Detection: Internal Horizontal Port Scan NMAP Top 20

Updated Date: 2026-01-29 ID: 3141a041-4f57-4277-9faa-9305ca1f8e5b Author: Dean Luxton Type: TTP Product: Splunk Enterprise Security

Description

This analytic identifies instances where an internal host has attempted to communicate with 250 or more destination IP addresses using on of the NMAP top 20 ports. Horizontal port scans from internal hosts can indicate reconnaissance or scanning activities, potentially signaling malicious intent or misconfiguration. By monitoring network traffic logs, this detection helps detect and respond to such behavior promptly, enhancing network security and preventing potential threats.

Search

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5        dc(All_Traffic.dest_ip) as totalDestIPCount
 6        values(All_Traffic.action) as action
 7        values(All_Traffic.dest_zone) as dest_zone
 8        values(All_Traffic.rule) as rule
 9        values(All_Traffic.src_category) as src_category
10        values(All_Traffic.src_port) as src_port
11        values(All_Traffic.src_zone) as src_zone
12
13from datamodel=Network_Traffic where
14
15All_Traffic.src_ip IN (
16  "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "100.64.0.0/10",
17  "127.0.0.0/8", "169.254.0.0/16", "192.0.0.0/24", "192.0.0.0/29", "192.0.0.8/32",
18  "192.0.0.9/32", "192.0.0.10/32", "192.0.0.170/32", "192.0.0.171/32", "192.0.2.0/24",
19  "192.31.196.0/24", "192.52.193.0/24", "192.88.99.0/24", "224.0.0.0/4", "192.175.48.0/24",
20  "198.18.0.0/15", "198.51.100.0/24", "203.0.113.0/24", "240.0.0.0/4"
21)
22All_Traffic.dest_port IN (
23  21, 22, 23, 25, 53, 80, 110, 111,
24  135, 139, 143, 443, 445, 993, 995,
25  1723, 3306, 3389, 5900, 8080
26)
27
28by span=1h _time
29   All_Traffic.src_ip All_Traffic.dest_port
30   All_Traffic.transport
31
32
33| `drop_dm_object_name("All_Traffic")`
34
35| where totalDestIPCount>=250
36
37| eval dest_port=transport + "/" + dest_port
38
39| stats min(firstTime) as firstTime
40        max(lastTime) as lastTime
41        dc(dest_port) as num_ports_scanned
42        sum(totalDestIPCount) as totalDestIPCount
43        values(action) as action
44        values(dest_port) as dest_ports
45        values(dest_zone) as dest_zone
46        values(rule) as rule
47        values(src_category) as src_category
48        values(src_zone) as src_zone
49  by _time src_ip
50
51| fields - _time
52
53| `security_content_ctime(lastTime)`
54
55| `security_content_ctime(firstTime)`
56
57| `internal_horizontal_port_scan_nmap_top_20_filter`

Data Source

Name	Platform	Sourcetype	Source
AWS CloudWatchLogs VPCflow	AWS	`'aws:cloudwatchlogs:vpcflow'`	`'aws_cloudwatchlogs_vpcflow'`
Cisco Secure Firewall Threat Defense Connection Event	Other	`'cisco:sfw:estreamer'`	`'not_applicable'`

Macros Used

Name	Value
security_content_ctime	`convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)`
internal_horizontal_port_scan_nmap_top_20_filter	`search *`

internal_horizontal_port_scan_nmap_top_20_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
T1046	Network Service Discovery	Discovery

Exploitation

DE.CM

CIS 13

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 properly run this search, Splunk needs to ingest data from networking telemetry sources such as firewalls like Cisco Secure Firewall, NetFlow, or host-based networking events. Ensure that the Network_Traffic data model is populated to enable this search effectively.

Known False Positives

No false positives have been identified at this time.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

$src_ip$ has scanned for ports $dest_ports$ across $totalDestIPCount$ destination IPs

Risk Object	Risk Object Type	Risk Score	Threat Objects
src_ip	system	50	No Threat Objects

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`aws:cloudwatchlogs:vpcflow`	`aws:cloudwatchlogs:vpcflow`
Integration	✅ Passing	Dataset	`aws:cloudwatchlogs:vpcflow`	`aws:cloudwatchlogs:vpcflow`

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: 9