shadow provides a set of utilities to deal with user accounts.
When the mailbox is created in useradd, the "open()" function does not receive the three arguments it expects while O_CREAT is present, which leads to random permissions on the created file, before fchmod() is executed.
Depending on the random permissions given to the mailbox file which is at this time owned by root, a local user may be able to open this file for reading or writing, or even executing it, maybe as the root user.
There is no known workaround at this time.
All shadow users should upgrade to the latest version:
# emerge --sync # emerge --ask --oneshot --verbose ">=sys-apps/shadow-4.0.15-r2"