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title laurel - Advice for writing Audit rulesets for use with `laurel(8)`
section 7
header System Administration Utilities
footer laurel 0.7.3

NAME

laurel-audit-rules - Advice for writing Audit rulesets for use with laurel(8)

SYNOPSIS

This page contains suggestions for Linux Audit rulesets that are useful to aid in detecting common attacker's tactics.

Note about auditctl(8) error messages

It is not possible for /auditctl(8)/ ro load file watches for files or directories that are not present. Depending on the rule set, it will spam possibly lots of error messages to standard error. The specific file watches are not installed, but those error messages can be ignored otherwise.

File watches

Authentication/authorization

-w /etc/group           -p wa -k wr_group
-w /etc/passwd          -p wa -k wr_passwd
-w /etc/shadow          -p wa -k wr_passwd
-w /etc/pam.conf        -p wa -k wr_pam
-w /etc/pam.d/          -p wa -k wr_pam
-w /etc/ssh/sshd_config -p wa -k wr_sshd
-w /etc/sudoers         -p wa -k wr_sudo
-w /etc/sudoers.d       -p wa -k wr_sudo

cron, at

-w /etc/crontab              -p wa -k wr_cron
-w /etc/cron.d/              -p wa -k wr_cron
-w /etc/cron.daily/          -p wa -k wr_cron
-w /etc/cron.hourly/         -p wa -k wr_cron
-w /etc/cron.monthly/        -p wa -k wr_cron
-w /etc/cron.weekly/         -p wa -k wr_cron
-w /etc/cron.yearly/         -p wa -k wr_cron
-w /etc/cron.allow           -p wa -k wr_cron
-w /etc/cron.deny            -p wa -k wr_cron
-w /var/spool/cron/crontabs/ -p wa -k wr_cron
-w /etc/at.allow             -p wa -k wr_cron
-w /etc/at.deny              -p wa -k wr_cron
-w /var/spool/cron/atjobs/   -p wa -k wr_cron

systemd

Systemd also has cron-like timer mechanism. udev triggers have also been abused for persistence. Note that watching the files in /etc is not sufficient.

-w /etc/systemd     -p wa -k wr_systemd
-w /lib/systemd     -p wa -k wr_systemd
-w /usr/lib/systemd -p wa -k wr_systemd
-w /etc/udev        -p wa -k wr_systemd
-w /lib/udev        -p wa -k wr_systemd
-w /usr/lib/udev    -p wa -k wr_systemd

Dynamic linkers

-w /lib/ld-linux.so.2          -p wa -k wr_ldso
-w /lib64/ld-linux-x86-64.so.2 -p wa -k wr_ldso
-w /lib/ld-musl-x86_64.so.1    -p wa -k wr_ldso
-w /lib/ld-musl-i386.so.1      -p wa -k wr_ldso
-w /etc/ld.so.conf             -p wa -k wr_ldso
-w /etc/ld.so.conf.d           -p wa -k wr_ldso
-w /etc/ld.so.preload          -p wa -k wr_ldso

Mandatory access control (SELinux, AppArmor) manipulation

-w /etc/selinux         -p wa -k wr_selinux
-w /usr/share/selinux   -p wa -k wr_selinux
-w /usr/libexec/selinux -p wa -k wr_selinux

-w /etc/apparmor.d              -p wa -k wr_apparmor
-w /usr/lib/apparmor            -p wa -k wr_apparmor
-w /usr/share/apparmor          -p wa -k wr_apparmor
-w /usr/share/apparmor-features -p wa -k wr_apparmor

Kernel modules

-w /etc/modprobe.conf -p wa -k wr_modules
-w /etc/modprobe.d/   -p wa -k wr_modules
-w /lib/modules/      -p wa -k wr_modules

Auditd + Laurel

-w /etc/audit/        -p wa -k wr_audit_config
-w /etc/libaudit.conf -p wa -k wr_audit_config
-w /etc/audisp/       -p wa -k wr_audit_config
-w /etc/laurel/       -p wa -k wr_laurel_confg

Log specific program executions

Possible tampering with auditd, laurel

-w /sbin/auditctl   -p x -k wr_audit_tools
-w /sbin/auditd     -p x -k wr_audit_tools
-w /usr/sbin/laurel -p x -k wr_audit_tools

Log specific "harmless" programs executions

Adding context to system service activities is useful because together with Laurel's process labels (label-process.label-keys, label-process.propagate-labels), it enables more accurate detection rules that can help recognize benign system management activity.

