cis-ubuntu1604-v200-4-1-10

DevOps & Security

Ensure unsuccessful unauthorized file access attempts are collected

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CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 4.1.10

Profile

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

Description

Monitor for unsuccessful attempts to access files. The parameters below are associated with system calls that control creation (creat), opening (open, openat) and truncation (truncate, ftruncate) of files. An audit log record will only be written if the user is a non-privileged user (auid >= 1000), is not a Daemon event (auid=4294967295) and if the system call returned EACCES (permission denied to the file) or EPERM (some other permanent error associated with the specific system call). All audit records will be tagged with the identifier "access."

Note: Reloading the auditd config to set active settings requires the auditd service to be restarted, and may require a system reboot.

Rationale

Failed attempts to open, create or truncate files could be an indication that an individual or process is trying to gain unauthorized access to the system.

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 access /etc/audit/rules.d/*.rules

Verify output matches:

-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access

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

auditctl -l | grep access

Verify output matches:

-a always,exit -F arch=b32 -S open,creat,truncate,ftruncate,openat -F exit=-EACCES -F auid>=1000 -F auid!=-1 -F key=access
-a always,exit -F arch=b32 -S open,creat,truncate,ftruncate,openat -F exit=-EPERM -F auid>=1000 -F auid!=-1 -F key=access

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 access /etc/audit/rules.d/*.rules

Verify output matches:

-a always,exit -F arch=b64 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b64 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access

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

auditctl -l | grep access

Verify output matches:

-a always,exit -F arch=b64 -S open,truncate,ftruncate,creat,openat -F exit=-EACCES -F auid>=1000 -F auid!=-1 -F key=access
-a always,exit -F arch=b32 -S open,creat,truncate,ftruncate,openat -F exit=-EACCES -F auid>=1000 -F auid!=-1 -F key=access
-a always,exit -F arch=b64 -S open,truncate,ftruncate,creat,openat -F exit=-EPERM -F auid>=1000 -F auid!=-1 -F key=access
-a always,exit -F arch=b32 -S open,creat,truncate,ftruncate,openat -F exit=-EPERM -F auid>=1000 -F auid!=-1 -F key=access

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-access.rules

Add the following lines:

-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access

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-access.rules

Add the following lines:

-a always,exit -F arch=b64 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EACCES -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b64 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access
-a always,exit -F arch=b32 -S creat -S open -S openat -S truncate -S ftruncate -F exit=-EPERM -F auid>=1000 -F auid!=4294967295 -k access

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 unauthorized file access attempts.

References

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Section 4.1.10

CIS Controls

Controls Version	Control
v7	14.9 Enforce Detail Logging for Access or Changes to Sensitive Data - Enforce detailed audit logging for access to sensitive data or changes to sensitive data (utilizing tools such as File Integrity Monitoring or Security Information and Event Monitoring).