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linux_auditd

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Best practice configuration for Linux auditd for CIS and STIG standards, enhanced with LOTL detection rules. If you need help with implementation, contact@criticalmissions.be

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# linux_auditd

Description
===========
This repo provides as a reference the best practice configuration for Linux auditd logging by harmonizing the CIS and STIG standards for RedHat 9 and Oracle Linux 9. Most of the rules will cover Linux 7 and 8 too, perhaps with minor adaptations to a very small number of rules. The rules are supplemented with monitoring for LOTL activity, as explained below, for more effective security monitoring.

The CIS (Center for Internet Security) produces various cyber security related services. In particular, it produces benchmarks, which are “configuration guidelines for various technology groups to safeguard systems against today evolving cyber threat" in the words of the CIS.

Security Technical Implementation Guides are published as a tool to improve the security of Department of Defense (DOD) information systems. The requirements are derived from the National Institute of Standards and Technology (NIST) 800-53 and related documents.

References
==========
* [Center for Internet Security (CIS) compliance in Red Hat Enterprise Linux using OpenSCAP](https://www.redhat.com/en/blog/center-internet-security-cis-compliance-red-hat-enterprise-linux-using-openscap)
* [Center for Internet Security (CIS) Red Hat Enterprise Linux](https://www.cisecurity.org/benchmark/red_hat_linux)
* [Red Hat Enterprise Linux 8 Security Technical Implementation Guide (STIG)](https://www.stigviewer.com/stig/red_hat_enterprise_linux_8/2023-09-11/MAC-1_Classified/)
* [Red Hat Enterprise Linux 9 Security Technical Implementation Guide (STIG)](https://www.stigviewer.com/stig/red_hat_enterprise_linux_9/)
* [Best practices for event logging and threat detection](https://www.cyber.gov.au/resources-business-and-government/maintaining-devices-and-systems/system-hardening-and-administration/system-monitoring/best-practices-event-logging-threat-detection?utm_source=linkedin&utm_campaign=event-logging-2024&utm_medium=social&utm_content=publication-2)
* [Guide to the Secure Configuration of Red Hat Enterprise Linux 8 with profile CIS Red Hat Enterprise Linux 8 Benchmark for Level 2 - Server](https://static.open-scap.org/ssg-guides/ssg-rhel8-guide-index.html)
* [Guide to the Secure Configuration of Red Hat Enterprise Linux 9 with profile CIS Red Hat Enterprise Linux 9 Benchmark for Level 2 - Server](https://static.open-scap.org/ssg-guides/ssg-rhel9-guide-index.html)

Living Off The Land (LOTL)
==========================

<blockquote>
Living Off The Land (LOTL) attacks are a sophisticated cyber threat where malicious actors exploit legitimate system tools and processes to carry out their objectives. Instead of deploying custom malware, attackers leverage built-in utilities like PowerShell, WMI, and command-line tools, allowing them to blend their activity with normal system operations. This technique makes it exceptionally difficult for traditional security measures to detect intrusions, as the tools used are inherently trusted. LOTL tactics enable attackers to minimize their digital footprint, evade signature-based detection, and increase their chances of successful privilege escalation and lateral movement within a network. Robust logging, behavioral analysis, and strict adherence to the principle of least privilege are essential for mitigating the risks posed by these stealthy attacks. In essence, LOTL represents a shift towards attacks that rely on the environment itself, rather than external malicious software.
</blockquote>

The auditing rules in this package defined for STIG and CIS compliance are optionally augmented by including the auditd best practices for tracking LOTL techniques used by threat actors on the Linux OS, as advised by the combined advisory of:
* Australian Signals Directorate’s Australian Cyber Security Centre (ASD’s ACSC)
* United States (US) Cybersecurity and Infrastructure Security Agency (CISA), the Federal Bureau of Investigation (FBI) and the National Security Agency (NSA)
* United Kingdom (UK) National Cyber Security Centre (NCSC-UK)
* Canadian Centre for Cyber Security (CCCS)
* New Zealand National Cyber Security Centre (NCSC-NZ) and Computer Emergency Response Team (CERT NZ)
* Japan National Center of Incident Readiness and Strategy for Cybersecurity (NISC) and Computer Emergency Response Team Coordination Center (JPCERT/CC)
* The Republic of Korea National Intelligence Services (NIS) and NIS’s National Cyber Security Center (NCSC-Korea)
* Singapore Cyber Security Agency (CSA)
* The Netherlands General Intelligence and Security Service (AIVD) and Military Intelligence and Security Service (MIVD).