-w /usr/sbin/sshd -p x -k sshd

-w /usr/bin/yum                -p x -k pkg_mgmt
-w /usr/bin/rpm                -p x -k pkg_mgmt
-w /usr/bin/dnf                -p x -k pkg_mgmt
-w /usr/bin/dpkg               -p x -k pkg_mgmt
-w /usr/bin/apt                -p x -k pkg_mgmt
-w /usr/bin/apt-get            -p x -k pkg_mgmt
-w /usr/bin/apt-key            -p x -k pkg_mgmt
-w /usr/bin/apt-add-repository -p x -k pkg_mgmt
-w /usr/bin/aptitude           -p x -k pkg_mgmt
-w /usr/bin/aptitude-curses    -p x -k pkg_mgmt
-w /usr/bin/wajig              -p x -k pkg_mgmt
-w /usr/bin/snap               -p x -k pkg_mgmt
-w /usr/sbin/yast2             -p x -k pkg_mgmt
-w /usr/bin/zypper             -p x -k pkg_mgmt

-w /usr/bin/containerd        -p x -k container
-w /usr/bin/podman            -p x -k container
-w /usr/bin/runc              -p x -k container
-w /usr/bin/dockerd           -p x -k container
-w /usr/bin/docker            -p x -k container
-w /usr/bin/docker-containerd -p x -k container
-w /usr/bin/docker-runc       -p x -k container

-w /usr/sbin/cron -p x -k sched_task
-w /usr/sbin/atd  -p x -k sched_task

-w /usr/sbin/httpd              -p x -k apache-httpd
-w /usr/local/apache2/bin/httpd -p x -k apache-httpd

-w /usr/sbin/nginx                         -p x -k nginx
-w /usr/local/nginx/sbin/nginx             -p x -k nginx
-w /usr/local/openresty/nginx/sbin/nginx   -p x -k nginx

Syscalls

Log all fork and exec calls for reliable process tracking

For reliable process tracking that is required for assigning and propagating process labels, it is useful to have the Linux Audit subsystem produce events for all fork/exec style syscalls.

## Ignore clone( flags=CLONE_VM|… ), log other process-creating calls
-a never,exit  -F arch=b32 -S clone -F a2&0x100
-a never,exit  -F arch=b64 -S clone -F a2&0x100
-a always,exit -F arch=b32 -S fork,vfork,clone,clone3 -k fork
-a always,exit -F arch=b64 -S fork,vfork,clone,clone3 -k fork
-a always,exit -F arch=b32 -S execve,execveat
-a always,exit -F arch=b64 -S execve,execveat

It is only important that Laurel gets to observe these events. To reduce log volume, Laurel's filtering settings should be used, e.g.:

[filter]
filter-keys = ["fork"]
filter-action = drop
keep-first-per-process = true

Log usage of ptrace

We are interested in ptrce usage, but not in every transaction (PEEK, POKE, CONT)

-a never,exit -F arch=b32 -S ptrace -F a0>=1 -F a0<=7
-a never,exit -F arch=b64 -S ptrace -F a0>=1 -F a0<=7
-a always,exit -F arch=b32 -S ptrace
-a always,exit -F arch=b64 -S ptrace

Log BPF usage

Usage of BPF should be restricted to few processes; log everything except data transfer operations because they would put too much load on the system.

-a never,exit -F arch=b32 -S bpf -F a0>=1 -F a0<=4
-a never,exit -F arch=b64 -S bpf -F a0>=1 -F a0<=4
-a never,exit -F arch=b32 -S bpf -F a0>=0xb -F a0<=0xf
-a never,exit -F arch=b64 -S bpf -F a0>=0xb -F a0<=0xf
-a never,exit -F arch=b32 -S bpf -F a0=0x13
-a never,exit -F arch=b64 -S bpf -F a0=0x13
-a always,exit -F arch=b32 -S bpf -F success=1
-a always,exit -F arch=b64 -S bpf -F success=1

Log kernel module loading, unloading

-a always,exit -F arch=b32 -S init_module,finit_module,delete_module -k module
-a always,exit -F arch=b64 -S init_module,finit_module,delete_module -k module

SEE ALSO

audit.rules(7), laurel(8)

AUTHOR