Dimitrios Valsamaras

A cybersecurity professional with expertise in mobile, web, and network penetration testing. Dimitrios holds a degree in Computer Science, majoring in Cryptography and Security, and has worked with top companies like Microsoft and Google. He is frequent speaker at prominent security conferences such as BlackHat, Nullcon, Insomni'hack, and Troopers. He is passionate about reverse engineering and was a member of one of Greece's first reverse engineering research groups.


Permissionless Universal Overlays
Dimitrios Valsamaras

Both Android and iOS operating systems interact with the users using a constrained graphical interface, typically occupied at its majority by one application at a time while many of them can run in the background. That being said, a user must rely on the GUI provided by the application itself to verify its legitimacy. This type of behavior has raised concerns within the security research community that have been proved to be well founded, judging from the fact that multiple malware campaigns use GUI confusion as their main attack vector.

In this paper we present a novel GUI attack that leverages the fact that an Android activity maintains its graphical state and can receive touches, while it's in the top of the back stack of the device home screen. Whilst most of the techniques that have been introduced so far require the SYSTEM_ALERT_WINDOW permission, the one we present is permissionless and makes use only of the FLAG_NOT_TOUCH_MODAL flag.

By using this technique, we were able to create overlapping views over system dialogues, luring the user to unintentionally approve dangerous permissions and access to system services. Third party applications are also at risk, as it is possible to garble their UI by projecting fraudulent views that ostensibly belong to the targeted application's context. For the latter to be successful, the PACKAGE_USAGE_STATS permission must be obtained in order to identify the application that is currently in the foreground.

Google addressed the issue (CVE-2021-39617) by not dispatching touches to critical decision windows which are fully or partially obscured, but 3rd party applications are still affected.

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