PAM Module

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Table of Contents

• PAM Module
  • Password hashing algorithm
  • Failed password attempts

PAM Module

Linux-PAM is a library that enables the local system administrator to choose how individual applications authenticate users. It offers multiple low-level authentication schemes into a high-level application programming interface (API).

Password hashing algorithm

Rationale

Currently more used are the SHA-256 and SHA-512 based hashes, sha256crypt and sha512crypt, which are similar in structure to md5crypt but support variable amounts of iteration. They're marked with $5$ and $6$ respectively. sha512crypt ($6$) is what at least RedHat/CentOS and Debian (generally most modern distros) currently use by default.

Solution

Set properly password hashes in /etc/shadow

# C2S/CIS: CCE-27104-9 (Medium)

password  sufficient  pam_unix.so sha512 shadow nullok try_first_pass use_authtok

Policies

C2S/CIS: CCE-27104-9 (Medium)

Useful resources

• Using Pluggable Authentication Modules (PAM) ^[Official]
• About Secure Password Hashing

Failed password attempts

Rationale

This option provides the capability to lock out user accounts after a number of failed login attempts.

Locking out user accounts presents the risk of a denial-of-service attack.

Solution

Set lockout time

Edit AUTH and ACCOUNT (for the last parameter) section of both /etc/pam.d/system-auth and /etc/pam.d/password-auth:

# C2S/CIS: CCE-26884-7 (Medium)

# Add the following line immediately before the pam_unix.so
auth required pam_faillock.so preauth silent deny=5 unlock_time=900 fail_interval=900

# Add the following line immediately after the pam_unix.so
auth [default=die] pam_faillock.so authfail deny=5 unlock_time=900 fail_interval=900

# Add the following line immediately before the pam_unix.so
account required pam_faillock.so

Policies

C2S/CIS: CCE-26884-7 (Medium)

Comments

If you want, you can use a more restrictive configuration (I personally prefer this way):

auth required pam_faillock.so preauth silent deny=3 unlock_time=1800 fail_interval=900

auth [default=die] pam_faillock.so authfail deny=3 unlock_time=1800 fail_interval=900

Other guides recommend setting the FAILLOG_ENAB and FAIL_DELAY params in /etc/login.defs configuration file. It's incorrect solution beacuse login.defs is no longer used by login, su and passwd (see manpage) unless you use pam_pwcheck.

Useful resources

• How to Lock User Accounts After Failed Login Attempts