Internal Horizontal Port Scan

SPLUNK detection rule by Dean Luxton · Apache-2.0

This analytic identifies instances where an internal host has attempted to communicate with 250 or more destination IP addresses using the same port and protocol. 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.

name: Internal Horizontal Port Scan
id: 1ff9eb9a-7d72-4993-a55e-59a839e607f1
version: 14
creation_date: '2024-07-01'
modification_date: '2026-05-13'
author: Dean Luxton
status: production
type: TTP
description: This analytic identifies instances where an internal host has attempted to communicate with 250 or more destination IP addresses using the same port and protocol. 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.
data_source:
    - AWS CloudWatchLogs VPCflow
    - Cisco Secure Firewall Threat Defense Connection Event
search: |-
    | tstats `security_content_summariesonly` values(All_Traffic.action) as action values(All_Traffic.src_category) as src_category values(All_Traffic.dest_zone) as dest_zone values(All_Traffic.src_zone) as src_zone values(All_Traffic.src_port) as src_port count FROM datamodel=Network_Traffic
      WHERE All_Traffic.src_ip IN ("10.0.0.0/8","172.16.0.0/12","192.168.0.0/16")
      BY All_Traffic.src_ip All_Traffic.dest_port All_Traffic.dest_ip
         All_Traffic.transport All_Traffic.rule span=1s
         _time
    | `drop_dm_object_name("All_Traffic")`
    | eval gtime=_time
    | bin span=1h gtime
    | stats min(_time) as _time values(action) as action dc(dest_ip) as totalDestIPCount values(src_category) as src_category values(dest_zone) as dest_zone values(src_zone) as src_zone
      BY src_ip dest_port gtime
         transport
    | where totalDestIPCount>=250
    | eval dest_port=transport + "/" + dest_port
    | stats min(_time) as _time values(action) as action sum(totalDestIPCount) as totalDestIPCount values(src_category) as src_category values(dest_port) as dest_ports values(dest_zone) as dest_zone values(src_zone) as src_zone
      BY src_ip gtime
    | fields - gtime
    | `internal_horizontal_port_scan_filter`
how_to_implement: To properly run this search, Splunk needs to ingest data from networking telemetry sources such as firewalls, 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.
references: []
drilldown_searches:
    - name: View the detection results for - "$src_ip$"
      search: '%original_detection_search% | search  src_ip = "$src_ip$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$src_ip$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$src_ip$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
finding:
    title: $src_ip$ has scanned for ports $dest_ports$ across $totalDestIPCount$ destination IPs
    entity:
        field: dest_ports
        type: system
        score: 50
threat_objects:
    - field: src_ip
      type: ip_address
analytic_story:
    - Network Discovery
    - Cisco Secure Firewall Threat Defense Analytics
    - China-Nexus Threat Activity
    - Scattered Lapsus$ Hunters
asset_type: Endpoint
mitre_attack_id:
    - T1046
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: network
security_domain: network
tests:
    - name: AWS CloudWatch True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1046/nmap/horizontal.log
          source: aws:cloudwatchlogs:vpcflow
          sourcetype: aws:cloudwatchlogs:vpcflow
      test_type: unit
    - name: Cisco Secure Firewall True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/cisco_secure_firewall_threat_defense/connection_event/connection_events.log
          source: not_applicable
          sourcetype: cisco:sfw:estreamer
      test_type: unit

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

a55a42cb-881a-4b49-80ab-833a0cda97c0

Propose edits, view related rules, and engage with the community through comments.

Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

Community Discussion