Some apps hide on the devices as calculator or calendar. However, they could track all the typing, including web searches, text messages and emails. Such apps are called stalkerware. These are available on Google’s Play Store, and on App Store. They have innocuous names such as MobileTool, Agent, and Cerberus.

Increasingly large number of devices are being infected by stalkerware. An app called SpyFone gets access to a victim’s location, photos and messages.

These apps are invasive. Some surveillance apps have legitimate uses, e.g. parental control software for kids online to protect them from predators. However, when this software sneaks into the phone furtively, it becomes stalkerware. Thus this is a grey area.

Android phones are prone to having such apps since it is open source. Google has banned apps which violate its policies, e.g. the Flash Key logger app.

Previously, these apps were installed on PCs, but they are now focused on cell phones. Cell phones have more personal and intimate data.

Some apps record phone calls, some log key strokes, some spot the location and some upload photos to a remote server. These apps lurk in the background.

Always look for the unusual behaviour on the device, say a rapidly draining battery. It means a stalkerware is running in the background. There are some apps which detect stalkerware, e.g. Certo, Lookout, Norton LifeLock and MalwareBytes.

It is necessary to keep changing the passwords for important online accounts.