Trojan:Android/Koler is an Android app being promoted as part of a ransomware scheme. On installation, the app displays a (fake) notice stating the device has been 'locked' and demanding payment.
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Trojan:Android/Koler.A is reportedly a mobile-targeted extension of the Reveton ransomware network.
If a user browsing online on an Android device lands on a malicious webpage, they will be prompted to download an app. Unlike similar attacks against users browsing on a PC or Mac machine, download of the file is neither silent nor automatic; the user must confirm the download and manually install the app. To encourage download and installation, the app is promoted as providing access to adult materials.
On installation, the app sends the device' International Mobile Subscriber Identity (IMEI) number to a remote server. It then opens a browser page that displays a (fake) notice over the Home screen saying the device has been 'locked due to security violations and all files have been encrypted', or words to that effect.
The specific wording of the message varies depending on the user's geographical location. For example, users in Finland will see a Finnish-localized message, users in Sweden will see Swedish-language text and users in the UK or US will see English text.
The message also demands payment of a 'fine', supposedly to decrypt the files. The app does not actually encrypt files on the device, but does interfere with normal usage of the device by displaying the notice repeatedly until the 'fine' is paid.