The Janus vulnerability was discovered in 2017 and was named after the Roman god Janus, who is depicted with two faces. It allows an attacker to modify the APK (Android application package) file without invalidating its digital signature, essentially allowing them to add malicious code to an existing app without being detected. This type of attack is known as a "code injection" or "Janus attack."

By exploiting the Janus vulnerability, an attacker can potentially gain access to sensitive data, perform unauthorized actions, or even take control of the affected Android device. The vulnerability affects devices running Android versions prior to 7.0 (Nougat).

To mitigate the Janus vulnerability, it is essential to keep your Android device up to date with the latest security patches and firmware updates. Android has addressed this vulnerability in subsequent updates, so ensuring that your device is running a recent version of Android is crucial.

If you suspect that your device may be affected, it is recommended to run a reputable mobile security application to scan for malware or take the device to a professional for further inspection and assistance.

Please note that my information might be outdated since my knowledge cutoff is in September 2021. It's always a good idea to refer to the latest security resources and advisories for the most up-to-date information on vulnerabilities.