The culprit is actually Samsung's S-Memo app, and all the passwords stored in it can be accessed by anyone who has root access to the device.
Normally, that would mean only its owner, but since some users root their devices in order to customize their mobile experience as they see fit, the passwords contained in the app can be seen by anyone who has physical access to the device.
The fact was discovered by a XDA Recognized Developer and Forum Moderator, after he poked around his own rooted Samsung Galaxy S3 device one afternoon and discovered that the app's SQLite files contained his Google account password in plaintext format.
The great majority of users don't bother with rooting their smartphones, so they are safe for now. Still, the S-Memo app should encrypt passwords stored in it, and I'm sure we can expect an update that will fix the vulnerability soon.