PAM – Pluggable Authentication Modules for Linux and how to edit the defaults

Originally posted on She ITs and Giggles blog.

Most of us have been using PAM when authenticating without really thinking about it, but for the few of us that have actually tried to make sense of it, PAM is the partner that always says “no”, unless otherwise stated. It’s the bane of any sysadmin’s existence when it comes to making system x secure, and it becomes a major pain point on and off when I forget about the normal rules of engagement.

Rules of Engagement

Session windows

To engage with PAM in any combative situation, please ensure belts and braces are on, and keep your arms and legs inside the vehicle at all times. Backup the /etc/pam.d/ directory, and make sure that you have one or two non-terminating sessions open on your system – ideally a console, and an ssh session.

The unexpected overwrite

In RHEL/Fedora like systems, PAM is configured to have two main files which are included by the rest of the PAM configuration: /etc/pam.d/system-auth-ac and /etc/pam.d/password-auth-ac.  Normally, system-auth and password-auth in the same /etc/pam.d directory are links to the above files. authconfig tools will overwrite the configuration in the files with a suffix of -ac. This means that if the changes need to be persistent and not overwritten, the symlinks can be set to the new location As follows:

ls -l /etc/pam.d/*-auth

lrwxrwxrwx. 1 root root 19 Feb 19 12:57 /etc/pam.d/fingerprint-auth -> fingerprint-auth-ac
lrwxrwxrwx. 1 root root 16 Feb 19 12:57 /etc/pam.d/password-auth -> password-auth-ac
lrwxrwxrwx. 1 root root 17 Feb 19 12:57 /etc/pam.d/smartcard-auth -> smartcard-auth-ac
lrwxrwxrwx. 1 root root 14 Feb 19 12:57 /etc/pam.d/system-auth -> system-auth-ac

[root@node1 pam.d]# rm password-auth
rm: remove symbolic link ‘password-auth’? y
[root@node1 pam.d]# ln -s password-auth-local password-auth

The Language

When the usual PAM translator (authconfig) is not enough to achieve the right system authentication, one has to start thinking about communicating directly with it. PAM has four different keywords for controlling authentication with the system:

awk '!/^[#\-]/{print $1}' /etc/pam.d/*  | sort | uniq

account
auth
password
session

auth is used for providing some kind of challenge/response (depending on the module)  – usually username/password.

account is used to time or otherwise restrict the user account -i.e. user must use faillock, load all the sssd account requirements etc.

password is used to update the authtoken associated with the user account. This is mainly used to change passwords and it can be where the rules around local password strength can be formulated.

session is used to determine what the user needs before they are allowed a session: working home directory, to which system limits apply (open filehandles, number of terminals, etc.), and user keyring.

Each of these keywords has  these common modes:

required - if this fails return failure but continue executing anyway
[success=ok new_authtok_reqd=ok ignore=ignore default=bad]

requisite - if this fails return failure and die (don't even attempt to preauth)
[success=ok new_authtok_reqd=ok ignore=ignore default=die]

sufficient - this is enough for success and exit if nothing previously has failed
[success=done new_authtok_reqd=done default=ignore]

optional - we don't care unless this is the only module in the stack associated with this type
[success=ok new_authtok_reqd=ok default=ignore]

PAM execution stack

PAM executes everything sequentially unless told otherwise. The following snippet of password-auth-ac will:

# set environment variables
auth required pam_env.so
 
# delay (ms) by this amount if last time this user failed
# otherwise if absent check login.defs for specified delay
auth required pam_faildelay.so delay=2000000
 
# check user authentication from the system 
# try_first_pass - use the password that has already been entered if any
# nullok allow - allow blank password
auth sufficient pam_unix.so nullok try_first_pass
 
# succeed if uid is greater than 1000 and don't log success to the system log
auth requisite pam_succeed_if.so uid >= 1000 quiet_success
 
# deny everything else
auth required pam_deny.so

If anything is sufficient and it succeeds then the execution stack exits for the component – i.e. auth successful when local user signs in with username/password.

PAM if statements

PAM doesn’t have very obvious if statements, but given the right parameters it allows jumps of execution. Below is a way of incorporating an SSSD back-end with PAM to allow users with IdM logins access to the system:

# check if the user is allowed to log in with preauthorisation (i.e. has faillock entries)
auth        required      pam_faillock.so preauth silent audit deny=5 unlock_time=900
 
# skip two rules if successful 
# NOTE: default ignore  means sufficient
# and check if it's a unix user - use the password provided by the auth stack
auth        [success=2 new_authtok_reqd=done default=ignore] pam_unix.so try_first_pass
 
# if it's not a unix user, then use sssd backend for logging in
auth        sufficient   pam_sss.so forward_pass
 
# otherwise fail 
auth        [default=die] pam_faillock.so authfail audit deny=5 unlock_time=900
 
# this is the skip step from pam_unix module
# it allows for resetting the faillock when necessary
auth        sufficient    pam_faillock.so authsucc audit deny=5 unlock_time=900

The comments in line explain what each module is doing. The execution sequence reminds me a bit of jump statements in assembly language and it helps me to think about them in that manner.

Putting them all together gives us this auth section:

auth        required      pam_env.so
auth        required      pam_faildelay.so delay=2000000
auth        required      pam_faillock.so preauth silent audit deny=5 unlock_time=900
auth        [success=2 new_authtok_reqd=done default=ignore] pam_unix.so try_first_pass 
auth        sufficient pam_sss.so forward_pass
auth        [default=die] pam_faillock.so authfail audit deny=5 unlock_time=900 
auth        sufficient pam_faillock.so authsucc audit deny=5 unlock_time=900
auth        required      pam_deny.so

If we were to make slight adjustments to the above snippet, it may have the frightening effect of allowing users to log in without having the correct password:

auth        required      pam_env.so
auth        required      pam_faildelay.so delay=2000000
auth        required      pam_faillock.so preauth silent audit deny=5 unlock_time=900
# reducing this number from 2 to 1 (success=1)
auth        [success=1 new_authtok_reqd=done default=ignore] pam_unix.so try_first_pass
auth        sufficient pam_sss.so forward_pass
# swapping these two lines
auth        sufficient pam_faillock.so authsucc audit deny=5 unlock_time=900
auth        [default=die] pam_faillock.so authfail audit deny=5 unlock_time=900
auth        required      pam_deny.so

Conclusion

PAM is a very powerful, yet quite obscure tool. It can be configured to allow people in without even a valid password, or it can deny everyone access apart from every alternate Tuesday between 19:00 and 20:00 (in combination with other tools). Whenever I have configured it, I have found it useful to test for access allowed, access denied, and access locked in order to ensure predictable operation.

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