Cisco Secure Firewall - Bits Network Activity

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

The following analytic detects the use of the Background Intelligent Transfer Service (BITS) client application in allowed outbound connections. It leverages logs from Cisco Secure Firewall Threat Defense devices and identifies instances where BITS is used to initiate downloads from non-standard or unexpected domains. While BITS is a legitimate Windows service used for downloading updates, it is also commonly abused by adversaries to stealthily retrieve payloads or tools. This analytic filters out known Microsoft Edge update URLs and focuses on connections that may indicate suspicious or unauthorized file transfers. If confirmed malicious, this could represent a command and control (C2) channel or a download of malware or tooling as part of an attack chain.

name: Cisco Secure Firewall - Bits Network Activity
id: b08e69d4-b42d-494c-bd30-abaaa3571ba4
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 the use of the Background Intelligent Transfer Service (BITS) client application in allowed outbound connections. It leverages logs from Cisco Secure Firewall Threat Defense devices and identifies instances where BITS is used to initiate downloads from non-standard or unexpected domains. While BITS is a legitimate Windows service used for downloading updates, it is also commonly abused by adversaries to stealthily retrieve payloads or tools. This analytic filters out known Microsoft Edge update URLs and focuses on connections that may indicate suspicious or unauthorized file transfers. If confirmed malicious, this could represent a command and control (C2) channel or a download of malware or tooling as part of an attack chain.
data_source:
    - Cisco Secure Firewall Threat Defense Connection Event
search: |
    `cisco_secure_firewall` EventType=ConnectionEvent action IN ("Trust", "Allow", "allowed") ClientApplication="BITS" AND NOT url IN ("*://msedge.b.tlu.dl*")
    | stats count min(_time) as firstTime max(_time) as lastTime by src, dest, dest_port, transport, rule, url, EVE_Process, ClientApplication, ClientApplicationVersion, action
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `cisco_secure_firewall___bits_network_activity_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: |
    BITS is a legitimate Windows component used by Microsoft services such as Windows Update or Microsoft Edge for downloading updates.
    Although this analytic filters known Microsoft Edge update URLs, false positives may still occur from other legitimate enterprise applications or software distribution platforms that utilize BITS.
    Additional tuning may be required to account for internal application distribution systems or approved update mechanisms that also rely on BITS.
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: $src$ downloaded a file from $url$ via BITS Service
        - field: dest
          type: system
          score: 20
          message: $src$ downloaded a file from $url$ via BITS Service
threat_objects:
    - field: url
      type: url
analytic_story:
    - Cisco Secure Firewall Threat Defense Analytics
asset_type: Network
mitre_attack_id: []
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
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

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