cis-ubuntu1604-v200-4-1-13

DevOps & Security

Ensure file deletion events by users are collected

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_16.04_LTS_Benchmark_v2.0.0/cis-ubuntu1604-v200-4-1-13/SKILL.md

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CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 4.1.13

Profile

Level: 2 - Server
Level: 2 - Workstation
Assessment Status: Automated

Description

Monitor the use of system calls associated with the deletion or renaming of files and file attributes. This configuration statement sets up monitoring for the unlink (remove a file), unlinkat (remove a file attribute), rename (rename a file) and renameat (rename a file attribute) system calls and tags them with the identifier "delete".

Note:

At a minimum, configure the audit system to collect file deletion events for all users and root.
Reloading the auditd config to set active settings requires the auditd service to be restarted, and may require a system reboot.

Rationale

Monitoring these calls from non-privileged users could provide a system administrator with evidence that inappropriate removal of files and file attributes associated with protected files is occurring. While this audit option will look at all events, system administrators will want to look for specific privileged files that are being deleted or altered.

Impact

None.

Audit Procedure

Command Line

On a 32 bit system:

Run the following command to verify the rules are contained in a .rules file in the /etc/audit/rules.d/ directory:

grep delete /etc/audit/rules.d/*.rules

Verify output matches:

-a always,exit -F arch=b32 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete

Run the following command to verify that rules are in the running auditd config:

auditctl -l | grep delete

Verify output matches:

-a always,exit -F arch=b32 -S unlink,rename,unlinkat,renameat -F auid>=1000 -F auid!=-1 -F key=delete

On a 64 bit system:

Run the following command to verify the rules are contained in a .rules file in the /etc/audit/rules.d/ directory:

grep delete /etc/audit/rules.d/*.rules

Verify output matches:

-a always,exit -F arch=b64 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete
-a always,exit -F arch=b32 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete

Run the following command to verify that rules are in the running auditd config:

auditctl -l | grep delete

Verify output matches:

-a always,exit -F arch=b64 -S rename,unlink,unlinkat,renameat -F auid>=1000 -F auid!=-1 -F key=delete
-a always,exit -F arch=b32 -S unlink,rename,unlinkat,renameat -F auid>=1000 -F auid!=-1 -F key=delete

Expected Result

Output should match the rules listed above for the appropriate architecture.

Remediation

Command Line

For 32 bit systems: Edit or create a file in the /etc/audit/rules.d/ directory ending in .rules.

Example: vi /etc/audit/rules.d/50-delete.rules

Add the following lines:

-a always,exit -F arch=b32 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete

For 64 bit systems: Edit or create a file in the /etc/audit/rules.d/ directory ending in .rules.

Example: vi /etc/audit/rules.d/50-delete.rules

Add the following lines:

-a always,exit -F arch=b64 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete
-a always,exit -F arch=b32 -S unlink -S unlinkat -S rename -S renameat -F auid>=1000 -F auid!=4294967295 -k delete

Additional Information: Systems may have been customized to change the default UID_MIN. To confirm the UID_MIN for your system, run the following command:

awk '/^\s*UID_MIN/{print $2}' /etc/login.defs

If your systems' UID_MIN is not 1000, replace auid>=1000 with auid>=<UID_MIN for your system> in the Audit and Remediation procedures.

Default Value

By default, no audit rules are configured for file deletion events.

References

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Section 4.1.13

CIS Controls

Controls Version	Control
v7	13 Data Protection - Data Protection