Detection: Cisco Secure Firewall - Repeated Blocked Connections

Updated Date: 2025-05-02 ID: 1f57f10e-1dc5-47ea-852c-2e85b2503d79 Author: Nasreddine Bencherchali, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic detects repeated blocked connection attempts from the same initiator to the same responder within a short time window. It leverages Cisco Secure Firewall Threat Defense logs and identifies connections where the action is set to Block, and the number of occurrences reaches or exceeds a threshold of ten within a one-minute span. This pattern may indicate a misconfigured application, unauthorized access attempts, or early stages of a brute-force or scanning operation. If confirmed malicious, this behavior may represent an attacker probing the network, attempting lateral movement, or testing firewall rules for weaknesses.

 1`cisco_secure_firewall` EventType=ConnectionEvent action=Block 
 2
 3| bin _time span=1m 
 4
 5| stats count min(_time) as firstTime max(_time) as lastTime
 6    Values(dest_port) as dest_port
 7    Values(url) as url
 8    by src_ip, dest, transport, rule, action
 9
10| where count >= 10
11
12| `security_content_ctime(firstTime)`
13
14| `security_content_ctime(lastTime)`
15
16| `cisco_secure_firewall___repeated_blocked_connections_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___repeated_blocked_connections_filter search *
cisco_secure_firewall___repeated_blocked_connections_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
T1018 Remote System Discovery Discovery
T1046 Network Service Discovery Discovery
T1110 Brute Force Credential Access
T1203 Exploitation for Client Execution Execution
T1595.002 Vulnerability Scanning Reconnaissance
Exploitation
Installation
Reconnaissance
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.

Known False Positives

Misconfigured applications or automated scripts may generate repeated blocked traffic, particularly if attempting to reach decommissioned or restricted resources. Vulnerability scanners or penetration testing tools running in authorized environments may trigger this alert. Tuning may be required to exclude known internal tools or scanner IPs from detection.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

Repeated blocked connections detected from $src_ip$ to $dest$ according to the configured firewall rule $rule$

Risk Object Risk Object Type Risk Score Threat Objects
src_ip system 25 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