JSON Hijacking Mini Challenge

Ger Hobbelt edited this page Jan 21, 2017 · 2 revisions

URL

http://vwzq.net/challenge/jsonhijack.html

Expected solution

This is the solution that all solvers used (with little modifications). The first thing to note was that the page did not specify any charset in the "Content-Type" header, what should have given us a big clue about where were the shots.

Another hint was the title itself: "JSON hijacking". Or my last posts playing with charsets ;P So We just needed to take a quick view to these issues in the past [http://www.thespanner.co.uk/2011/05/30/json-hijacking/] (courtesy of Mr. @garethheyes) to get an idea of how these attacks usually work.

It seems that We require to "script source" the vulnerable page and then do some kind of obscure magic:

<script src="http://demo.vwzq.net/php/secret.php?uid="></script>

Unfortunately, there is a "while(1)" that will mess with us :) And here comes the "trick", since the remote page does not specify a charset, we can set it in the script tag. The requirement is that we should use a non-ASCII compliance encoding in order to avoid the infinite loop. The first option is trying UTF-16 or UTF-32, since they will consume 2 and 4 bytes for each character (breaking the "while" token).

<script charset="utf-16" src="http://demo.vwzq.net/php/secret.php?uid="></script>

Results in the following invalid Javascript: "桷汩⡥⤱嬻瑻歯湥∺敳牣瑥∱甬摩∺索".

<script charset="utf-32" src="http://demo.vwzq.net/php/secret.php?uid="></script>

Results in the following invalid Javascript: "���������".

UTF-32 will not generate any valid JS character (in this case we expect an identifier), but UTF-16 seems to give us some valid characters:

wh - 桷 (VALID) il - 汩 (VALID) e( - ⡥ (INVALID)

    • ⤱ (INVALID) ;[ - 嬻 (VALID) {t - 瑻 (VALID) ok - 歯 (VALID) en - 湥 (VALID) :" - ∺ (INVALID) se - 敳 (VALID) cr - 牣 (VALID) et - 瑥 (VALID) 1" - ∱ (INVALID) ,u - 甬 (VALID) id - 摩 (VALID) :" - ∺ (INVALID) ...

Most alphanumeric ASCII bytes results in a valid UTF-16 character, but some other pairs fail. We can also try with UTF-16BE (Big Endian representation, by default UTF-16 uses Little Endian).

<script charset="utf-16be" src="http://demo.vwzq.net/php/secret.php?uid="></script>

Which results in an almost valid Javascript identifier: "睨楬攨ㄩ㭛筴潫敮㨢獥捲整ㄢ楤㨢".

Now you can imagine what is going on.

Note: this scenario is not very realistic since the returned JSON is a bit custom.

Now, since we control the last part of the embedded javascript because of the parameter "uid", we just need to retrieve the information. Most solutions used an assignment "unicode_identifier=1//" and then tried to access the "window" object looking for the identifier name (which contains the secret inside its bytes).

<script charset="utf-16be" src="http://demo.vwzq.net/php/secret.php?uid=%00=%001%00/%00/"></script>

The "%00" is necessary because ASCII chars in UTF-16 have the same hex value plus a null less or more significant byte, depending on BE or LE.

.mario, filedescriptor and s3cur1tyrocks' solutions iterate over the window properties looking for the ones matching with /^W/, mine used "Object.keys(self).pop()" to get the last property setted (I'm that dirty).

Masato, in the other hand, did not need even an assigment. He set a "Proxy" to "window.proto" (only FF) and intercepted the access to the property in order to get its name ("睨楬攨ㄩ㭛筴潫敮㨢獥捲整ㄢ楤㨢䄢絝") :)

Once we get the Japanese identifier, only rests to get the byte representation and transform it back to ASCII:

<script>unescape(escape("睨楬攨ㄩ㭛筴潫敮㨢獥捲整ㄢ楤㨢䄢絝").replace(/%u(..)(..)/g,'%$1%$2'))</script>

After that we only need to substring the token an "alert" it. Easy peasy.

So, my complete solution for Chrome was:

<script charset="utf-16be" src="http://demo.vwzq.net/php/secret.php?uid=%00=%001%00%2f%00%2f"></script>
<script>alert(unescape(escape((Object.keys(window).pop())).replace(/%u(..)(..)/g,'%$1%$2')).substr(18,7))</script>

As I said, it wasn't a hard challenge and it didn't use anything new, in fact Masato blog about it in 2012 [http://masatokinugawa.l0.cm/2012/05/utf-16content-security-policy.html], though I had no idea (thanks to s3cur1tyrocks for the link).

In any case it will work just in very specific scenarios where the JSON casually forms a valid UTF-16BE and the server does not specify the charset.

@shafigullin also pointed out if "X-Content-Type-Options: nosniff" would mitigate the exploitation. And while in Chrome does it, it's not the case for Firefox (you can't test with "secret2.php"). So I guess we have a bug here.

Server misconfiguration + Windows/Linux "bug"? + HTML5

@BugRoast also came with a very out-of-the-box PoC abusing a misconfiguration on my server. It isn't a valid solution because of the required user interaction, but I found it interesting enough to worth the mention :) Btw, if you have not seen his paper [http://conference.hitb.org/hitbsecconf2015ams/wp-content/uploads/2015/02/WHITEPAPER-Exploiting-Browsers-the-Logical-Way.pdf] for HITB, take a look!

His idea consisted in making use of the HTML5 download attribute for anchors, in order to force the victim's browser to download the "secret.php" page as an HTML file with a XSS and to wait for the local opening of the file.

<a id="foo" href="http://demo.vwzq.net/php/secret.php?uid=%3Csvg/onload=alert(body.textContent)%3E" download="foo.html">Click</a>
<script>foo.click()</script>

Unfortunately, the filename is ignored on X-domain requests, so the browser will save the file as "secret.php" which should not be opened by the browser, right? Well, here comes the interesting part.

What @BugRoast did was to abuse the following error in my server which allows overwriting the path: http://demo.vwzq.net/php/secret.php/foo.html. With this trick, which also reminds me to @OrenHafif's Reflected File Download, he was able to force Chrome to downloading the JSON file like an HTML file containing an <script> element.

However, this is only true for Chrome on Windows and Linux, but not on OSX. On OSX, Chrome seems to use the "Content-Type" instead of the URL extension, which in this case is "application/json", to override the extension of the file. What in my opinion should be the secure and expected behaviour.

Finally, the full PoC was:

<a id="foo" href="http://demo.vwzq.net/php/secret.php/open.html?uid=%3Csvg/onload=alert(body.textContent)%3E" download="ignored_name.any">Click</a>
<script>foo.click()</script>

Inline PDF

While preparing the challenge, I also thought about the possibility of injecting a valid PDF inside the JSON (since there were no chars restrictions almost), and use Acrobat Reader to execute Javascript in the context of the server and ultimately read the secret token. @insertScript also mentioned it, but I'm not sure if it is really possible... A PoC would be very cool :) Volunteers?

Conclusion

And that's all. Thank you for reading and participating, congratulations to the solvers and I hope you have enjoyed playing or learned something new :)

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