Cisco Secure Firewall - Communication Over Suspicious Ports
SPLUNK detection rule by Nasreddine Bencherchali, Splunk · Apache-2.0
The following analytic detects potential reverse shell activity by identifying connections involving ports commonly associated with remote access tools, shell listeners, or tunneling utilities. It leverages Cisco Secure Firewall Threat Defense logs and monitors destination ports against a list of non-standard, high-risk port values often used in post-exploitation scenarios. Adversaries frequently configure tools like netcat, Meterpreter, or other backdoors to listen or connect over uncommon ports such as 4444, 2222, or 51820 to bypass standard monitoring and firewall rules. If confirmed malicious, this activity may represent command and control (C2) tunneling, lateral movement, or unauthorized remote access.
name: Cisco Secure Firewall - Communication Over Suspicious Ports
id: d85c05c8-42c0-4e4a-87e7-4e1bb3e844e3
version: 8
creation_date: '2025-04-03'
modification_date: '2026-05-13'
author: Nasreddine Bencherchali, Splunk
status: production
type: Anomaly
description: |
The following analytic detects potential reverse shell activity by identifying connections involving ports commonly associated with remote access tools, shell listeners, or tunneling utilities. It leverages Cisco Secure Firewall Threat Defense logs and monitors destination ports against a list of non-standard, high-risk port values often used in post-exploitation scenarios. Adversaries frequently configure tools like netcat, Meterpreter, or other backdoors to listen or connect over uncommon ports such as 4444, 2222, or 51820 to bypass standard monitoring and firewall rules. If confirmed malicious, this activity may represent command and control (C2) tunneling, lateral movement, or unauthorized remote access.
data_source:
- Cisco Secure Firewall Threat Defense Connection Event
search: |
`cisco_secure_firewall` EventType=ConnectionEvent dest_port IN ("888", "999", "2200", "2222", "4000", "4444", "6789", "8531", "50501", "51820") | fillnull value="unknown" url
| stats min(_time) as firstTime max(_time) as lastTime
values(src_port) as src_port
values(url) as url
values(rule) as rule
count by src, dest, dest_port, transport, action
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `cisco_secure_firewall___communication_over_suspicious_ports_filter`
how_to_implement: |
This search requires Cisco Secure Firewall Threat Defense Logs, which
includes the ConnectionEvent EventType. This search uses an input macro named `cisco_secure_firewall`.
We strongly recommend that you specify your environment-specific configurations
(index, source, sourcetype, etc.) for Cisco Secure Firewall Threat Defense logs. Replace the macro definition
with configurations for your Splunk environment. The search also uses a post-filter
macro designed to filter out known false positives.
The logs are to be ingested using the Splunk Add-on for Cisco Security Cloud (https://splunkbase.splunk.com/app/7404).
The access policy must also enable logging.
The search uses a hardcoded list of suspicious ports, you might want to tune those according to your environment
known_false_positives: |
Some legitimate services or custom applications may use non-standard ports for development, remote management, or internal communication.
Ephemeral ports in test environments may occasionally overlap with ports used in this detection.
Additional context such as process name, user behavior, or endpoint telemetry should be used to validate suspicious sessions before escalation.
references:
- https://www.cisco.com/c/en/us/td/docs/security/firepower/741/api/FQE/secure_firewall_estreamer_fqe_guide_740.pdf
drilldown_searches:
- name: View the detection results for - "$src$"
search: '%original_detection_search% | search src = "$src$"'
earliest_offset: $info_min_time$
latest_offset: $info_max_time$
- name: View risk events for the last 7 days for - "$src$"
search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$src$") | 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"
intermediate_findings:
entities:
- field: src
type: system
score: 20
message: Suspicious communication detected from $src$ to $dest$ over port $dest_port$.
threat_objects:
- field: url
type: url
analytic_story:
- Cisco Secure Firewall Threat Defense Analytics
asset_type: Network
mitre_attack_id:
- T1021
- T1055
- T1059.001
- T1105
- T1219
- T1571
product:
- Splunk Enterprise
- Splunk Cloud
- Splunk Enterprise Security
category: network
security_domain: network
tests:
- name: 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
CIRCL