cis-ubuntu1804-v220-5-2-3-10

DevOps & Security

Ensure unsuccessful unauthorized file access attempts are collected

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5.2.3.10 Ensure unsuccessful unauthorized file access attempts are collected

Profile Applicability

Level 2 - Server
Level 2 - Workstation

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 for non-privileged users and for a non-Daemon event. All audit records will be tagged with the identifier "access."

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

Run the following commands to verify auditd is configured in accordance with site policy. For on disk configuration, the -S options may be on one line or multiple lines. For running configuration, the system calls will be on one line.

On disk configuration

64 Bit systems

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 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

64 Bit systems

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 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

Edit or create a file in the /etc/audit/rules.d/ directory ending in .rules extension, with rules to monitor unauthorized 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>=%s -F auid!=unset -k access
-a always,exit -F arch=b64 -S creat,open,openat,truncate,ftruncate -F exit=-EPERM -F auid>=%s -F auid!=unset -k access
-a always,exit -F arch=b32 -S creat,open,openat,truncate,ftruncate -F exit=-EACCES -F auid>=%s -F auid!=unset -k access
-a always,exit -F arch=b32 -S creat,open,openat,truncate,ftruncate -F exit=-EPERM -F auid>=%s -F auid!=unset -k access
" "${UID_MIN}" "${UID_MIN}" "${UID_MIN}" "${UID_MIN}" >> /etc/audit/rules.d/50-access.rules || printf "ERROR: Variable 'UID_MIN' is unset.\n"

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

Version	Control	IG 1	IG 2	IG 3
v8	8.5 Collect Detailed Audit Logs		●	●
v7	14.9 Enforce Detail Logging for Access or Changes to Sensitive Data			●

MITRE ATT&CK Mappings

Techniques / Sub-techniques	Tactics	Mitigations
T1562, T1562.006	TA0007	M1047

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. This is important to understand for both the auditing and remediation sections as the examples given are optimized for performance as per the man page.