Detection: ESXi SSH Brute Force

Updated Date: 2025-05-12 ID: 68fe4efa-bbbb-44ee-9f09-d07d2f0f346b Author: Raven Tait, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

This detection identifies signs of SSH brute-force attacks by monitoring for a high number of failed login attempts within a short time frame. Such activity may indicate an attacker attempting to gain unauthorized access through password guessing.

Search

1`esxi_syslog` Message="*Authentication failure for*" 
2| rex "for (?<user>[\w]+) from (?<src_ip>\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})" 
3| rex field=_raw "Z (?<dest>[\w\.]+)\s" 
4| bin _time span=5m 
5| stats min(_time) as firstTime max(_time) as lastTime count by user, src_ip, dest 
6| where count > 10 
7| `security_content_ctime(firstTime)` 
8| `security_content_ctime(lastTime)` 
9| `esxi_ssh_brute_force_filter`

Data Source

Name	Platform	Sourcetype	Source
VMWare ESXi Syslog	N/A	`'vmw-syslog'`	`'vmware:esxlog'`

Macros Used

Name	Value
esxi_syslog	`sourcetype=vmw-syslog OR sourcetype=vmware:esxlog*`
esxi_ssh_brute_force_filter	`search *`

esxi_ssh_brute_force_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
T1110	Brute Force	Credential Access

Exploitation

DE.AE

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

This is based on syslog data generated by VMware ESXi hosts. To implement this search, you must configure your ESXi systems to forward syslog output to your Splunk deployment. These logs must be ingested with the appropriate Splunk Technology Add-on for VMware ESXi Logs, which provides field extractions and CIM compatibility.

Known False Positives

Limited false positives in most environments, however tune as needed.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

Attempted SSH brute force on ESXi host $dest$.

Risk Object	Risk Object Type	Risk Score	Threat Objects
dest	system	25	No Threat Objects

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`vmware:esxlog`	`vmw-syslog`
Integration	✅ Passing	Dataset	`vmware:esxlog`	`vmw-syslog`

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: 1