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Table of Contents

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

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).

Modifications of the PAM can have unexpected consequences. To re-write the PAM configuration files, destroying any manually made changes and replacing them with a series of system defaults please use authconfig or system-config-authentication.

Before start this chapter please read:

• Using Pluggable Authentication Modules (PAM) ^[Official]
• The Linux-PAM System Administrators' Guide

Useful resources

• Set a password policy in Red Hat Enterprise Linux 7 ^[Official]
• How to configure password complexity [...] using pam_passwdqc.so ^[Official]
• Hardening Your System With Tools and Services ^[Official]
• Configuring Services: PAM ^[Official]

Password hashing algorithm

Rationale

Currently more used is SHA-512 based hash (sha512crypt), which is similar in structure to md5crypt and sha256crypt and but support variable amounts of iteration. It's marked with 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/pam.d/system-auth

password sufficient pam_unix.so sha512 shadow nullok try_first_pass use_authtok

^{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:

# 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

^{C2S/CIS: CCE-26884-7 (Medium); C2S/CIS: 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:

# For the pam_unix.so:
password sufficient pam_unix.so sha512 shadow nullok try_first_pass use_authtok remember=5

# For the pam_pwhistory.so:
password required pam_pwhistory.so debug use_authtok remember=5

^{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

• RHEL 7 and pam_pwhistory - old password can still be re-used ^[Official]
• OWASP (OTG-AUTHN-007)

Password quality requirements

Rationale

The pam_pwquality PAM module can be configured to meet requirements for a variety of policies.

C2S/CIS allows modified these arguments to ensure compliance with your organization's security policy.

Use of a complex or strength password helps to increase the time and resources required to compromise the password.

• minlen parameter controls requirements for minimum characters required in a password. The shorter the password, the lower the number of possible combinations that need to be tested before the password is compromised.
• dcredit parameter controls requirements for usage of digits in a password.
• lcredit parameter controls requirements for usage of lowercase letters in a password.
• ucredit parameter controls requirements for usage of uppercase letters in a password.

Solution

Check pam_pwquality and set password retry prompts

Setting the password retry prompts that are permitted on a per-session basis to a low value requires some software, such as SSH, to re-connect.

The DoD requirement is a maximum of 3 prompts per session.

# Edit /etc/pam.d/system-auth:
password requisite pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type=

^{C2S/CIS: CCE-27160-1 (Unknown)}

Set password minimum length

# Edit /etc/security/pwquality.conf:
minlen = 14

^{C2S/CIS: CCE-27293-0 (Medium)}

Set password strength

# Edit /etc/security/pwquality.conf:
dcredit = -1
lcredit = -1
ucredit = -1

^{C2S/CIS:CCE-27214-6 (Medium); C2S/CIS:CCE-27345-8 (Medium); C2S/CIS:CCE-27200-5 (Medium)}

Comments

Official C2S/CIS standard also explain the following parameters. However, it doesn't specify identifiers for them.

• difok sets the minimum number of characters that must be different from the previous password. If you increase minlen, you may also want to increase this value as well.
• ocredit sets the maximum credit for having other characters in the new password.
• maxrepeat reject passwords which contain more than N same consecutive characters.

difok = 4
ocredit = -1
maxrepeat = 3

If you want to check password strengths, you should use cracklib-check:

cat > ~/passwd.test << __EOF__
for i in aaa password \$RANDOM \$(pwgen 12) ighu6zaivoomahPhah ; do

  echo -en "Check password: \$i\\n"
  echo "\$i" | cracklib-check

done
__EOF__

$ bash ~/passwd-test
Check password: aaa
aaa: it is WAY too short
Check password: password
password: it is based on a dictionary word
Check password: 29997
29997: it is too short
Check password: Ociechai2moh
Ociechai2moh: OK
Check password: ighu6zaivoomahPhah
ighu6zaivoomahPhah: OK

Useful resources

• How to set password complexity in redhat 7.5?