Sigma rule: Suspicious Outbound SMTP Connections

Rule

Title

Suspicious Outbound SMTP Connections

Status

test

Level

medium

Author

frack113

Date

2022-01-07

Modified

2022-09-21

File

rules/windows/network_connection/net_connection_win_susp_outbound_smtp_connections.yml

Commit

ccbb0369507f796db0b0ae6854c8a2be471b9d84

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.

Log source

product

windows

category

network_connection

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_*

False positives (as the rule lists them)

• Other SMTP tools

Tags

• attack.exfiltration
• attack.t1048.003

MITRE ATT&CK ids this rule names

• T1048.003

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

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/windows/network_connection/net_connection_win_susp_outbound_smtp_connections.yml