Researchers from Avast the antivirus firm are investigating the discovery of pre-installed malware found in 141 low-cost Android devices in over 90 countries.
The operation is linked to the discovery made in December 2016 by researchers at antivirus firm Dr. Web, when the experts reported a crime gang that had compromised the supply-chain of several mobile carriers, infecting mobile devices with malware.
“The adware we analyzed has previously been described by Dr. Web and goes by the name “Cosiloon.” As can be seen in the screenshots below, the adware creates an overlay to display an ad over a webpage within the users’ browser. The adware has been active for at least three years, and is difficult to remove as it is installed on the firmware level and uses strong obfuscation.” reads the analysis published by Avast.
“Thousands of users are affected, and in the past month alone we have seen the latest version of the adware on around 18,000 devices belonging to Avast users located in more than 100 countries including Russia, Italy, Germany, the UK, as well as some users in the U.S.”
Avast published a list of over 140 Android smartphones and tablets on which it says it found the group’s malware —which they named Cosiloon.
The Cosiloon malware is the same that was spotted in 2015 by Dr. Web and according to the experts it hasn’t received any updates.
The malware is composed of two separate APKs, the dropper, and the payload. In the older versions of the malware, the experts noticed a separate adware app pre-installed in the /system partition, in most recent variants the researchers found a new dropped payload.
“A second variant of the dropper is a bit more interesting. The code is pretty much the same as the first variant, but it is not a separate system application. The code is embedded in SystemUI.apk, an integral part of the Android OS. This makes the dropper pretty much impossible to remove by the user.” continues the analysis.
The dropper runs from the “/system” folder with full root privileges, it downloads an XML file from a remote server and then installs other malicious apps.
In almost any infection, the malicious codes were used to display ads on top of mobile apps or the Android OS interface.
The experts noticed the pre-installed malware doesn’t drop any malicious app if the device language is set to Chinese, when the device’s public IP address is also from a Chinese IP range, and when the number of installed apps is below three (a circumstance that could indicate that the malware is running in a test environment).
Avast researchers confirmed that the infection point is still a mystery due to the large number of vendors involved, the detection of the dropper in very complicated as explained in the analysis.
“Detecting the dropper is further complicated by the fact that it is a system app, part of the devices’ read-only firmware, which is integrated in the device shipped from the factory.” continues the analysis.
“Also, it is likely odexed in most firmwares, meaning the app’s code was removed from the original APK file, optimized and stored separately during the firmware’s build process. As a result, cybersecurity firms are likely missing many of the dropper samples and have to rely on the payload for detection and statistics.”
Experts believe the attackers are opportunistic and target in some way the supply chain at random, every time they have the possibility to compromise the firmware of the vendors.
The control server was up until April 2018, crooks have produced new payloads over the time while new devices were shipped by several manufacturers with the pre-installed dropper.
The experts have attempted to disable Cosiloon’s C&C server by sending takedown requests to the domain registrar and server providers. While the ZenLayer provider quickly shut down the server, but crooks moved their activities to another provider that did not respond to Avast’s request.
“Avast Mobile Security can detect and uninstall the payload, but it cannot acquire the permissions required to disable the dropper, so Google Play Protect has to do the heavy lifting.” concluded Avast.
“If your device is infected, it should automatically disable both the dropper and the payload. We know this works because we have observed a drop in the number of devices infected by new payload versions after Play Protect started detecting Cosiloon.”
Further details, including IoCs for the Cosiloon pre-installed malware are reported in the Avast analysis.
(Security Affairs – pre-installed malware, Cosiloon malware)