学步园

An Evaluation of the Application ("App") Verification Service in Android 4.2

By Xuxian Jiang, Associate Professor, Department of Computer Science, NC State University

With the latest Android 4.2 (JellyBean) release on November 13, 2012, Google announced an exciting security feature called the"application
verification service" to protect against harmful Android applications. As stated in a recent Google+ post by
a member of the Google Android team, "Now, with Jelly Bean Android 4.2 devices that have Google Play installed have the option of using Google as an application verifier. We will check for potentially harmful applications no matter where you are installing
them from." It is indeed an exciting security feature! We think it was a really good move by Google to directly face Android malware threats and take such measures to better protect Android users.

Meanwhile, because of the introduction of this service, people may start to wonder, "are third-party security apps still necessary with Android 4.2?" -- see the links here, here, here,
and there. We are no exception! To demystify this service, we perform the following study
for two main purposes: (1) We want to understand better how the app verification service works; (2) We also want to quantify the effectiveness of this service and compare it with existing third-party anti-virus engines.

1. Google's App Verification Service (in Android 4.2) -- An Overview

 
Figure 1: Google's new app verification service (in Android 4.2)

The new service is implemented inside the official Google Play app, but is designed to work with apps from all app stores, including the official Google Play marketplace and other alternative ones. A user
can turn the service on/off by going to "Settings," "Security," and then "Verify apps." When an app is being installed (Step 1), the service, if turned on, will be invoked (Step 2) to collect and send information about the
app (e.g., the app name, size, SHA1 value, version, and the URL associated with it) as well as information about the device (e.g., the device ID and IP address) back to the Google cloud (Step 3). After that, the Google cloud will respond with
a detection result (Step 4). If the app is not safe, the user is then shown a warning popup (Step 5) flagging the app as eitherdangerous or potentially dangerous. Dangerous apps are blocked from
being installed, while potentially dangerous ones instead alert users and provide an option to either continue or abort the installation (Step 6) with a warning popup. In Figure 1, we show the key steps behind the service.

Figure 2(a): Enabling the app verification service

Figure 2(b): Detecting a potentially dangerousapp

Figure 2(c): Detecting a dangerous app

We note that this app verification service is optional, but turned on by default. The first time an app is side-loaded, a popup window will ask whether to "Allow Google to check all apps installed to this
device for harmful behavior?" (Figure 2(a)). If yes and the app is detected as potentially dangerous, the user will be prompted with a warning (Figure 2(b)) and given the option to continue or abort the installation. If detected as dangerous,
the app will be blocked from installation (Figure 2(c)).

2. Assessment Setup and Evaluation Results

To quantify the effectiveness of this service, we leverage our prior Android Malware Genome Project and
check whether the collected Android malware samples can be detected. We conducted our experiments on November 30, 2012, by using the same dataset of 1260 samples (belonging to 49 different families) that has been widely shared within the research community,
including Google. Our study involves semi-automated installation of them on a few of latest Nexus 10 tablets (16GB) running Android 4.2 (build number:JOP40C). The detailed detection results are shown in Table 1. Overall, among these 1260 samples, 193 of them
can be detected, indicating a low detection rate of 15.32%.

In addition, we perform another set of experiments to compare the Google's app verification service with existing third-party anti-virus engines. Specifically, we randomly picked up a sample from each malware
family and test it with the VirusTotal service (acquired by Google in September 2012). In Table 2, we show the comparison with ten representative anti-virus
engines from VirusTotal (i.e., Avast, AVG, TrendMicro, Symantec, BitDefender, ClamAV, F-Secure, Fortinet, Kaspersky, and Kingsoft). Overall, the detection rates of these representative anti-virus engines range from 51.02% to 100% while the detection rate of
this new service is 20.41%. Figure 3 shows the average detection rates of this new app verification service and ten other representative anti-virus engines.

Figure 3: The comparison between the new app verification service (in Android 4.2) and ten other representative anti-virus engines

3. Look Ahead

By introducing this new app verification service in Android 4.2, Google has shown its commitment to continuously improve security on Android. However, based on our evaluation results, we feel this service
is still nascent and there exists room for improvement.

Specifically, our study indicates that the app verification service mainly uses an app's SHA1 value and the package name to determine whether it is dangerous or potentially dangerous.
This mechanism is fragile and can be easily bypassed. It is already known that attackers can change with ease the checksums of existing malware (e.g., by repackaging or mutating it). To be more effective, additional information about the app may need to be
collected. However, how to determine the extra information for collection is still largely unknown -- especially given user privacy concerns.

In addition, the new app verification service largely relies on the server component (in the Google cloud) to determine whether an app is malicious or not. Unfortunately, it is not realistic to assume that
the server side has all existing malware samples (especially with limited information such as app checksums and package names). From another perspective, the client side, in the current implementation, does not have any detection capability, which suggests
possible opportunity for enhancement. However, due to the limited processing and communication power on mobile devices, we need to strike a delicate balance on how much detection capability can and should be offloaded.

Last but not least, we notice that VirusTotal (owned by Google) has not been integrated yet into
this app verification service. From our measurement results, VirusTotal performs much better than this standalone service. For improved detection results, we expect such integration in the future will be helpful.

本文转自：http://www.cs.ncsu.edu/faculty/jiang/appverify/

其他参考：

http://www.csdn.net/article/2012-12-12/2812698-application-verification-service

https://support.google.com/accounts/answer/2812853?hl=zh-Hans&ref_topic=3100928#survey=true