6.2.3.9 Ensure discretionary access control permission modification events are collected
Audit#
Note: Output showing all audited syscalls, e.g. (-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat,chmod,fchmod,fchmodat,setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=unset -F key=perm_mod) is also acceptable. These have been separated by function on the displayed output for clarity.
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/ \
&&/ -F *auid>=${UID_MIN}/ \
&&(/chmod/||/fchmod/||/fchmodat/ \
||/chown/||/fchown/||/fchownat/||/lchown/ \
||/setxattr/||/lsetxattr/||/fsetxattr/ \
||/removexattr/||/lremovexattr/||/fremovexattr/) \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)" /etc/audit/rules.d/*.rules \
|| printf "ERROR: Variable 'UID_MIN' is unset.\n"
}
Verify the output matches:
-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=1000 -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=1000 -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=unset -F key=perm_mod
-a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=unset -F key=perm_mod
Running configuration Run the following command to check loaded rules:
# {
UID_MIN=$(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/ \
&&/ -F *auid>=${UID_MIN}/ \
&&(/chmod/||/fchmod/||/fchmodat/ \
||/chown/||/fchown/||/fchownat/||/lchown/ \
||/setxattr/||/lsetxattr/||/fsetxattr/ \
||/removexattr/||/lremovexattr/||/fremovexattr/) \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)" \
|| printf "ERROR: Variable 'UID_MIN' is unset.\n"
}
Verify the output matches:
-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
-a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=-1 -F key=perm_mod
-a always,exit -F arch=b32 -S lchown,fchown,chown,fchownat -F auid>=1000 -F auid!=-1 -F key=perm_mod
-a always,exit -F arch=b64 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
-a always,exit -F arch=b32 -S setxattr,lsetxattr,fsetxattr,removexattr,lremovexattr,fremovexattr -F auid>=1000 -F auid!=-1 -F key=perm_mod
32 Bit systems
Follow the same procedures as for 64 bit systems and ignore any entries with b64.
Remediation#
Create audit rules Edit or create a file in the /etc/audit/rules.d/ directory, ending in .rules extension, with the relevant rules to monitor discretionary access control permission modification events. Example:
# {
UID_MIN=$(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.\n"
}
Load audit rules Merge and load the rules into active configuration:
Check if reboot is required.