A flaw was found in Linux-Pam in versions prior to 1.5.1 in the way it handle empty passwords for non-existing users. When the user doesn't exist PAM try to authenticate with root and in the case of an empty password it successfully authenticate.
Gentoo Linux Security Advisory 202012-6 - A vulnerability has been found in Linux-PAM, allowing attackers to bypass the authentication process. Versions less than 1.5.1 are affected.