How do I configure Linux audit rules to track user activity, file changes, and directory changes on my EC2 Linux instance?

Short description

Auditd is the user-space component to the Linux Audit system. Auditd is useful for tracking suspicious activity, and can help you identify areas where you can take additional security measures.

Auditd is used to:

• Track the user or application that is accessing or modifying files and directories.
• Track which user is running specific commands.

Resolution

1.    Connect to your EC2 instance using SSH as ec2-user/ubuntu/root user. Replace ubuntu with the user name for your AMI.

2.    Run the following command to install the audit package:

RHEL and CentOS:

# sudo yum install audit

SUSE Linux:

# sudo zypper install audit

Ubuntu:

# sudo apt install auditd

3.    Create audit rules.

Audit rules are defined in the /etc/audit/audit.rules file. Custom audit rules are defined in the /etc/audit/rules.d/custom.conf file. Audit rules defined in files are persistent. Rules can also be implemented at runtime.

Run the auditctl command to create your audit rules.

Example rules

Track the user or application that is accessing or modifying a certain file or directory:

# sudo auditctl -a always,exit -F arch=b64 -S rename,rmdir,unlink,unlinkat,renameat -F auid\>=500 -F auid\!=-1 -F dir=/root/test/ -F key=delete

Track which user is running a specific command. In the following example, the command is sudo.

# sudo auditctl -w /bin/sudo -p rwxa -k sudo

The following is a list of the syntax used in the preceding examples:

-a - Add a new rule
-w - Insert a watch for the file system object at a specific path, for example, /etc/shadow.
-p- Set permissions filters for a file system.
-k - Set a filter key on an audit rule. The filter key uniquely identifies the audit records produced by a rule.
-F - Use this field to specify additional options such, as architecture, PID, GID, auid, and so on.
-S - Is a system call. This is a name or number.

For a complete list of syntax and switches, see auditctl(8) and audit.rules(7) on the Linux man page.

Note: To make sure that your rules persist after a reboot, edit audit.rules, and then add the following rule to the file:

RHEL 6, CentOS 6, or Amazon Linux 1:

#sudo vi /etc/audit/audit.rules

-a always,exit -F arch=b64 -S rename,rmdir,unlink,unlinkat,renameat -F auid>=500 -F auid!=-1 -F dir=/root/test/ -F key=delete
-w /bin/sudo -p rwxa -k sudo

RHEL 7, CentOS 7, or Amazon Linux 2:

# sudo vi /etc/audit/rules.d/audit.rules

-a always,exit -F arch=b64 -S rename,rmdir,unlink,unlinkat,renameat -F auid>=500 -F auid!=-1 -F dir=/root/test/ -F key=delete
-w /bin/sudo -p rwxa -k sudo

4.    Restart the auditd service after making any changes. Be sure that the auditd service is set to run on boot.

# sudo chkconfig auditd on
# sudo service auditd start
# sudo service auditd stop
# sudo service auditd restart

Note: It's a best practice to use the service command instead of the systemctl command in CentOS and RHEL 7 when restarting the auditd service. Using the systemctl command might cause errors.

5.    Run the ausearch command to read the audit logs.

Example audit logs

In the following example, user ec2-user (uid = ec2-user) deleted the file /root/test/example.txt.

# sudo ausearch -i -k delete
type=PROCTITLE msg=audit(04/04/20 19:41:51.231:3303) : proctitle=rm -rf /root/test/example.txt
type=PATH msg=audit(04/04/20 19:41:51.231:3303) : item=1 name=/root/test/example.txt inode=16777349 dev=ca:01 mode=file,777 ouid=root ogid=root
rdev=00:00 nametype=DELETE cap_fp=none cap_fi=none cap_fe=0 cap_fver=0
type=PATH msg=audit(04/04/20 19:41:51.231:3303) : item=0 name=/tmp/test/ inode=16777328 dev=ca:01 mode=dir,777 ouid=root ogid=root rdev=00:00 nametype=PARENT cap_fp=none cap_fi=none
cap_fe=0 cap_fver=0
type=CWD msg=audit(04/04/20 19:41:51.231:3303) : cwd=/home/ec2-user
type=SYSCALL msg=audit(04/04/20 19:41:51.231:3303) : arch=x86_64 syscall=unlinkat success=yes exit=0 a0=0xffffff9c a1=0xc5f290 a2=0x0 a3=0x165 items=2 ppid=3645
pid=933 auid=ec2-user uid=ec2-user gid=ec2-user euid=ec2-user suid=ec2-user fsuid=ec2-user egid=ec2-user sgid=ec2-user fsgid=ec2-user tty=pts0 ses=1 comm=rm exe=/usr/bin/rm key=delete

In the following example, user ec2-user (uid= 1000) ran the command sudo su - with sudo privilege.

# sudo ausearch -k sudo

time->Mon Apr  6 18:33:26 2020
type=PROCTITLE msg=audit(1586198006.631:2673): proctitle=7375646F007375002D type=PATH msg=audit(1586198006.631:2673): item=1 name="/lib64/ld-linux-x86-64.so.2" inode=5605 dev=103:05 mode=0100755 ouid=0 ogid=0 rdev=00:00 objtype=NORMAL cap_fp=0000000000000000 cap_fi=0000000000000000 cap_fe=0 cap_fver=0
type=PATH msg=audit(1586198006.631:2673):
 item=0 name="/usr/bin/sudo" inode=12800710 dev=103:05 mode=0104111 ouid=0 ogid=0 rdev=00:00 objtype=NORMAL cap_fp=0000000000000000 cap_fi=0000000000000000 cap_fe=0 cap_fver=0
type=CWD msg=audit(1586198006.631:2673):  cwd="/home/ec2-user"
type=EXECVE msg=audit(1586198006.631:2673): argc=3 a0="sudo" a1="su" a2="-"
type=SYSCALL msg=audit(1586198006.631:2673): arch=c000003e syscall=59 success=yes exit=0 a0=e8cce0 a1=e8c7b0 a2=e61720 a3=7ffde58ec0a0 items=2 ppid=2658 pid=3726 auid=1000 uid=1000 gid=1000 euid=0 suid=0 fsuid=0 egid=1000 sgid=1000 fsgid=1000
tty=pts0 ses=4 comm="sudo" exe="/usr/bin/sudo" key="script"