Windows: PsExec Service Child Process Execution as LOCAL SYSTEM

Rule ID

PH_Rule_SIGMA_2575

Default Status

Enabled

Description

Detects suspicious launch of the PSEXESVC service on this system and a sub process run as LOCAL_SYSTEM (-s), which means that someone remotely started a command on this system running it with highest privileges and not only the privileges of the login user account (e.g. the administrator account). This rule is adapted from https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_sysinternals_psexesvc_as_system.yml

Severity

MITRE ATT&CK® Tactics

Persistence

Persistence consists of techniques that adversaries use to keep access to systems across restarts, changed credentials, and other interruptions that could cut off their access. Techniques used for persistence include any access, action, or configuration changes that let them maintain their foothold on systems, such as replacing or hijacking legitimate code or adding startup code.

https://attack.mitre.org/tactics/TA0003

MITRE ATT&CK® Techniques

No Technique Specified

Impacts

Server

Data Source

Windows Sysmon via FortiSIEM Agent

Detection

Correlation

Remediation Guidance

Investigate whether the user needs to really run PAExec or PsExec. Typically system administrators need these utilities to do their work. Investigate why legitimate PsExec needs to run with highest privilege. PAExec may be bundled with other software. If necessary, rebuild the host from a known, good source and have the user change their password.

Time Window

If the following pattern or patterns match an ingested event within the given time window in seconds, trigger an incident.

300 seconds

Trigger Conditions

If the following defined pattern/s occur within a 300 second time window.

Filter

SubPattern Definitions

SubPattern Name: Filter

This is the named definition of the event query, this is important if multiple subpatterns are defined to distinguish them.

SubPattern Query

This is the query logic that matches incoming events

eventType="Win-Sysmon-1-Create-Process" AND parentProcName="C:\\Windows\\PSEXESVC.exe" AND user REGEXP ".*AUTHORI.*|.*AUTORI.*"

Group by Attributes

This defines how matching events are aggregated, only events with the same matching attribute values are grouped into one unique incident ID

hostName,parentProcName,user

Aggregate Constraint

This is most typically a numerical constraint that defines when the rule should trigger an incident

COUNT(*) >= 1

Incident Attribute Mapping

This section defines which fields in matching raw events should be mapped to the incident attributes in the resulting incident.

The available raw event attributes to map are limited to the group by attributes and the aggregate event constraint fields for each subpattern

hostName = Filter.hostName,
parentProcName = Filter.parentProcName,
user = Filter.user

FortiSIEM Rules