Detection: Cisco Secure Firewall - Communication Over Suspicious Ports

Updated Date: 2025-05-02 ID: d85c05c8-42c0-4e4a-87e7-4e1bb3e844e3 Author: Nasreddine Bencherchali, Splunk Type: Anomaly Product: Splunk Enterprise Security

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.

 1`cisco_secure_firewall` EventType=ConnectionEvent dest_port IN ("888", "999", "2200", "2222", "4000", "4444", "6789", "8531", "50501", "51820") 
 2| fillnull value="unknown" url
 3
 4| stats min(_time) as firstTime max(_time) as lastTime 
 5      values(src_port) as src_port
 6      values(url) as url
 7      values(rule) as rule
 8      count by src_ip, dest, dest_port, transport, action
 9
10| `security_content_ctime(firstTime)`
11
12| `security_content_ctime(lastTime)`
13
14| `cisco_secure_firewall___communication_over_suspicious_ports_filter`

Data Source

Name Platform Sourcetype Source
Cisco Secure Firewall Threat Defense Connection Event N/A 'cisco:sfw:estreamer' 'not_applicable'

Macros Used

Name Value
cisco_secure_firewall sourcetype="cisco:sfw:estreamer"
cisco_secure_firewall___communication_over_suspicious_ports_filter search *
cisco_secure_firewall___communication_over_suspicious_ports_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
T1021 Remote Services Lateral Movement
T1055 Process Injection Defense Evasion
T1059.001 PowerShell Privilege Escalation
T1105 Ingress Tool Transfer Execution
T1219 Remote Access Tools Command And Control
T1571 Non-Standard Port Command And Control
Command and Control
Exploitation
Installation
DE.AE
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 Risk Event True
This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

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.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

Suspicious communication detected from $src_ip$ to $dest$ over port $dest_port$.

Risk Object Risk Object Type Risk Score Threat Objects
src_ip system 20 url

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation ✅ Passing N/A N/A N/A
Unit ✅ Passing Dataset not_applicable cisco:sfw:estreamer
Integration ✅ Passing Dataset not_applicable cisco:sfw:estreamer

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