Detect Mail Protocols in CrowdStrike LogScale
Adversaries may communicate using application layer protocols associated with electronic mail delivery to avoid detection/network filtering by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server. Protocols such as SMTP/S, POP3/S, and IMAP that carry electronic mail may be very common in environments. Packets produced from these protocols may have many fields and headers in which data can be concealed. Data could also be concealed within the email messages themselves. An adversary may abuse these protocols to communicate with systems under their control within a victim network while also mimicking normal, expected traffic.
MITRE ATT&CK
- Tactic
- Command and Control
- Technique
- T1071 Application Layer Protocol
- Sub-technique
- T1071.003 Mail Protocols
- Canonical reference
- https://attack.mitre.org/techniques/T1071/003/
LogScale Detection Query
#event_simpleName=NetworkConnectIP4
| in(field=RemotePort, values=[25, 465, 587, 110, 995, 143, 993])
| !regex(field=ImageFileName, regex="(?i)(outlook|thunderbird|MailClient|eM.Client|msedge|chrome|firefox)\\.exe$")
| Protocol := case {
in(field=RemotePort, values=[25, 465, 587]) => "SMTP";
in(field=RemotePort, values=[110, 995]) => "POP3";
in(field=RemotePort, values=[143, 993]) => "IMAP";
* => "Unknown"
}
| groupBy(
[ComputerName, ImageFileName, RemoteAddressIP4, RemotePort, Protocol, UserName],
function=[
count(as=ConnectionCount),
min(ContextTimeStamp, as=FirstSeen),
max(ContextTimeStamp, as=LastSeen)
]
)
| ConnectionCount > 2
| sort(ConnectionCount, order=desc, limit=100)CrowdStrike LogScale (Falcon) query using NetworkConnectIP4 telemetry from the Falcon sensor to detect non-standard processes making repeated outbound connections to mail protocol ports. Groups events by host, process image, destination IP/port, and user, then filters for more than 2 connections to surface persistent C2 activity blending into mail protocol traffic. Excludes common mail client binaries via regex on ImageFileName.
Data Sources
Required Tables
False Positives & Tuning
- Scripting runtime processes (powershell.exe, python.exe, wscript.exe, cscript.exe) used by legitimate IT automation to send SMTP notifications or read mail via IMAP for ticketing integrations
- Custom enterprise thick clients or internal tools built on .NET or Java frameworks that handle email but do not match standard mail client binary naming conventions
- Vulnerability scanners or network asset discovery tools (e.g., Nessus, OpenVAS agent processes) probing mail ports as part of scheduled or on-demand authorized scans
Other platforms for T1071.003
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1SMTP C2 Exfiltration via PowerShell
Expected signal: Sysmon Event ID 1: PowerShell process creation with Send-MailMessage in command line. Sysmon Event ID 3: Network connection attempt to 127.0.0.1:25. PowerShell ScriptBlock Log Event ID 4104 with SMTP command details.
- Test 2POP3 Command Retrieval Simulation
Expected signal: Sysmon for Linux Event ID 3: Network connection from nc to 127.0.0.1:110. Process creation event for nc with POP3 commands piped via stdin.
- Test 3IMAP C2 Channel Simulation
Expected signal: Sysmon for Linux Event ID 3: Network connection from curl to 127.0.0.1:143. Process creation event for curl with imap:// URL and credentials in command line.
References (6)
- https://attack.mitre.org/techniques/T1071/003/
- https://web.archive.org/web/20151022204649/https://www.fireeye.com/content/dam/fireeye-www/global/en/current-threats/pdfs/rpt-apt28.pdf
- https://www.welivesecurity.com/wp-content/uploads/2020/05/ESET_Turla_ComRAT.pdf
- https://www.welivesecurity.com/2019/05/29/turla-powershell-usage/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1071.003/T1071.003.md
- https://pwc.github.io/issues/2023/yellow-liderc.html
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