Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that communicates between the user and an application. PAM modules are available on a system wide basis, so they can be requested by any application. Technically, PAM is used by many UNIX (Solaris/BSD/AIX/HP-UX) operating systems as well as Linux for configuring authentication related services.
PAM allows for applications, such as system daemons, to share a single, unified authentication widget. Otherwise, each application would have to include an individually written system which would result in weaker overall security and issues with compatibility. PAM modules, which are a set of shared libraries for a specific authentication mechanism.
PAM is not itself a password database, but rather its configuration tells the system how exactly to do the authentication. There is a PAM module that is used to authenticate a user from /etc/.shadow called pam_unix.so - a module used to authenticate all users local and remote during login.
- Options can be passed to pam_unix in the /etc/pam.d/login file
Linux use following two files:
/etc/pam/common-auth: authentication settings common to all services /etc/pam.d/common-password: password-related modules common to all services
sample PAM application configuration file:
#%PAM-1.0 auth required pam_securetty.so auth required pam_unix.so nullok auth required pam_nologin.so account required pam_unix.so password required pam_cracklib.so retry=3 password required pam_unix.so shadow nullok use_authtok session required pam_unix.so
KEY Configuration lines:
auth required pam_securetty.so — This module ensures that if the user is trying to log in as root, the tty on which the user is logging in is listed in the /etc/securetty file, if that file exists.
auth required pam_unix.so nullok — This module prompts the user for a password and then checks the password using the information stored in /etc/passwd and, if it exists, /etc/shadow.
auth required pam_nologin.so — This is the final authentication step. It checks whether the /etc/nologin file exists. If it exists and the user is not root, authentication fails.
disable null passwords
Open /etc/pam/common-auth and find the line that reads:
password required pam_unix.so nullok obscure min=4 max=8 md5
and remove the following parameter:
nullok
Save the file and exit to shell prompt. Now open file /etc/pam.d/common-password and find the line that reads:
auth required pam_unix.so nullok_secure
Remove the following parameter:
nullok_secure
Now save the file and exit to shell prompt. These changes will disable any user's able to login using null password.
source: Linux or UNIX disable null passwords by by NIX Craft.
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