Suspicious Outbound SMTP Connections — Sigma Rule

Rule metadata and linked tags

Author: frack113. Source status: test; severity: medium. Source dates: 2022-01-07 / 2022-09-21.

{
  "category": "network_connection",
  "product": "windows"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 4b00aa101fb2992d5ec7d68a1eece882266289a4f15774b7f9fdcda2eb07e891

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

title: Suspicious Outbound SMTP Connections
id: 9976fa64-2804-423c-8a5b-646ade840773
status: test
description: |
    Adversaries may steal data by exfiltrating it over an un-encrypted network protocol other than that of the existing command and control channel.
    The data may also be sent to an alternate network location from the main command and control server.
references:
    - https://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1048.003/T1048.003.md#atomic-test-5---exfiltration-over-alternative-protocol---smtp
    - https://www.ietf.org/rfc/rfc2821.txt
author: frack113
date: 2022-01-07
modified: 2022-09-21
tags:
    - attack.exfiltration
    - attack.t1048.003
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationPort:
            - 25
            - 587
            - 465
            - 2525
        Initiated: 'true'
    filter_clients:
        Image|endswith:
            - \thunderbird.exe
            - \outlook.exe
    filter_mailserver:
        Image|startswith: 'C:\Program Files\Microsoft\Exchange Server\'
    filter_outlook:
        Image|startswith: 'C:\Program Files\WindowsApps\microsoft.windowscommunicationsapps_'
        Image|endswith: '\HxTsr.exe'
    condition: selection and not 1 of filter_*
falsepositives:
    - Other SMTP tools
level: medium

Original YAML and metadata in JSON

False positives

• Other SMTP tools

Source references

• https://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1048.003/T1048.003.md#atomic-test-5---exfiltration-over-alternative-protocol---smtp
• https://www.ietf.org/rfc/rfc2821.txt