A warning has been issued to Android users over a 'malicious bug' that has affected millions of devices.

The bug, called Necro, has infiltrated Android devices via certain apps and is capable of stealing personal data and signing people up for costly subscriptions without their consent.

According to the cyber security firm, Kaspersky, who first identified Necro in 2019, the malware tends to be installed on apps known as mods.

Mods are modified versions of official apps such as WhatsApp and Spotify that claim to offer additional features, Metro reports.

Most are unofficial, not on Google Play and tend to be offered at a cheaper price than their counterparts or for free.

However, a few apps on Google Play have also been infiltrated.

According to Kaspersky, among them is a photo editing app called Wuta Camera, which was downloaded 10 million times before Necro was discovered and removed.

How to keep your Android phone safe from Necro

Kaspersky offers a few tips on how you can stay safe from Necro.

Firstly, they strongly advise against downloading apps from unofficial sources, because these apps are often much easier to infiltrate with bugs.

Although you can download apps from official platforms such as Google Play, it’s best to approach each app with caution.

Check the app’s page in the store first and look to see if it has reviews with low ratings, as these can offer warnings about possible issues.

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Additionally, it's helpful to check if an app has an exceptionally high user rating with lots of rave reviews.

This is because these ratings and reviews could have been paid for and could mean the app is fake.

Finally, avoid using modified apps or hacked versions as they could have been infiltrated.