A recent investigation into an active phishing campaign uncovered a malware family that enables attackers to turn infected Android devices into network proxies without users’ knowledge.

According to the McAfee Mobile Research team, the campaign uses text messages to trick users into downloading the malware, known as TimpDoor, which is disguised as a fake voice messaging app. Once the user closes the app after installation completes, the app’s icon disappears from the home screen and secretly starts running a background service that allows the malware to gather device information, including the operating system (OS) version and connection type.

At that point, the malware initiates a secure shell (SSH) connection to the control server to retrieve an assigned remote port. The port enables the malware to use the device as a local Socket Secure (SOCKS) proxy server for port forwarding, which attackers can then employ to access corporate networks, send spam and phishing emails, perform ad-click fraud, and launch distributed denial-of-service (DDoS) attacks.

According to McAfee’s findings, TimpDoor has been active since March 2018 and has thus far infected at least 5,000 devices.

How Threat Actors Turn Infected Android Devices Into Network Proxies

TimpDoor isn’t the first malware that’s turned infected Android devices into network proxies. In April 2017, researchers at Trend Micro detected MilkyDoor malware — a successor to DressCode malware discovered by Check Point a year earlier — which masqueraded as apps available for download on the Google Play Store.

MilkyDoor also uses remote port forwarding via SSH to protect the connection and bypass network security restrictions. But while TimpDoor was designed to keep the SSH tunnel open and the proxy server running, MilkyDoor added backdoor functionality on top of its main role as an adware integrator.

How to Defend Against TimpDoor Malware

Security professionals can defend against TimpDoor malware by following mobile threat prevention best practices, such as using an app to scan approved devices and configurations for anomalous connections. Such a strategy enables security professionals to monitor and analyze how apps behave across user devices, flag suspicious events and take action accordingly.

Sources: McAfee, Trend Micro, Check Point