A flaw was found in Linux-Pam 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.
Introduced in version 1.5.0.
Created pam tracking bugs for this issue:
Affects: fedora-all [bug 1901173]
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):