cis-ubuntu2004-v300-5-4-1-1

5.4.1.1 Ensure password expiration is configured (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

The PASS_MAX_DAYS parameter in /etc/login.defs allows an administrator to force passwords to expire once they reach a defined age.

PASS_MAX_DAYS - The maximum number of days a password may be used. If the password is older than this, a password change will be forced. If not specified, -1 will be assumed (which disables the restriction).

Rationale

The window of opportunity for an attacker to leverage compromised credentials or successfully compromise credentials via an online brute force attack is limited by the age of the password. Therefore, reducing the maximum age of a password also reduces an attacker's window of opportunity.

We recommend a yearly password change. This is primarily because for all their good intentions users will share credentials across accounts. Therefore, even if a breach is publicly identified, the user may not see this notification, or forget they have an account on that site. This could leave a shared credential vulnerable indefinitely. Having an organizational policy of a 1-year (annual) password expiration is a reasonable compromise to mitigate this with minimal user burden.

Impact

The password expiration must be greater than the minimum days between password changes or users will be unable to change their password.

Excessive password expiration requirements do more harm than good, because these requirements make users select predictable passwords, composed of sequential words and numbers that are closely related to each other. In these cases, the next password can be predicted based on the previous one (incrementing a number used in the password for example). Also, password expiration requirements offer no containment benefits because attackers will often use credentials as soon as they compromise them. Instead, immediate password changes should be based on key events including, but not limited to:

• Indication of compromise
• Change of user roles
• When a user leaves the organization.

Not only does changing passwords every few weeks or months frustrate the user, but it's also been suggested that it does more harm than good, because it could lead to bad practices by the user such as adding a character to the end of their existing password.

Audit Procedure

Command Line

Run the following command and verify PASS_MAX_DAYS is set to 365 days or less and conforms to local site policy:

# grep -Pi -- '^\h*PASS_MAX_DAYS\h+\d+\b' /etc/login.defs

Example output:

PASS_MAX_DAYS 365

Run the following command to verify all /etc/shadow passwords PASS_MAX_DAYS:

• is greater than 0 days
• is less than or equal to 365 days
• conforms to local site policy

# awk -F: '($2~/^\$.+\$/) {if($5 > 365 || $5 < 1)print "User: " $1 " PASS_MAX_DAYS: " $5}' /etc/shadow

Expected Result

Nothing should be returned

Remediation

Command Line

Set the PASS_MAX_DAYS parameter to conform to site policy in /etc/login.defs:

PASS_MAX_DAYS 365

Modify user parameters for all users with a password set to match:

# chage --maxdays 365 <user>

Edit /etc/login.defs and set PASS_MAX_DAYS to a value greater than 0 that follows local site policy: Example:

PASS_MAX_DAYS 365

Run the following command to modify user parameters for all users with a password set to a maximum age no greater than 365 or less than 1 that follows local site policy:

# chage --maxdays <N> <user>

Example:

# awk -F: '($2~/^\$.+\$/) {if($5 > 365 || $5 < 1)system ("chage --maxdays 365 " $1)}' /etc/shadow

Warning: If a password has been set at system install or kickstart, the last change date field is not set, In this case, setting PASS_MAX_DAYS will immediately expire the password. One possible solution is to populate the last change date field through a command like: chage -d "$(date +%Y-%m-%d)" root

Default Value

PASS_MAX_DAYS 99999

References

1. CIS Password Policy Guide
2. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

CIS Controls

v8 - 5.2 Use Unique Passwords: Use unique passwords for all enterprise assets. Best practice implementation includes, at a minimum, an 8-character password for accounts using MFA and a 14-character password for accounts not using MFA. (IG 1, IG 2, IG 3)

v7 - 4.4 Use Unique Passwords: Where multi-factor authentication is not supported (such as local administrator, root, or service accounts), accounts will use passwords that are unique to that system. (IG 2, IG 3)

MITRE ATT&CK Mappings: T1078, T1078.001, T1078.002, T1078.003, T1078.004, T1110, T1110.001, T1110.002, T1110.003, T1110.004