Cis Ubuntu1804 V220 5 2 3 7

5.2.3.7 Ensure unsuccessful file access attempts are collected

Profile Applicability

• Level 2 - Server
• Level 2 - Workstation

Description

Monitor for unsuccessful attempts to access files. The following parameters are associated with system calls that control files:

• creation - creat
• opening - open, openat
• truncation - truncate, ftruncate

An audit log record will only be written if all of the following criteria is met for the user when trying to access a file:

• a non-privileged user (auid>=UID_MIN)
• is not a Daemon event (auid=4294967295/unset/-1)
• if the system call returned EACCES (permission denied) or EPERM (some other permanent error associated with the specific system call)

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.

Audit

64 Bit systems

On disk configuration

Run the following command to check the on disk rules:

UID_MIN=$(awk '/^\s*UID_MIN/{print $2}' /etc/login.defs)
[ -n "${UID_MIN}" ] && awk "/^ *-a *always,exit/ \
&&/ -F *arch=b(32|64)/ \
&&(/ -F *auid!=unset/||/ -F *auid!=-1/||/ -F *auid!=4294967295/) \
&&/ -S/ \
&&(/creat/ \
 ||/open/ \
 ||/truncate/) \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/" /etc/audit/rules.d/*.rules \
|| printf "ERROR: Variable 'UID_MIN' is unset.\n"

Verify the output includes:

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

Running configuration

Run the following command to check loaded rules:

UID_MIN=$(awk '/^\s*UID_MIN/{print $2}' /etc/login.defs)
[ -n "${UID_MIN}" ] && auditctl -l | awk "/^ *-a *always,exit/ \
&&/ -F *arch=b(32|64)/ \
&&(/ -F *auid!=unset/||/ -F *auid!=-1/||/ -F *auid!=4294967295/) \
&&/ -S/ \
&&(/creat/ \
 ||/open/ \
 ||/truncate/) \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/" \
|| printf "ERROR: Variable 'UID_MIN' is unset.\n"

Verify the output includes:

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

32 Bit systems

Follow the same procedures as for 64 bit systems and ignore any entries with b64.

Remediation

Create audit rules

Edit or create a file in the /etc/audit/rules.d/ directory, ending in .rules extension, with the relevant rules to monitor unsuccessful file access attempts.

64 Bit systems

Example:

UID_MIN=$(awk '/^\s*UID_MIN/{print $2}' /etc/login.defs)
[ -n "${UID_MIN}" ] && printf "
-a always,exit -F arch=b64 -S creat,open,openat,truncate,ftruncate -F exit=-EACCES -F auid>=${UID_MIN} -F auid!=unset -k access
-a always,exit -F arch=b64 -S creat,open,openat,truncate,ftruncate -F exit=-EPERM -F auid>=${UID_MIN} -F auid!=unset -k access
-a always,exit -F arch=b32 -S creat,open,openat,truncate,ftruncate -F exit=-EACCES -F auid>=${UID_MIN} -F auid!=unset -k access
-a always,exit -F arch=b32 -S creat,open,openat,truncate,ftruncate -F exit=-EPERM -F auid>=${UID_MIN} -F auid!=unset -k access
" >> /etc/audit/rules.d/50-access.rules || printf "ERROR: Variable 'UID_MIN' is unset.\n"

Load audit rules

Merge and load the rules into active configuration:

augenrules --load

Check if reboot is required:

if [[ $(auditctl -s | grep "enabled") =~ "2" ]]; then printf "Reboot required to load rules\n"; fi

32 Bit systems

Follow the same procedures as for 64 bit systems and ignore any entries with b64.

References

• NIST SP 800-53 Rev. 5: AU-3

CIS Controls

MITRE ATT&CK Mappings

Additional Information

Potential reboot required: If the auditing configuration is locked (-e 2), then augenrules will not warn in any way that rules could not be loaded into the running configuration. A system reboot will be required to load the rules into the running configuration.

System call structure: For performance (man 7 audit.rules) reasons it is preferable to have all the system calls on one line. However, your configuration may have them on one line each or some other combination.