/etc/pam.d/lightdm-greeter is in lightdm 1.18.3-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | #%PAM-1.0
# Load environment from /etc/environment and ~/.pam_environment
session required pam_env.so readenv=1
session required pam_env.so readenv=1 envfile=/etc/default/locale
# Always let the greeter start without authentication
auth required pam_permit.so
# No action required for account management
account required pam_permit.so
# Can't change password
password required pam_deny.so
# Setup session
session required pam_unix.so
session optional pam_systemd.so
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