PAM Module

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Table of Contents

• PAM Module
  • Password hashing algorithm
  • Failed password attempts
  • Limit password reuse

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).

Before start this chapter please see official RHEL documentation - Using Pluggable Authentication Modules (PAM).

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

• 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), CCE-27350-8 (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); CCE-27350-8 (Medium)

Comments

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 man for login.defs(5)) unless you use pam_pwcheck.

Useful resources

• How to Lock User Accounts After Failed Login Attempts

Limit password reuse

Rationale

Password history policy will set how often an old password can be reused so do not allow users to reuse recent passwords. Preventing re-use of previous passwords helps ensure that a compromised password is not re-used by a user.

The DoD STIG requirement is 5 passwords.

Solution

Set password reuse limit

Edit pam_unix.so or pam_pwhistory.so lines in /etc/pam.d/system-auth:

# C2S/CIS: CCE-26923-3 (Medium)

# For the pam_unix.so:
password sufficient pam_unix.so ...existing_options... remember=5

# For the pam_pwhistory.so:
password requisite pam_pwhistory.so ...existing_options... remember=5

Policies

C2S/CIS: CCE-26923-3 (Medium)

Comments

OWASP-OTG-AUTHN-007 provide great password policy solutions (sorry for copy-paste but it's really amazing):

• What characters are permitted and forbidden for use within a password? Is the user required to use characters from different character sets such as lower and uppercase letters, digits and special symbols?

• How often can a user change their password? How quickly can a user change their password after a previous change? Users may bypass password history requirements by changing their password 5 times in a row so that after the last password change they have configured their initial password again.

• When must a user change their password? After 90 days? After account lockout due to excessive log on attempts?

• How often can a user reuse a password? Does the application maintain a history of the user's previous used 8 passwords?

• How different must the next password be from the last password?

• Is the user prevented from using his username or other account information (such as first or last name) in the password?

Useful resources

• OWASP (OTG-AUTHN-007)