<blockquote>
Living off the Land Binaries (LOLBins) involves misusing legitimate system tools and processes to blend malicious activity with normal operations. Threat actors utilize LOLBins to operate stealthily, reducing the chances of detection. Since these tools are already trusted within the system, it's challenging for security to distinguish between legitimate and malicious use. LOLBins are effective across various environments, including on-premises, cloud, and hybrid systems, as well as Windows, Linux, and macOS. By using LOLBins, attackers avoid the need to develop and deploy custom tools. System administrators should define and enforce policies for the responsible use of LOLBins to mitigate risks.
</blockquote>

These auditing rules are found in these `/etc/audit/rules.d/` files. The rules are categorized and labelled according to a corresponding MITRE ATT&CK index of Tactics and Techniques identified by the intelligence community.

```
+-----------------------+-----------------------+---------------------------------------------------+
| rules.d File          | ATT&CK Tactic         | ATT&CK Techniques                                 |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0002.rules  | Execution             | T1059 - Command and Scripting Interpreter         |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0003.rules  | Persistence           | T1574 - Hijack Execution Flow                     |
|                       |                       | T1543 - Create or Modify System Process           |
|                       |                       | T1546 - Event Triggered Execution                 |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0005.rules  | Defense Evasion       | T1070 - Indicator Removal on Host                 |
|                       |                       | T1562 - Impair Defenses                           |
|                       |                       | T1218 - Signed Binary Proxy Execution             |
|                       |                       | T1070.002 - Clear Linux or Mac System Logs        |
|                       |                       | T1070.003 - Clear Command History                 |
|                       |                       | T1070.004 - File Deletion                         |
|                       |                       | T1070.005 - Network Share Connection Removal      |
|                       |                       | T1070.006 - Timestomp                             |
|                       |                       | T1070.007 - Clear Network Connection History      |
|                       |                       | T1070.008 - Clear Mailbox Data                    |
|                       |                       | T1070.009 - Clear Persistence                     |
|                       |                       | T1070.010 - Clear Data from Cloud Storage         |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0006.rules  | Credential Access     | T1003 - OS Credential Dumping                     |
|                       |                       | T1552 - Unsecured Credentials                     |
|                       |                       | T1552.001 - Credentials In Files                  |
|                       |                       | T1552.003 - Bash History                          |
|                       |                       | T1552.004 - Private Keys                          |
|                       |                       | T1552.007 - Container API                         |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0007.rules  | Discovery             | T1082 - System Information Discovery              |
|                       |                       | T1083 - File and Directory Discovery              |
|                       |                       | T1007 - System Service Discovery                  |
|                       |                       | T1016 - System Network Configuration Discovery    |
|                       |                       | T1033 - System Owner/User Discovery               |
|                       |                       | T1057 - Process Discovery                         |
|                       |                       | T1518 - Software Discovery                        |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0008.rules  | Lateral Movement      | T1021.001 - Remote Desktop Protocol               |
|                       |                       | T1021.004 - SSH                                   |
|                       |                       | T1021.005 - VNC                                   |
|                       |                       | T1071 - Application Layer Protocol                |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0009.rules  | Collection            | T1560 - Archive Collected Data                    |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0010.rules  | Exfiltration          | T1567 - Exfiltration Over Web Services            |
|                       |                       | T1567.001 - Exfiltration to Code Repository       |
|-----------------------+-----------------------+---------------------------------------------------|
| 41-lotl-TA0011.rules  | Command and Control   | T1071 - Application Layer Protocol                |
|                       |                       | T1090 - Proxy                                     |
|                       |                       | T1021.004 - SSH                                   |
|                       |                       | T1105 - Ingress Tool Transfer                     |
|                       |                       | T1562.001 - Disable or Modify Tools               |
|                       |                       | T1562.004 - Disable or Modify System Firewall     |
+-----------------------+-----------------------+---------------------------------------------------+

```

These rules require adaptation as follows:
* Adapting auditing rules for your known specific application accounts, because wildcards are not permitted in directory paths (e.g. `/home/*/.bash_history`)
* Adapting auditing rules for custom software packages that you install. It is important to audit config file changes, access to files containing secrets, etc.
* Adapting auditing rules for security software that you install for tracking evasion techniques that try to disable them.
* Adapting auditing rules to suppress whitelisted activity to reduce log volumes. You want to know if a hacker is using grep to mine for credentials in files, but not when your scripts are using grep as part of normal activities.
* Adapting auditing rules to incorporate new tactics and techniques as they are disseminated.

