LoFP / 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.

Techniques

• T1041
• T1048.003
• T1567.002

Sample rules

Cisco Secure Firewall - Potential Data Exfiltration

• source: splunk
• technicques:
  • T1041
  • T1567.002
  • T1048.003

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.

Detection logic

`cisco_secure_firewall` EventType=ConnectionEvent `cisco_secure_firewall_inside_to_outside`

| eval total_bytes = InitiatorBytes + ResponderBytes

| eval total_mb = round(total_bytes / 1024 / 1024, 2)

| where total_mb >= 100

| eval Exfiltrated = total_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_ip, dest, Exfiltrated, transport, action

| `security_content_ctime(firstTime)`

| `security_content_ctime(lastTime)`

| `cisco_secure_firewall___potential_data_exfiltration_filter`