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Apple Safari URI spoofing (CVE-2015-5764)

tl;dr Apple Safari for OS X was prone to URI spoofing vulnerability  (and more general a user interface spoofing). Apple released security updates for Safari 9 on OS X and assigned CVE-2015-5764. Accidentally this vulnerability was also present in iOS.

Instant demo

In Safari up to 8.0.8 :
  • go to
  • click "click me!"
  • notice the address bar being "data:text/html,%3CH1%3EHi!!%3C/H1%3E"
  • go back using the browser button
  • click "click me!"
  • notice the address bar being !!!! 

Well this looks a clear caching problem to me, right :) ?

The Introduction (Oldie but goldie)

Several months ago (almost a year!!) I was reading the great book written by lcamtuf (aka Michal Zalewski) named The Tangled Web .  I know, I know I was a bit late for the party :)
Said that, this book contained a really interesting Chapter (for the record Chapter 10) that is dedicated almost entirely to pseudo-URLs such as (about:, javascript:, or data:). 
As for almost all the parts of this book I wanted to try what it was written and I started a bit to poke around.  The followers of this blog know that I kind of like to "play" with OAuth. Hence I combined the two things and started to see what I could do.

The Issue

The first issue I found was the one mentioned in the Instant demo section above.
Now,  in order to understand the issue we need to look at the code of

<a href="">click me! </a>

As you can see this is simply pointing to a PHP page in my website ( So let's take a look at it:

header("Location: data:text/html,<H1>Hi!!</H1>");

This is simply an HTTP 302 redirect to data:text/html,data:text/html,<H1>Hi!!</H1>

Now when clicking at the link all the browser but Safari showed properly the address bar being data:text/html,%3CH1%3EHi!!%3C/H1%3E . Safari instead from the second visit onward would show the original website namely but with the HTML contained in the data:text/html pseudo-URI!!!

Well well this looks like an URI spoofing according to my book ;)
Safari was not unkown to this kind of vulnerability in 2015, same as Google Chrome

The Vulnerability(ies)

At this point your question can easily be why on earth should exists a website that allow an attacker to manipulate a 302 redirect versus a data:text/html URI and how did you find this weird vulnerability  :D ? The answer is: because of some  OAuth 2.0 implementations!!!
One of steps in order to obtain an OAuth client is to register a client application providing client name and a list of redirect_uri. 

If you are not too much familiar with OAuth 2.0 here you can find a book on OAuth that Justin Richer and myself have been writing on the subject.

Below a  little reminder on how a typical OAuth flow would look like

It turns out that exists some OAuth Autorization server that allows the registration of redirect_uri of the form of data:text/html. One example is (was as Facebook fixed this in the meantime) Moves on of Facebook acquisitions

The final piece of the puzzle is an Open Redirect vulnerability that exists in rfc6749 aka 'The OAuth 2.0 Authorization Framework' and some of its implementation . You can see an example of it clicking one of the links below:
The link will redirect to the registered_uri (without any user interaction). Before Facebook fixed the data:text/html redirect_uri clicking the following URL

 would have redirected you to  data:text/html,a&state=<script>alert('hi')</script>

The Attack

So far so good. Now let's try to sum up. Indeed we have all we need for a real attack. For an attacker would be enough to:

- find a website that offers OAuth support
- this website needs to allow registration of redirect_uri also of the type data:text/html
- this website implements OAuth 2 verbatim hence has an open redirect
- craft a URI of the form<script>alert('hi')</script>

And here we go, you would have a spoofed website (thanks to CVE-2015-5764)

The Fix

Apple released two security updates this month that include a fix for this issue:

Beyond URI spoofing

It looks like Apple Security team broke down the fix  for this issue in two tranches. Indeed if you careful read the description of CVE-2015-5764

Description: Multiple user interface inconsistencies may have allowed a malicious website to display an arbitrary URL. These issues were addressed through improved URL display logic.

This means that also other user interface were vulnerable and not only the address bar. One of the vulnerable component was the title of the alert box (that is commonly used for anti-phishing) .

But looks like Apple fixed this part of vulnerability in the previous security update.
If you want to give a look to it  with Safari 8.0.7 :



Lee Wei said…
Not working for me.

I'm on Mac OSX 10.10.5, Safari web browser 8.0.8 (10600.8.9).
ll said…
@Lee Wei, I reported this issue to Apple in November 2014 (almost a year ago) as said in the blog post Apple Security fixed this in progressively so might well be ....
Lee Wei said…
I'm writing that comment because in your blog post, under "Instant demo", it reads "In Safari up to 8.0.8 :...."

you don't mean to say 8.0.8 (inclusive), since at the end of your article you state 8.0.7...

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