Detections
Analytics

4.1.3.9 Ensure discretionary access control permission modification events are collected

Information

Monitor changes to file permissions, attributes, ownership and group. The parameters in this section track changes for system calls that affect file permissions and attributes. The following commands and system calls effect the permissions, ownership and various attributes of files.

chmod

fchmod

fchmodat

chown

fchown

fchownat

lchown

setxattr

lsetxattr

fsetxattr

removexattr

lremovexattr

fremovexattr

In all cases, an audit record will only be written for non-system user ids and will ignore Daemon events. All audit records will be tagged with the identifier 'perm_mod.'

Rationale:

Monitoring for changes in file attributes could alert a system administrator to activity that could indicate intruder activity or policy violation.

Solution

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 discretionary access control permission modification events.

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 chmod,fchmod,fchmodat -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=${UID_MIN} -F auid!=unset -F key=perm_mod
' >> /etc/audit/rules.d/50-perm_mod.rules || printf 'ERROR: Variable 'UID_MIN' is unset.
'
}

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
'; fi

32 Bit systems

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

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.

See Also

https://workbench.cisecurity.org/files/4115

Item Details

References: CSCv7|5.5

Plugin: Unix

Control ID: 350aecbf41bdb2569bacfd57c21e01ac71aa8f255b9b8c7592344cf88a705254