Clever Billing Fraud Applications on Google Play: Etinu

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A new wave of fraudulent apps has made its way to the Google Play store, targeting Android users in Southwest Asia and the Arabian Peninsula as well—to the tune of more than 700,000 downloads before detection by McAfee Mobile Research and co-operation with Google to remove the apps.

Figure 1. Infected Apps on Google Play

Posing as photo editors, wallpapers, puzzles, keyboard skins, and other camera-related apps, the malware embedded in these fraudulent apps hijack SMS message notifications and then make unauthorized purchases. While apps go through a review process to ensure that they are legitimate, these fraudulent apps made their way into the store by submitting a clean version of the app for review and then introducing the malicious code via updates to the app later.

Figure 2. Negative reviews on Google Play

McAfee Mobile Security detects this threat as Android/Etinu and alerts mobile users if they are present. The McAfee Mobile Research team continues to monitor this threat and is likewise continuing its co-operation with Google to remove these and other malicious applications on Google Play.

Technical analysis

In terms of details, the malware embedded in these apps takes advantage of dynamic code loading. Encrypted payloads of malware appear in the assets folder associated with the app, using names such as “cache.bin,” “settings.bin,” “data.droid,” or seemingly innocuous “.png” files, as illustrated below.

Figure 3. Encrypted resource sneaked into the assets folder

Figure 4. Decryption flow

The figure above shows the decryption flow. Firstly, the hidden malicious code in the main .apk opens “1.png” file in the assets folder, decrypts it to “loader.dex,” and then loads the dropped .dex. The “1.png” is encrypted using RC4 with the package name as the key. The first payload creates HTTP POST request to the C2 server.

Interestingly, this malware uses key management servers. It requests keys from the servers for the AES encrypted second payload, “2.png”. And the server returns the key as the “s” value of JSON. Also, this malware has self-update function. When the server responds “URL” value, the content in the URL is used instead of “2.png”. However, servers do not always respond to the request or return the secret key.

Figure 5. Updated payload response

As always, the most malicious functions reveal themselves in the final stage. The malware hijacks the Notification Listener to steal incoming SMS messages like Android Joker malware does, without the SMS read permission. Like a chain system, the malware then passes the notification object to the final stage. When the notification has arisen from the default SMS package, the message is finally sent out using WebView JavaScript Interface.

Figure 6. Notification delivery flow

As a result of our additional investigation on C2 servers, following information was found, including carrier, phone number, SMS message, IP address, country, network status, and so forth—along with auto-renewing subscriptions:

Figure 7. Leaked data

Further threats like these to come?

We expect that threats which take advantage of Notification Listener will continue to flourish. The McAfee Mobile Research team continues to monitor these threats and protect customers by analyzing potential malware and working with app stores to remove it. Further, using McAfee Mobile Security can detect such threats and protect you from them via its regular updates. However, it’s important to pay attention to apps that request SMS-related permissions and Notification Listener permissions. Simply put, legitimate photo and wallpaper apps simply won’t ask for those because they’re not necessary for such apps to run. If a request seems suspicious, don’t allow it.

Technical Data and IOCs

MITRE ATT&CK Matrix

IoCs

08C4F705D5A7C9DC7C05EDEE3FCAD12F345A6EE6832D54B758E57394292BA651 com.studio.keypaper2021
CC2DEFEF5A14F9B4B9F27CC9F5BBB0D2FC8A729A2F4EBA20010E81A362D5560C com.pip.editor.camera
007587C4A84D18592BF4EF7AD828D5AAA7D50CADBBF8B0892590DB48CCA7487E org.my.favorites.up.keypaper
08FA33BC138FE4835C15E45D1C1D5A81094E156EEF28D02EA8910D5F8E44D4B8 com.super.color.hairdryer
9E688A36F02DD1B1A9AE4A5C94C1335B14D1B0B1C8901EC8C986B4390E95E760 com.ce1ab3.app.photo.editor
018B705E8577F065AC6F0EDE5A8A1622820B6AEAC77D0284852CEAECF8D8460C com.hit.camera.pip
0E2ACCFA47B782B062CC324704C1F999796F5045D9753423CF7238FE4CABBFA8 com.daynight.keyboard.wallpaper
50D498755486D3739BE5D2292A51C7C3D0ADA6D1A37C89B669A601A324794B06 com.super.star.ringtones

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