As documented in pam_unix(8), the nullok_secure option means that null passwords are allowed for this module only when connected via one of the "secure" (that is, generally speaking, "local") ttys listed in /etc/securetty. The tty is not determined directly by pam_unix, but is instead provided to the PAM stack by the calling application setting the PAM_TTY value with pam_set_item(). This is further documented in pam_set_item(3) to be:
PAM_TTY The terminal name: prefixed by /dev/ if it is a device file; for graphical, X-based, applications the value for this item should be the $DISPLAY variable. If gdm is not setting the PAM_TTY item, then that's a bug in gdm, not in PAM. The /etc/securetty list of values is regrettably not comprehensive (because e.g. it doesn't support pattern matching), so if your system has been changed to run gdm on an X display beyond :3, the default value will not work for you. However, I don't see any reason this would occur unless the settings of gdm itself had been changed away from the defaults, and if you've done this you will need to adjust the contents of /etc/securetty to match. ** Changed in: pam (Ubuntu) Status: Triaged => Invalid -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to pam in Ubuntu. https://bugs.launchpad.net/bugs/1561302 Title: gdm won't allow passwordless login Status in gdm: Confirmed Status in Ubuntu GNOME: Triaged Status in casper package in Ubuntu: New Status in gdm3 package in Ubuntu: Triaged Status in gnome-shell package in Ubuntu: New Status in pam package in Ubuntu: Invalid Bug description: The Live session user does not have a password set, however gdm won't allow login in this case since the "Sign In" button is disabled. Normally this should be too much of an issue, since the login screen should only be hit on logout from the live session, some people are reporting hitting the gdm login screen on boot due to ubiquity-dm failing to boot. ProblemType: Bug DistroRelease: Ubuntu 16.04 Package: gdm3 3.18.2-1ubuntu1 ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6 Uname: Linux 4.4.0-15-generic x86_64 ApportVersion: 2.20-0ubuntu3 Architecture: amd64 CurrentDesktop: GNOME Date: Thu Mar 24 13:55:19 2016 InstallationDate: Installed on 2016-03-23 (0 days ago) InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Beta amd64 (20160323) SourcePackage: gdm3 UpgradeStatus: No upgrade log present (probably fresh install) To manage notifications about this bug go to: https://bugs.launchpad.net/gdm/+bug/1561302/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp