useradd in shadow-utils before 4.0.3, and possibly other versions before 4.0.8, does not provide a required argument to the open function when creating a new user mailbox, which causes the mailbox to be created with unpredictable permissions and possibly allows attackers to read or modify the mailbox.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.001
EPSS Ranking 28.3%