cis-ubuntu2004-v300-5-2-7

DevOps & Security

Ensure access to the su command is restricted

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Ensure access to the su command is restricted (Automated)

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Description

The su command allows a user to run a command or shell as another user. The program has been superseded by sudo, which allows for more granular control over privileged access. Normally, the su command can be executed by any user. By uncommenting the pam_wheel.so statement in /etc/pam.d/su, the su command will only allow users in a specific groups to execute su. This group should be empty to reinforce the use of sudo for privileged access.

Rationale

Restricting the use of su, and using sudo in its place, provides system administrators better control of the escalation of user privileges to execute privileged commands. The sudo utility also provides a better logging and audit mechanism, as it can log each command executed via sudo, whereas su can only record that a user executed the su program.

Audit Procedure

Command Line

Run the following command:

# grep -Pi '^\h*auth\h+(?:required|requisite)\h+pam_wheel\.so\h+(?:[^#\n\r]+\h+)?(?:(?!\2)(use_uid\b|group=\H+\b))\h+(?:[^#\n\r]+\h+)?(?:(?!\1)(use_uid\b|group=\H+\b))(\h+.*)?#x27; /etc/pam.d/su

Verify the output matches:

auth required pam_wheel.so use_uid group=<group_name>

Run the following command and verify that the group specified in <group_name> contains no users:

# grep <group_name> /etc/group

Verify the output does not contain any users in the relevant group:

<group_name>:x:<GID>:

Expected Result

The pam_wheel.so line should be configured with use_uid and a group= parameter, and the specified group should contain no users.

Remediation

Command Line

Create an empty group that will be specified for use of the su command. The group should be named according to site policy. Example:

# groupadd sugroup

Add the following line to the /etc/pam.d/su file, specifying the empty group:

auth required pam_wheel.so use_uid group=sugroup

References

NIST SP 800-53 Rev. 5: AC-3, MP-2

CIS Controls

v8 - 3.3 Configure Data Access Control Lists Configure data access control lists based on a user's need to know. Apply data access control lists, also known as access permissions, to local and remote file systems, databases, and applications.

v7 - 4.3 Ensure the Use of Dedicated Administrative Accounts Ensure that all users with administrative account access use a dedicated or secondary account for elevated activities. This account should only be used for administrative activities and not internet browsing, email, or similar activities.

MITRE ATT&CK Mappings: T1548 | TA0004 | M1026