Root fails to login with login incorrect

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Environment

SUSE Linux Enterprise Server 12 Service Pack 4 (SLES 12 SP4)
SUSE Linux Enterprise Server 12 Service Pack 3 (SLES 12 SP3)

Situation

When attempting to login as root or any other user, the error message "login incorrect" appears. Some systems fail to boot entirely.

The following error messages were observed.

login: pam_unix(login:auth): check pass: user unknown
login: pam_unix(login:auth): authentication failure; logname=LOGIN uid=0 euid=0 tty=tty1 ruser= rhost=
login: FAILED LOGIN 1 FROM tty1 FOR (unknown), User not known to the underlying authentication module

Some system services may be affected with similar errors:

[FAILED] Failed to start OpenSSH Daemon.
[FAILED] Failed to start wicked AutoIPv4 supplicant service.
[FAILED] Failed to start wicked DHCPv6 supplicant service.
[FAILED] Failed to start wicked DHCPv4 supplicant service.

There was no /etc/nologin present. However, the root user is found correctly in the authentication files.

# grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash

# ls -al /etc/passwd
-rw------- 1 root root 1060 Oct 26  2018 /etc/passwd

# grep passwd /etc/nsswitch.conf
passwd: compat

# getent passwd | grep root
root:x:0:0:root:/root:/bin/bash

Resolution

Change the /etc/passwd permissions from 600 to 644.

# chmod 644 /etc/passwd

Cause

Invalid permissions on the /etc/passwd file.

Additional Information

Disclaimer

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