Cisco Secure Firewall - Potential Data Exfiltration

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

The following analytic detects potentially suspicious large volumes of data sent by the connection initiator on flows from internal to external networks. It leverages Cisco Secure Firewall Threat Defense ConnectionEvent logs and thresholds on InitiatorBytes (bytes transmitted by the initiator), which for typical inside-initiated client sessions approximates upload or outbound payload from the internal host and avoids flagging large downloads where most bytes appear in ResponderBytes. Connections where the initiator sent at least 100 MB are flagged, as these may indicate unauthorized data exfiltration, especially if associated with unusual users, hosts, or processes. This analytic is scoped to inside-to-outside flows using a macro (cisco_secure_firewall_inside_to_outside) to abstract environment-specific zone definitions. If confirmed malicious, this behavior may reflect data staging and exfiltration over an encrypted or stealthy transport.

name: Cisco Secure Firewall - Potential Data Exfiltration
id: 3d8536b6-52b4-4c3e-b695-3f2e90bb22be
version: 7
creation_date: '2025-04-03'
modification_date: '2026-05-13'
author: Nasreddine Bencherchali, Splunk
status: production
type: Anomaly
description: |
    The following analytic detects potentially suspicious large volumes of data sent by the connection initiator on flows from internal to external networks. It leverages Cisco Secure Firewall Threat Defense ConnectionEvent logs and thresholds on InitiatorBytes (bytes transmitted by the initiator), which for typical inside-initiated client sessions approximates upload or outbound payload from the internal host and avoids flagging large downloads where most bytes appear in ResponderBytes. Connections where the initiator sent at least 100 MB are flagged, as these may indicate unauthorized data exfiltration, especially if associated with unusual users, hosts, or processes. This analytic is scoped to inside-to-outside flows using a macro (cisco_secure_firewall_inside_to_outside) to abstract environment-specific zone definitions. If confirmed malicious, this behavior may reflect data staging and exfiltration over an encrypted or stealthy transport.
data_source:
    - Cisco Secure Firewall Threat Defense Connection Event
search: |
    `cisco_secure_firewall` EventType=ConnectionEvent `cisco_secure_firewall_inside_to_outside`
    | eval initiator_mb = round(InitiatorBytes / 1024 / 1024, 2)
    | where initiator_mb >= 100
    | eval Potentially_Exfiltrated = initiator_mb + " MB"
    | stats min(_time) as firstTime max(_time) as lastTime
        Values(url) as url
        Values(rule) as rule
        Values(dest_port) as dest_port
        by src, dest, Potentially_Exfiltrated, transport, action
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `cisco_secure_firewall___potential_data_exfiltration_filter`
how_to_implement: |
    This search requires Cisco Secure Firewall Threat Defense Logs, which
    includes the ConnectionEvent EventType. This search uses two input macros named `cisco_secure_firewall` and `cisco_secure_firewall_inside_to_outside`.
    We strongly recommend that you specify your environment-specific configurations
    (index, source, sourcetype, etc.) for Cisco Secure Firewall Threat Defense logs. Replace the macro definitions
    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: |
    Large initiator-side transfers may occur due to legitimate activities such as cloud backups, file syncing, or developer build deployments.
    Backup servers, CI/CD pipelines, and enterprise sync tools (e.g., OneDrive, Dropbox) may exhibit similar patterns.
    Uncommon connection initiation (e.g., certain server-initiated or asymmetric paths) can change which side is logged as the initiator; tune or filter those scenarios if needed.
    Additional validation using user context, scheduled task windows, or endpoint telemetry is recommended to reduce false positives.
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: Potential data exfiltration activity from $src$ to $dest$ — With $Potentially_Exfiltrated$ transferred (initiator bytes)
threat_objects:
    - field: url
      type: url
analytic_story:
    - Cisco Secure Firewall Threat Defense Analytics
asset_type: Network
mitre_attack_id:
    - T1041
    - T1567.002
    - T1048.003
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

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Executable code, not a declarative rule

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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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