Your Android device may have malware installed without you even knowing it. However, these four signs may help you identify if your phone has one. CNET reported that a malware installed on your device might be hard to spot. Pesky mobile ads were claimed to be removed by a mobile app called "Ads Blocker".
Check what permission do your applications have, and check your apps that ask for permission, which are not necessary, such as games that sends a request for SMS messages access. Installing an antivirus app is also one of the most effective ways to prevent malware.
Some of the best antivirus apps are Norton, Lookout, Malwarebytes, or Bitdefender. You can also consider deleting those applications that were installed from third-party app stores.