Some of the audit rules that monitor for the execution of imported LOLBins are best detected using malware scanning tools rather than auditd. This is partly because the binary path is dynamic, which makes auditd detection difficult.

References
==========
* [Identifying and Mitigating Living Off the Land Techniques](https://www.cyber.gov.au/about-us/view-all-content/alerts-and-advisories/identifying-and-mitigating-living-off-the-land-techniques)

List of Rules:
===========================

The `/etc/audit/rules.d` directory contains separate files that define individual audit rules.

The files in this directory are organized into groups with following meanings:
```
10 - Kernel and auditctl configuration
20 - Rules that could match general rules but you want a different match
30 - Main rules
40 - Optional rules
50 - Server-specific rules
70 - System local rules
90 - Finalize (immutable)
```

The following rules files exist in this repo:
```
10-base-config.rules - initial setup
30-actions.rules - monitor sudo activity
31-audit_rules_usergroup_modification.rules - monitor access to user and group configuration files
32-activity.rules - log commands with elevated privileges or emulating another user
33-access.rules - monitor unsuccessful file modification attempts
34-delete.rules - log all deletion operations performed by users with auid (audit user ID) >= 1000,
35-logins.rules - tracking user logins attempts, logouts, and current logins
36-session.rules - log any attempts to alter or access critical session files
41-lotl-TA0002.rules - rules to track LOTL tactics for Execution https://attack.mitre.org/tactics/TA0002/
41-lotl-TA0003.rules - rules to track LOTL tactics for Persistence https://attack.mitre.org/tactics/TA0003/
41-lotl-TA0005.rules - rules to track LOTL tactics for Defense Evasion https://attack.mitre.org/tactics/TA0005/
41-lotl-TA0006.rules - rules to track LOTL tactics for Credential Access https://attack.mitre.org/tactics/TA0006/
41-lotl-TA0007.rules - rules to track LOTL tactics for Discovery https://attack.mitre.org/tactics/TA0007/
41-lotl-TA0008.rules - rules to track LOTL tactics for Lateral Movement https://attack.mitre.org/tactics/TA0008/
41-lotl-TA0009.rules - rules to track LOTL tactics for Collection https://attack.mitre.org/tactics/TA0009/
41-lotl-TA0010.rules - rules to track LOTL tactics for Exfiltration https://attack.mitre.org/tactics/TA0010/
41-lotl-TA0011.rules - rules to track LOTL tactics for Command and Control https://attack.mitre.org/tactics/TA0011/
41-lotl.rules - rules to track LOTL techniques commonly used by threat actors
42-activity.rules - OPTIONAL rules to track all user activity
50-server-specific.rules.example - OPTIONAL rules to track access to installed applications
70-system_local.rules - monitor home dirs, local cron job configs, local firewall configs, sensitive files, local scripts.
71-MAC_policy.rules - tracking attempts to modify Mandatory Access Controls
72-maintenance.rules - monitor all privileged maintenance activities
73-modules.rules - monitor the loading and unloading of kernel modules
73-mounts.rules - audit mount operations on the system
74-perm_mod.rules - monitor changes to file permissions, ownership, and extended attributes
75-privileged.rules - monitor the use of privileged commands and activities performed by users with elevated privileges
76-setgid.rules - monitor and log changes to the set group ID (setgid) permissions on files and directories
77-setuid.rules - monitor and log changes to the set user ID (setuid) permissions on files and directories
78-time_change.rules - tracking attempts to change the sytem time
99-finalize.rules - making the configuration immutable
```

Make sure the config and rules files have the correct permisions:
```bash
# chmod 0640 /etc/audit/rules.d/*.rules
# chmod 0640 /etc/audit/auditd.conf
```

To view logs that match these records, use the key which is specified within the corresponding *.rules file:
```bash
# ausearch -k logins
```

To generate the rules file:
===========================

The Linux Audit daemon auditd can be configured to use the augenrules program to read audit rules files (*. rules) located
in /etc/audit/rules.d location and compile them to create the resulting form of the /etc/audit/audit.rules configuration
file during the daemon startup (default configuration). Alternatively, the auditd daemon can use the auditctl utility to read
audit rules from the /etc/audit/audit.rules configuration file during daemon startup, and load them into the kernel. The
expected behavior is configured via the appropriate ExecStartPost directive setting in the /usr/lib/systemd/system/auditd.service
configuration file.

