Detection: Cisco Secure Firewall - Potential Data Exfiltration

Updated Date: 2025-04-02 ID: 3d8536b6-52b4-4c3e-b695-3f2e90bb22be Author: Nasreddine Bencherchali, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic detects potentially suspicious large outbound data transfers from internal to external networks. It leverages Cisco Secure Firewall Threat Defense logs and calculates the total volume of data exchanged per connection by summing InitiatorBytes and ResponderBytes. Connections exceeding 100 MB are flagged, as these may indicate unauthorized data exfiltration, especially if initiated by 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.

 1`cisco_secure_firewall` EventType=ConnectionEvent `cisco_secure_firewall_inside_to_outside`
 2
 3| eval total_bytes = InitiatorBytes + ResponderBytes
 4
 5| eval total_mb = round(total_bytes / 1024 / 1024, 2)
 6
 7| where total_mb >= 100
 8
 9| eval Exfiltrated = total_mb + " MB"
10
11| stats min(_time) as firstTime max(_time) as lastTime
12    Values(url) as url
13    Values(rule) as rule
14    Values(dest_port) as dest_port
15    by src_ip, dest, Exfiltrated, transport, action
16
17| `security_content_ctime(firstTime)`
18
19| `security_content_ctime(lastTime)`
20
21| `cisco_secure_firewall___potential_data_exfiltration_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___potential_data_exfiltration_filter search *
cisco_secure_firewall___potential_data_exfiltration_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
T1041 Exfiltration Over C2 Channel Exfiltration
T1567.002 Exfiltration to Cloud Storage Exfiltration
T1048.003 Exfiltration Over Unencrypted Non-C2 Protocol Exfiltration
Actions on Objectives
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 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 outbound transfers may occur due to legitimate activities such as cloud backups, file syncing, OS or application updates, or developer build deployments. Backup servers, CI/CD pipelines, and enterprise sync tools (e.g., OneDrive, Dropbox) may exhibit similar patterns. Additional validation using user context, scheduled task windows, or endpoint telemetry is recommended to reduce false positives.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

Potential data exfiltration from $src_ip$ to $dest$ with $Exfiltrated$ MB of data exfiltrated"

Risk Object Risk Object Type Risk Score Threat Objects
src_ip system 40 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: 1