Your important personal or financial data is also on the stake once your device has been hit by any malware / virus or backdoor. Its not recommended to install the applications from unknown sources until you trust them. Therefore Google had already disabled the installations of APK from unknown sources and it could only be enabled from security settings of the phone, which makes your Android smartphone or tablet more vulnerable to the malware threats. One should only install the apps directly from Google Play Store, as it will not only support the developers of apps but also keep you protected from viruses / malware and backdoors. Some websites do use malicious code and take the benefit of web browser vulnerability to exploit user's data. Its also recommended to keep your apps updated with their latest versions, as developers continuously keep on fixing any vulnerabilities / bugs being found in their apps.
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