HTML5 Mobile Apps – Injection Heaven, Security Hell

Three weeks ago was fined for stupidly stealing their user’s contact list and saving it onto their servers. Path’s doing was obviously wrong but I’m not sure that their punishment was really justified, needing to pay this enormous bribe to the FTC using COPPA as an excuse. The lesson here is to always comply with COPPA.

Anyhow, in that same techcrunch article you can also find that “The FTC also took the opportunity to introduce a new set of guidelines for mobile developers“. Although they explain early in that article that it’s not meant to be a guideline, I still feel they misses a lot.

When it comes to HTML5 apps even the simplest app can greatly compromise the user privacy and security. If we’ll take the FTC example of a simple and harmless alarm clock app, If that app is built using HTML5 its size and complexity doesn’t matter. All that is needed is one javascript injection that will pass thorough.

How will that code be injected you may ask – all that is needed is for the app to load some content from a remote server the simplest example will be the “Terms And Condition” page which is mostly loaded into a WebView. It can be a more “complex” settings, like choosing the favorite color or loading the saved alarms. Any kind of sharing will probably be way more open to be exploited, i.e. “share your favorite alarms”. Push messages might also bring malicious code. ETC’

The bottom line is that any injection of javascript will give an attacker a lot of control over the device, more often than not it’ll be persistant. HTML5 apps usually use the localStorage that is rarely flushed, and leverage native DBs and the file system. The “page” or webview is rarly refreshed, so even if the injection is not persistant it’ll be alive for a long time.
Things like stealing the user’s contact list and tracking the user location are pretty common. Enabled by default in iPhone PhoneGap for example.

It’s only limited by the native API that is opened to Javascript, generally it’s very open, even more than the PhoneGap default API. I know of at least 1 popular HTML5 app that opens almost all of the Android native API.

You see, Javascript is one tough beast – it can run almost anywhere.
Javascript was designed basically as a none important sidekick to the browser’s HTML, “it should not cause any problems by being poorly written and should fail silently and not interfere with the main thing that is HTML.” Seriously that how it was, we’re lucky it’s not case insensitive. I’m sure that back than some people though it’ll make it simpler and better.
So, Javascript will run in any dom element no matter how naive you may think it is, it will run in unexpected parts of the element without needing the <script> tag, i.e. onerror=”attack()”. It used to even run from CSS and from images, but we’re over that now asfaik in mobile browsers.

As opposed to that, it’ll take a very special case for injection to be able to execute arbitrary native code. You can make a native android app that will run anything – even get root, but I doubt that any legitimate app regularly download strings and run it as commands. (basically on rooted Android you can do exec(“su”) and everything else)

With Javascript the app does not need to be designed in any special way, an unsanitizes string will likely to execute.

These kind of injection are not the sole problem of PhoneGap based applications.
Any app that uses HTML5, even if it’s mostly native, any API that is opened to javascript can be leveraged by an attacker.

Phonegap (Cordova) has a mechanism to white list remote hosts which is really only effective on the iOS. It adds a little bit of security, but many apps anyway uses a wildecard “*” to allow all hosts. The wildcard is used by default in the phonegap cloud (saas solution to build phonegap apps)

As you can see the option for an attacker are enourmoe, all it needs is one vector of injection and there is an open path (no phan) to take over all of the devices of all of the users.

HTML5 apps that runs inside the mobile browser are also a nice target for injection attacks, althouygh it’s lacking most of the native api, there is still access to location in all mobile browsers. It’s less powerful for the attacker since it’ll prompt the user way more vigusly.
The Dolphin Mobile Browser implement the full phonegap native api, for example (which is generally a good thing), but it makes in-the-browser websites and apps more exposed to attacks.

So what to do than?!
– Sanitize sanitize sanitize all user input, server and client!

  • Basically what you are saying is that in the world of hybrid apps and web apps with permissions, JavaScript injections become dramatically more potent. I totally agree – excellent point. I think vendors should modify the permissions of script includes in these conditions. Perhaps enforce same origin policy, or something along these lines.

  • guya

    Hi Dan,
    Thanx for your comment,
    Indeed, policies, whitelisting and input sanitation will all help.
    I believe that eventually it’s up to us to be more vigilant and make sure it’s all properly applied.

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