuth core spec you might end up having an Open Redirect. Full story here . Interesting attack here .

#7 Native apps - Which OAuth flow ?

In a nutshell

  • It is NOT recommended that native applications use the implicit flow.
  • Native clients CAN NOT protect a client_secret unless it is configured at runtime as in the dynamic registration case (RFC 7591).
If you do not follow this suggestions then you risk this.

#6 Cross-site request forgery for OAuth Clients

Defined  the the Most Common OAuth2 Vulnerability. So do you the state anti CSRF parameter, as long as you use the right library to check and not a broken one :)

#5 Cross-site request forgery for Authorization Servers

As per any other website part is important to not forget Cross Site Request Forgery aka CSRF protection in your OAuth provider impelemtation. Some examples are:

#4 On Bearer Tokens

DO NOT  (if you can avoid) pass the access_token as a URI parameter a la

GET /resource?access_token=mF_9.B5f-4.1Jq HTTP/1.1                  
Host: server.example.com


since:

#3 The Devil Wears Prada

 If you are an OAuth client that use OAuth for authentication (do NOT). If you absolutely have to, you'd better read User Authentication with OAuth 2.0 . Specially if you are using the OAuth Implicit Grant flow (aka Client side).
More about the topic in here and here

#2 Lassie Come Home for OAuth clients

If you are building an OAuth client,  
Thou shall register a redirect_uri as much as specific as you can

#1 Lassie Come Home for Authorization Server

 ....and the winner is (again) 'Lassie Come Home'. Well this is hell of a danger.
There are way too many example of provider vulnerable to this attack. Just listing few here:

At least the mitigation for this issue is damn simple:  use exact matching against registered redirect uri to validate the redirect_uri parameter

BTW the slides are here.

<snip>
//SHAMELESS SELF ADVERTISEMENT
If you like OAuth 2.0 and/or you want to know more about it here you can find a book on OAuth that Justin Richer and myself have been writing on the subject.
https://www.manning.com/books/oauth-2-in-action

</snip>

Comments

Popular posts from this blog

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 …

The Curious Case of WebCrypto Diffie-Hellman on Firefox - Small Subgroups Key Recovery Attack on DH

tl;dr Mozilla Firefox prior to version 72 suffers from Small Subgroups Key Recovery Attack on DH in the WebCrypto's API. The Firefox's team fixed the issue removing completely support for DH over finite fields (that is not in the WebCrypto standard). If you find this interesting read further below. Premise In this blog post I assume you are already knowledgeable about Diffie-Hellman over finite fields and related attacks. If not I recommend to read any cryptography book that covers public key cryptography. Here is a really cool simple explanation by David Wong:
I found a cooler way to explain Diffie-Hellman :D pic.twitter.com/DlPvGwZbto — David Wong (@cryptodavidw) January 4, 2020 If you want more details about Small Subgroups Key Recovery Attack on DH I covered some background in one of my previous post (OpenSSL Key Recovery Attack on DH small subgroups (CVE-2016-0701) ). There is also an academic paper where we examine the issue with some more rigors. If you want to read th…

All your Paypal OAuth tokens belong to me - localhost for the win

tl;dr  I was able to hijack the OAuth tokens of EVERYPaypal OAuth application with a really simple trick.
Introduction If you have been following this blog you might have got tired of how many times  I have stressed out the importance of the redirect_uri parameter in the OAuth flow.
This simple parameter might be source of many headaches for any maintainer of OAuth installations being it a client or a server.
Accepting the risk of repeating myself here is two simple suggestions that may help you stay away from troubles (you can always skip this part and going directly to the Paypal Vulnerability section):
If you are building an OAuth client,   Thou shall register a redirect_uri as much as specific as you can
i.e. if your OAuth client callback is https://yourouauthclient.com/oauth/oauthprovider/callback then

DO register https://yourouauthclient.com/oauth/oauthprovider/callbackNOT JUST https://yourouauthclient.com/ or https://yourouauthclient.com/oauth If you are still not convinced here…