Kaspersky has discovered a malicious version of a popular WhatsApp messenger mod (unofficial modification of the app) called FMWhatsapp.

The mod spreads the Triada mobile Trojan, which downloads other Trojans and can launch ads, issue subscriptions, and intercept a user’s SMSs.

Even though WhatsApp is one of the most popular apps for instant mobile messaging, not all users are satisfied with its features. Looking for the most user-friendly version, people can be tempted to install modified versions of WhatsApp, which provide many more options than the official one (such as choosing dynamic templates or the ability to read deleted messages).

In such apps, creators often publish various ads to monetise their work. On the flip side, there are fraudsters taking advantage of this, often distributing malicious code through advertising. One example of this is FMWhatsapp – the 16.80.0 version – which includes the Triada Trojan and one of the ad libraries.

In the dangerous version of the FMWhatsapp mod, the Triada Trojan acts as a mediator. First, it collects data about the user’s mobile device and then, at the owner’s command, downloads one of the other Trojans to the smartphone.

These Trojans can independently launch ads, issue paid subscriptions to the device owner and even log into the WhatsApp account, intercepting the SMS to confirm login – leaving the victim vulnerable to illegal activity through their phone.

“With this app, it is hard for users to recognise the potential threat because the mod application actually does what is proposed – it adds additional features,” comments Igor Golovin, security expert at Kaspersky. “However, we have observed how cybercriminals have started to spread malicious files through the ad blocks in such apps.

“That is why we recommend you only use messenger software downloaded from official app stores. They may lack some additional functions, but they will not install a bunch of malware on your smartphone.”

Kaspersky solutions detected the malicious implant as Trojan.AndroidOS.Triada.ef.