Level 31
A hidden web server always running in the background was found by security researcher Robert Baptiste in ES File Explorer, an Android file manager with over 100,000,000 installs displayed on the Google Play store page and over 500 million users worldwide according to its developer.

As discovered by Baptiste, right after launching the app it will start a local HTTP server on port 59777 which will stay open until all the background services of ES File Explorer are killed:

Everytime a user is launching ES File Explorer, a HTTP server is started. This server is opening locally the port 59777. An attacker connected on the same local network than the victim, can obtain a lot of juicy information (device info, app installed, ...) about the victim's phone, remotely get a file from the victim's phone and remotely launch an app on the victim's phone.