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Android Trojans Exploit Marshmallow's Permission Model

The authors of Android Trojans have adapted their attack models to target the permission-granting model that Google has introduced in Android 6.0 Marshmallow, security firm Symantec has discovered.

Google improved the security of Android devices with the introduction of a new permission-granting model in the latest platform iteration, one designed to prompt users to grant permissions only when apps require them. As it turns out, Trojans such as Android.Bankosy and Android.Cepsohord have found a way to leverage the new restrictions.

As Symantec’s Principal Threat Analysis Engineer Dinesh Venkatesan explains in a blog post, apps now request permissions that could pose a privacy risk at runtime, instead of asking for all of them during installation.

However, there is an exception to this rule: applications with the “target_sdk” attribute set to less than 23 don’t trigger the permission requesting dialogue at runtime. Basically, if an app has the said attribute set to 22, all of the requested permissions could be granted during the app’s installation, albeit the user can manually revoke permissions for any app at any time.

Malware authors continue to use the older permission model that allows them to get authorization for all permissions at once, but Bankosy and Cepsohord switched to the new model, which allows them to effectively target Marshmallow devices. This is not surprising, given that the latest Android version is picking up market share.

Symantec researchers discovered that Bankosy adopted the new model by checking whether a permission to call a specific number is in a non-revoked state by using Marshmallow’s checkSelfPermission API. If the permission is granted, the malware can enable unconditional call forwarding on the compromised device.

Detailed in early January, Bankosy is a financial Trojan capable of deceiving voice call-based two-factor authorization (2FA) systems, and call forwarding is essential for its nefarious operations. The threat was also designed to intercept incoming SMS, delete SMS messages, wipe data, and perform other actions usually supported by financial Trojans.

It also works as a backdoor and includes support for enabling and disabling silent mode and for locking the device, so that the victim is not alerted on incoming calls. The Trojan steals user’s login credentials and, armed with this information and with the two-factor authentication and authorization codes, allows attackers to make fraudulent transactions from the victim’s accounts.

The second Trojan to have adopted the Marshmallow permissions model is click-fraud malware Cepsohord. Unlike Bankosy, however, this threat not only checks the permission’s authorization state, but also prompts the user to authorize the permission in case it is in a revoked state.

As always, it’s important that users download applications only from known, trusted sources and to pay close attention to the permissions that these programs request. Moreover, users are advised to keep their devices updated at all times and to install and maintain up-to-date malware protection.

Related: Upgraded Android Banking Trojan Targets Users in 200 Countries

Related: Android Trojan Steals Credit Card Info, Locks Devices Remotely

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