Detection: Linux Auditd Unix Shell Configuration Modification

Updated Date: 2024-09-30 ID: 66f737c6-3f7f-46ed-8e9b-cc0e5bf01f04 Author: Teoderick Contreras, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects suspicious modifications to Unix shell configuration files, which may indicate an attempt to alter system behavior or gain unauthorized access. Unix shell configuration files, such as .bashrc or .profile, control user environment settings and command execution. Unauthorized changes to these files can be used to execute malicious commands, escalate privileges, or hide malicious activities. By monitoring for unusual or unauthorized modifications to shell configuration files, this analytic helps identify potential security threats, allowing security teams to respond quickly and mitigate risks.

Search

1`linux_auditd` type=PATH name IN ("/etc/profile", "/etc/shells", "/etc/profile.d", "/etc/bash.bashrc", "/etc/bashrc", "/etc/zsh/zprofile", "/etc/zsh/zshrc", "/etc/zsh/zlogin", "/etc/zsh/zlogout", "/etc/csh.cshrc", "/etc/csh.login", "/root/.bashrc", "/root/.bash_profile", "root/.profile", "/root/.zshrc", "/root/.zprofile", "/home/*/.bashrc", "/home/*/.zshrc", "/home/*/.bash_profile", "/home/*/.zprofile", "/home/*/.profile", "/home/*/.bash_login", "/home/*/.bash_logout", "/home/*/.zlogin", "/home/*/.zlogout") 
2| rename host as dest 
3| stats count min(_time) as firstTime max(_time) as lastTime by name nametype OGID type dest 
4| `security_content_ctime(firstTime)`
5| `security_content_ctime(lastTime)`
6| `linux_auditd_unix_shell_configuration_modification_filter`

Data Source

Name	Platform	Sourcetype	Source
Linux Auditd Path	Linux	`'linux:audit'`	`'/var/log/audit/audit.log'`

Macros Used

Name	Value
linux_auditd	`sourcetype="linux:audit"`
linux_auditd_unix_shell_configuration_modification_filter	`search *`

linux_auditd_unix_shell_configuration_modification_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1546.004	Unix Shell Configuration Modification	Persistence
T1546	Event Triggered Execution	Privilege Escalation

KillChainPhase.EXPLOITAITON

KillChainPhase.INSTALLATION

NistCategory.DE_CM

Cis18Value.CIS_10

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Notable	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Risk Event	True

This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

To implement this detection, the process begins by ingesting auditd data, that consist SYSCALL, TYPE, EXECVE and PROCTITLE events, which captures command-line executions and process details on Unix/Linux systems. These logs should be ingested and processed using Splunk Add-on for Unix and Linux (https://splunkbase.splunk.com/app/833), which is essential for correctly parsing and categorizing the data. The next step involves normalizing the field names to match the field names set by the Splunk Common Information Model (CIM) to ensure consistency across different data sources and enhance the efficiency of data modeling. This approach enables effective monitoring and detection of linux endpoints where auditd is deployed

Known False Positives

Administrator or network operator can use this application for automation purposes. Please update the filter macros to remove false positives.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message	Risk Score	Impact	Confidence
A [$type$] event occured on host - [$dest$] to modify the unix shell configuration file.	64	80	80

The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`/var/log/audit/audit.log`	`linux:audit`
Integration	✅ Passing	Dataset	`/var/log/audit/audit.log`	`linux:audit`

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 2