To instruct the auditd daemon to use the augenrules program to read audit rules (default configuration), use the following setting
in the /usr/lib/systemd/system/auditd.service configuration file:
```
ExecStartPost=-/sbin/augenrules --load
```

To instruct the auditd daemon to use the auditctl utility to read audit rules, use the following setting
in the /usr/lib/systemd/system/auditd.service configuration file:
```
ExecStartPost=-/sbin/auditctl -R /etc/audit/audit.rules
```

The augenrules script reads rules located in the /etc/audit/rules.d/ directory and compiles them into an audit.rules file. This
script processes all files that end with .rules in a specific order based on their natural sort order.

```bash
# auditctl -D
# augenrules --load
# auditctl -l
# cat /etc/audit/audit.rules
```

Activate Rules
======================

Restarting auditd with systemctl is prevented if there are dependendies:
```bash
# systemctl status auditd.service
# systemctl list-dependencies auditd.service
```

Restarting the OS will activate the new auditing configuration:
```bash
# reboot
# systemctl status auditd
```

List the configured rules:
```bash
# sudo auditctl -l
```

To re-load the rules if they are modified:
```bash
# auditctl -R /etc/audit/audit.rules
```

Guidelines for Authoring Rules
==============================
The syntax for writing auditd rules is not well documented. These examples and rules are correct for RHEL 9 / Oracle Linux 9:

Monitor the execution of a specific binary requires the absolute path:
```
-a always,exit -F arch=b64 -S execve -F path=/usr/bin/ag -F auid>=1000 -F auid!=4294967295 -k T1552_Credentials_In_Files
```

Directories must exist if specified in -dir.

Monitor all files in a directory for changes (wa):
```
-a always,exit -F arch=b64 -S write,pwrite,truncate,ftruncate,creat,chmod,fchmod,chown,fchown,lchown,utime,utimes,futimesat -F dir=/etc/udev/rules.d/ -F auid>=1000 -F auid!=4294967295 -k T1546_Udev_Modification
```
Monitor all files in a directory for reads (r):
```
-a always,exit -F arch=b64 -S open,access,read -F dir=/etc/udev/rules.d/ -F auid>=1000 -F auid!=4294967295 -k T1546_Udev_Modification
```

Monitor file reads (r):
```
-a always,exit -F arch=b64 -S open,access,read -F path=/root/.ssh -F auid>=1000 -F auid!=4294967295 -k T1082_System_Discovery
```

Alternatively:
```
-a always,exit -F arch=b64 -F path=/root/.ssh -F perm=r -F auid>=1000 -F auid!=4294967295 -k T1546_Udev_Modification
```

Monitor file changes (wa):
```
-a always,exit -F arch=b64 -S write,pwrite,truncate,ftruncate,creat,chmod,fchmod,chown,fchown,lchown,utime,utimes,futimesat -F path=/etc/os-release -F auid>=1000 -F auid!=4294967295 -k T1546_Udev_Modification
```

The directory path (e.g. /etc/kubernetes/) must exist for its regular files to be audited, even if the regular files do not exist:
```
-a always,exit -F arch=b64 -F path=/etc/kubernetes/scheduler.conf -F perm=rw -F auid>=1000 -F auid!=4294967295 -k T1552_Container_API
```

Old style watch rules are slower and are deprecated. Do not use them even though they are described in CIS and STIG standards:
```
-w /etc/localtime -p wa -k audit_time_rules
```

The use of wildcards is only permitted for file names, not for directories:
* OK     ```path=/home/user1/*.bash_history```
* NOT OK ```path=/home/*/.bash_history```

Configure Log Rotation
======================

Like the Windows OS, Linux Auditd is designed to manage the log trail size based upon a specified size. It does not offer the capabilility
out-of-the-box to retain the last X days of logs online. It is expected and mandated that security logs be relayed in near real-time to
a centralized logging service (SIEM) for archiving and processing. Local online log storage is not intended for keeping a historical archive.
Additionally, a Log Storm caused by a repeating security event could overflow the disk storage and cause a loss of a service.
Therefore it is necessary to budget an amount of disk space in /var/log/audit for storing Auditd logs.

