Cisco Secure Firewall - Repeated Blocked Connections

SPLUNK detection rule by Nasreddine Bencherchali, Splunk · Apache-2.0

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.

name: Cisco Secure Firewall - Repeated Blocked Connections
id: 1f57f10e-1dc5-47ea-852c-2e85b2503d79
version: 9
creation_date: '2025-04-03'
modification_date: '2026-05-13'
author: Nasreddine Bencherchali, Splunk
status: production
type: Anomaly
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.
data_source:
    - Cisco Secure Firewall Threat Defense Connection Event
search: |
    `cisco_secure_firewall` EventType=ConnectionEvent action IN ("Block with reset", "Block", "blocked")
    | bin _time span=1m
    | stats count min(_time) as firstTime max(_time) as lastTime
        Values(dest_port) as dest_port
        Values(url) as url
        by src, dest, transport, rule, action
    | where count >= 10
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `cisco_secure_firewall___repeated_blocked_connections_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.
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.
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: Repeated blocked connections detected from $src$ to $dest$ according to the configured firewall rule $rule$
threat_objects:
    - field: url
      type: url
analytic_story:
    - Cisco Secure Firewall Threat Defense Analytics
asset_type: Network
mitre_attack_id:
    - T1018
    - T1046
    - T1110
    - T1203
    - T1595.002
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

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

b9872c7b-5dcb-46e8-8420-a845b464df35

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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.

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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.

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