IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.

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SMTP to the Internet

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SMTP to the Internet

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Detects events that may describe SMTP traffic from internal hosts to a host across the Internet. In an enterprise network, there is typically a dedicated internal host that performs this function. It is also frequently abused by threat actors for command and control, or data exfiltration.

Rule type: query

Rule indices:

filebeat-*
packetbeat-*
logs-endpoint.events.*

Severity: low

Risk score: 21

Runs every: 5 minutes

Searches indices from: now-6m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

Tags:

Elastic
Host
Network
Threat Detection
Command and Control

Version: 5 (version history)

Added (Elastic Stack release): 7.6.0

Last modified (Elastic Stack release): 7.10.0

Rule authors: Elastic

Rule license: Elastic License

Potential false positives

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NATed servers that process email traffic may false and should be excluded from this rule as this is expected behavior for them. Consumer and personal devices may send email traffic to remote Internet destinations. In this case, such devices or networks can be excluded from this rule if this is expected behavior.

Rule query

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event.category:(network or network_traffic) and network.transport:tcp
and (destination.port:(25 or 465 or 587) or event.dataset:zeek.smtp)
and source.ip:(10.0.0.0/8 or 172.16.0.0/12 or 192.168.0.0/16) and not
destination.ip:(10.0.0.0/8 or 127.0.0.0/8 or 172.16.0.0/12 or
192.168.0.0/16 or "::1")

Threat mapping

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Framework: MITRE ATT&CK^TM

Tactic:
- Name: Command and Control
- ID: TA0011
- Reference URL: https://attack.mitre.org/tactics/TA0011/
Technique:
- Name: Commonly Used Port
- ID: T1043
- Reference URL: https://attack.mitre.org/techniques/T1043/
Tactic:
- Name: Exfiltration
- ID: TA0010
- Reference URL: https://attack.mitre.org/tactics/TA0010/
Technique:
- Name: Exfiltration Over Alternative Protocol
- ID: T1048
- Reference URL: https://attack.mitre.org/techniques/T1048/

Rule version history

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Version 5 (7.10.0 release)

Formatting only

Version 4 (7.9.0 release)

Updated query, changed from:

network.transport:tcp and destination.port:(25 or 465 or 587) and
source.ip:(10.0.0.0/8 or 172.16.0.0/12 or 192.168.0.0/16) and not
destination.ip:(10.0.0.0/8 or 127.0.0.0/8 or 172.16.0.0/12 or
192.168.0.0/16 or "::1")

Version 3 (7.7.0 release)

Updated query, changed from:

network.transport: tcp and destination.port: (25 or 465 or 587) and (
network.direction: outbound or ( source.ip: (10.0.0.0/8 or
172.16.0.0/12 or 192.168.0.0/16) and not destination.ip: (10.0.0.0/8
or 172.16.0.0/12 or 192.168.0.0/16) ) )

Version 2 (7.6.1 release)

Removed auditbeat-*, packetbeat-*, and winlogbeat-* from the rule indices.

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