By default, the /etc/audit/auditd.conf in this package budgets for 1GB of disk space:
```
# Budget 1GB for total size size (128M * 8)
# Logs will cycle
max_log_file = 128
num_logs = 8
```

The package defaults to ROTATE to cycle through the log files by overwriting them:
```
## Configure auditd max_log_file_action Upon Reaching Maximum Log Size
## The default action to take when the logs reach their maximum size is to rotate the log files, discarding the oldest one.
## Possible values for ACTION are described in the auditd.conf man page. These include:
## ignore
## syslog
## suspend
## rotate
## keep_logs
## Set the ACTION to rotate to ensure log rotation occurs. This is the default. The setting is case-insensitive.

## Automatically rotating logs (by setting this to rotate) minimizes the chances of the system unexpectedly running out of
## disk space by being overwhelmed with log data. However, for systems that must never discard log data, or which use
## external processes to transfer it and reclaim space, keep_logs can be employed.

max_log_file_action = ROTATE
```

To configure auditd log rotation by scheduling Linux auditd service to rotate its logs every day at midnight:

```bash
# tee "/etc/cron.d/auditd" > /dev/null 2>&1 <<< "0 0 * * * root /bin/bash -lc 'service auditd rotate' > /dev/null 2>&1"
```

Test log rotation:
```bash
# ls -la /var/log/audit/
# tail -f /var/log/messages &
# service auditd rotate
```

View Hourly Statistics
======================

To view the number of audit records per hour by key in the last 24 hours, run the provided script:
```bash
 ./hourlystats.py
```

Sample output:
```
Processing file: /var/log/audit/audit.log.4
Processing file: /var/log/audit/audit.log.3
Processing file: /var/log/audit/audit.log.2
Processing file: /var/log/audit/audit.log.1
Processing file: /var/log/audit/audit.log
Key                                         00        01        02        03        04        05        06        07        08        09        10        11        12        13        14        15        16        17        18        19        20        21        22        23
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
audit_rules_usergroup_modification      0         0         0         0         0         0         0         0         0         0         0         0         10        0         0         0         0         0         0         0         0         0         0         0
delete                                  0         0         0         0         0         0         0         0         0         0         0         0         4291      9953      0         0         0         0         0         0         0         0         0         0
execall                                 0         0         0         0         0         0         0         0         0         0         0         0         482       1966      0         0         0         0         0         0         0         0         0         0
execpriv                                0         0         0         0         0         0         0         0         0         0         0         0         0         1         0         0         0         0         0         0         0         0         0         0
logins                                  0         0         0         0         0         0         0         0         0         0         0         0         6         3         0         0         0         0         0         0         0         0         0         0
perm_mod                                0         0         0         0         0         0         0         0         0         0         0         0         12112     805       0         0         0         0         0         0         0         0         0         0
session                                 0         0         0         0         0         0         0         0         0         0         0         0         4         0         0         0         0         0         0         0         0         0         0         0
system-locale                           0         0         0         0         0         0         0         0         0         0         0         0         0         620       0         0         0         0         0         0         0         0         0         0
time-change                             0         0         0         0         0         0         0         0         0         0         0         0         8         30        0         0         0         0         0         0         0         0         0         0
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total                                   0         0         0         0         0         0         0         0         0         0         0         0         16913     13378     0         0         0         0         0         0         0         0         0         0
```

Prerequisites:
Python must be setup to run this script, for example:

```bash
sudo yum update -y
sudo yum install -y epel-release
sudo yum groupinstall -y "Development Tools"
sudo yum install -y python36-devel libjpeg-devel zlib-devel
sudo yum install -y python3-pip
```

As a non-root user:
```bash
pip3 install matplotlib
```

Complementary Controls
======================

* RHEL 8: Ensure Sudo Logfile Exists - sudo logfile - CCE-83601-5
* RHEL 9: Ensure Sudo Logfile Exists - sudo logfile - CCE-83527-2

By default, RHEL and Oracle Linux do not create the logfile that succinctly records each command that is executed with sudo.
The following setting will effectively duplicate the logging of the Linux auditd rules provided in this package.

