czwartek, 22 maja 2014

Yandex - 2x XSS (duplicates) + Same-Origin Policy Bypass

Here are some writeups of few bugs I found in Yandex services last time.

1) Reflected XSS in

While uploading some file on the response would contains JSON with some informations about success or failure. The problem is the response headers:

HTTP/1.1 200 OK
Server: nginx
Date: Thu, 17 Apr 2014 17:13:57 GMT
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
X-Content-Type-Options: nosniff
Content-Encoding: gzip

As you can see - it’s html document. As far, as we are able to control part of response content and it’s unescaped - it would render as normal html. That’s typical XSS. JSON contains GET filename parameter unescaped, controlled by us. The Proof of Concept would look like this:

<form method="POST" enctype="multipart/form-data" action="<script>alert(1);</script>">
    <input type="submit">
    <input type="file" name="f"> <!-- just in case -->



2) Stored XSS in Yandex Maps Constructor (

This one is also simple - we are just creating new map calling it with some XSS payload.

PoC Request:

POST /maps/tools/constructor/proxy.xml HTTP/1.1 Host: Connection: keep-alive Content-Length: 422 Accept: application/json, text/javascript, */*; q=0.01 Origin: X-Requested-With: XMLHttpRequest User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.62 Safari/537.36 Content-Type: application/x-www-form-urlencoded; charset=UTF-8 Referer: Accept-Encoding: gzip,deflate,sdch Accept-Language: pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4,es;q=0.2 Cookie: [my-cookies-here] key=uc52c5955b971de4d17729378417f305d&v=1.0&action=create&map=%7B%22name%22%3A%22a%3Cscript%3Ealert(1)%3B%3C%2Fscript%3E%22%2C%22sid%22%3A%22%22%2C%22size%22%3A%5B600%2C450%5D%2C%22lang%22%3A%22ru-RU%22%2C%22boundedBy%22%3A%5B%5B16.87373%2C52.38846%5D%2C%5B16.97673%2C52.4357%5D%5D%2C%22type%22%3A%22MAP%22%2C%22geoObjects%22%3A%5B%5D%2C%22styles%22%3A%7B%7D%2C%22center%22%3A%5B16.92523%2C52.41209%5D%2C%22zoom%22%3A13%7D

URL decoded map parameter looks this way:

And the effect is of course alert(1):


Those two XSS were duplicates. Also as you can notice, both are unexploitable without having CSRF token (in my case: uc52c5955b971de4d17729378417f305d). That’s why I want to introduce...

3) Same-Origin Policy bypass I was working to find out how steal this token. At first I noticed, that the same CSRF token is used in multiple services (of course different tokens for different users ;)). One of places where this token appears in source code is Yandex Maps. After many days of trying to somehow get secret key - I checked crossdomain.xml. It's a special file used by Flash or Java for Same-Origin Policy purposes. Here’s how it looks for

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE cross-domain-policy SYSTEM "">
    <site-control permitted-cross-domain-policies="master-only"/>
    <allow-access-from domain="*"/>
    <allow-access-from domain="*"/>
    <allow-access-from domain="*"/>
    <allow-access-from domain="*"/>
    <allow-access-from domain="*"/>
    <allow-access-from domain=""/> 
    <allow-access-from domain="*"/> 
    <allow-access-from domain=""/> 
    <allow-access-from domain="*"/>
    <allow-access-from domain=""/>
    <allow-access-from domain="*"/>
    <allow-access-from domain=""/>
    <allow-access-from domain=""/> 
    <allow-access-from domain=""/>  
    <allow-access-from domain=""/>
    <allow-access-from domain=""/>
    <allow-access-from domain="*"/>
    <allow-access-from domain=""/>   
    <allow-access-from domain="*"/>
    <allow-access-from domain="*"/>
<!-- vim: set ts=4 sw=4 et: -->

It means, that Flash application hosted on any domain from above is able to send requests (with credentials) to and also gain full response content (the same that browsers renders for authenticated user). The problem was that I didn’t had possibility to upload my specially prepared SWF file to be hosted on any of this domains. After lots of research and analysis of swf files hosted on those domains, I found one interesting:

At the moment is turned off - but full source code of this file is available here.

I find out that it uses dangerous actionscript function loadBytes() that in short allows attacker to execute own flash bytecode. :) That was great for my case, because flash is able to execute my own code and still beeing hosted from (which is one of allowed domains in Yandex Maps crossdomain.xml).
After lots of source code analysis I finally understood what parameters I can control and what kind of external data I can provide to this application - so in the end the PoC for kraski-universal-blogplayer looks like this:

Now - servantUrl determine from where data should be loaded. Application make two requests - first one:

...which is normal JPEG file, and after clicking Play button (and it’s only one victim’s interaction) it make request to:

...which is XML file. As far as this service (kraski) is not working anymore, I had to determine how the XML file should looks like, to execute my own flash :) Effect:

<?xml version="1.0" encoding="UTF-8"?> <card picturesTotal="1" v="1.0"> <visual type="Picture"> <content>Q1dTDgoFAAB42l1TzW7b(........)TxP9U4ZKU=</content> </visual> </card>

The <content> node contains base64 encoded swf binary - kraski-universal-blogplayer decodes it, and loads using loader.loadBytes(). The source for my flash looks like this:

After running all - Chrome console showed me full content of


The full attack scenario goes this way:

1. Victim visits malicious site (
2. Attacker puts there <object> with kraski-universal-blogplayer 
3. Victim click on ‘play’ button.
4. Response from flash, through arrives to javascript console.log() on domain (SOP bypass)  
5. Because attacker have access to javascript in origin - he can parse response for CSRF token.
6. Attacker exploits both XSS issues, because now he know secret-key value :) (as far as first one is reflected XSS - it cannot be exploited on browsers that uses some Anti-XSS mechanism, but the second one works fine on all modern browsers. That is why on video I’ll show later only one alert will apear (as far as I'm Chrome user). 

The exploit is available here:

Of course - it’s not the end. :) Bypassing Same-Origin Policy can be exploited in worst scenario, because have the same crossdomain.xml file. ;-) So attacker is able to read sensitive data such as: user login, user phone number, list of contacts, e-mail subjects and content. Every domain that Yandex own and have similar crossdomain.xml file - is vulnerable to this attack. In there are two interactions that victim need to do - first is accept certificate for, and second is to click “Play” button.

Second exploit:

And at the end - video demonstration for stealing CSRF token and user e-mails:

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