Detection rules › Splunk
Windows Mail Protocol In Non-Common Process Path
The following analytic detects a Windows application establishing an SMTP connection from a non-common installation path. It leverages Sysmon EventCode 3 to identify processes not typically associated with email clients (e.g., Thunderbird, Outlook) making SMTP connections. This activity is significant as adversaries, including malware like AgentTesla, use such connections for Command and Control (C2) communication to exfiltrate stolen data. If confirmed malicious, this behavior could lead to unauthorized data exfiltration, including sensitive information like desktop screenshots, browser data, and system details, compromising the affected host.
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Command & Control | T1071.003 Application Layer Protocol: Mail Protocols |
Event coverage
| Provider | Event ID | Title |
|---|---|---|
| Sysmon | 3 | Network connection |
Stages and Predicates
Stage 1: search
search (DestinationPort IN (25, 587) OR DestinationPortName="smtp") NOT Image IN ("C:\\Program Files \(x86\)\\*", "C:\\Program Files\\*", "C:\\Windows\\SysWOW64\\*", "C:\\Windows\\System32\\*") EventCode=3
Stage 2: stats
stats BY action, app, dest, dest_ip, dest_port, direction, dvc, protocol, protocol_version, src, src_ip, src_port, transport, user, vendor_product, process_name, process_exec, process_guid, process_id
Stage 3: search
search
Stage 4: search
search
Stage 5: search
search `macro`
Exclusions
Top-level NOT(...) conjuncts — predicates this rule actively suppresses.
| Stage | Field | Kind | Excluded values |
|---|---|---|---|
| 1 | Image | in | "C:\\Program Files \(x86\)\\*", "C:\\Program Files\\*", "C:\\Windows\\SysWOW64\\*", "C:\\Windows\\System32\\*" |
Indicators
Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.
| Field | Kind | Values |
|---|---|---|
DestinationPort | in |
|
DestinationPortName | eq |
|
EventCode | eq |
|