To apply the change:
```bash
sudo visudo
Defaults logfile="/var/log/sudo.log"
```

Verify the configuration:
```bash
sudo visudo -c
```

Run a test to verify sudo commands are being logged:
```bash
sudo ls /root
tail -n 10 /var/log/sudo.log
```

To configure log rotation for this file:
```bash
# vi /etc/logrotate.d/sudo

/var/log/sudo.log {
    daily                   # Rotate logs on a daily basis
    rotate 8                # Keep at least 7 days of logs
    compress                # Compress old log files
    delaycompress           # Delay compression until the next rotation
    notifempty              # Do not rotate the log if it is empty
    missingok               # Continue without error if the log file is missing
    postrotate
        /usr/bin/killall -HUP rsyslogd # Send a SIGHUP to rsyslogd to reopen log files
    endscript
}

# chmod 655 /etc/logrotate.d/sudo
```

* RHEL 8: Set SSH Daemon LogLevel to VERBOSE - CCE-82420-1
* RHEL 9: Set SSH Daemon LogLevel to VERBOSE - CCE-27495-1

VERBOSE mode will log details of the SSH public key that was used to authenticate - the fingerprint, the key length, and the type of key.
This is strongly advised when multiple keys are stored in authorized_keys for an account that is shared.

To apply the change:
```bash
# vi /etc/ssh/sshd_config
LogLevel VERBOSE

# systemctl restart sshd
```

To verify that fingerprints of authentication keys are being logged, establish a new SSH session, and in the first session run:
```bash
# grep "Accepted key" /var/log/secure
```

The SSHD log /var/log/secure is already covered by syslog rotation:
```bash
# vi /etc/logrotate.d/syslog
/var/log/cron
/var/log/maillog
/var/log/messages
/var/log/secure
/var/log/spooler
{
    missingok
    sharedscripts
    postrotate
        /usr/bin/systemctl -s HUP kill rsyslog.service >/dev/null 2>&1 || true
    endscript
}
```

The system default logrotate settings will apply. For example:
```bash
# cat /etc/logrotate.conf
# see "man logrotate" for details
# rotate log files weekly
weekly

# keep 4 weeks worth of backlogs
rotate 4

# create new (empty) log files after rotating old ones
create

# use date as a suffix of the rotated file
dateext

# uncomment this if you want your log files compressed
#compress

# RPM packages drop log rotation information into this directory
include /etc/logrotate.d

# system-specific logs may be also be configured here.
```

An example to configure custom settings for syslog to retain only 1 week:
```bash
# vi /etc/logrotate.d/syslog
/var/log/cron
/var/log/maillog
/var/log/messages
/var/log/secure
/var/log/spooler
/var/log/boot.log {
    missingok
    daily
    rotate 8
    compress
    delaycompress
    notifempty
    create 0640 root utmp
    sharedscripts
    postrotate
        /bin/kill -HUP $(cat /var/run/syslogd.pid 2>/dev/null) 2> /dev/null || true
    endscript
}
```

Log Collection
==============

The auditd logs must be sent to a SIEM for processing and archiving. The main log to monitor is `/var/log/audit/audit.log`

It is supplemented by other Linux logs, including:
* Authentication from SSH, sudo, su, PAM: `/var/log/secure`
* Currently logged in users: `/var/run/utmp`
* Failed login attempts: `/var/log/btmp`
* Historical records of user logins and logouts: `/var/log/wtmp`
* Logging of sudo activity: `/var/log/sudo.log`
* Scheduled tasks from cron: `/var/log/cron`
* Mail server activity: `/var/log/maillog`
* General system messages and events: `/var/log/messages`
* Print spooler activity: `/var/log/spooler`
* Boot process messages: `/var/log/boot.log`
* Plus every application log.

Some SIEM options that could be considered include:
* Splunk
* Microsoft Sentinel
* AWS Security Lake in combination with AWS GuardDuty, AWS OpenSearch index, and AWS SageMaker (OCSF format from S3 bucket)
* [AWS CloudWatch](https://aws.amazon.com/blogs/mt/optimize-log-collection-with-amazon-cloudwatch-agent-log-filter-expressions/)
* AWS OpenSearch
* CRIBL
* Oracle Audit Vault and Database Firewall
* OCI Custom Logging
* OpenSearch
* CISA LME (Logging Made Easy) [LME github](https://github.com/cisagov/LME)
* ELK using [AuditBeat](https://www.elastic.co/guide/en/beats/auditbeat/current/auditbeat-module-auditd.html#audit-rules)

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