Windows: Suspicious Csi.exe Usage

Rule ID

PH_Rule_SIGMA_1549

Default Status

Enabled

Description

Csi.exe is a signed binary from Microsoft that comes with Visual Studio and provides C# interactive capabilities. It can be used to run C# code from a file passed as a parameter in command line. Early version of this utility provided with Microsoft ‚Äö√Ñ√∫Roslyn‚Äö√Ñ√π Community Technology Preview was named 'rcsi.exe'. This rule is adapted from https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_csi_execution.yml

Severity

MITRE ATT&CK® Tactics

Execution

Execution consists of techniques that result in adversary-controlled code running on a local or remote system. Techniques that run malicious code are often paired with techniques from all other tactics to achieve broader goals, like exploring a network or stealing data. For example, an adversary might use a remote access tool to run a PowerShell script that does Remote System Discovery.

https://attack.mitre.org/tactics/TA0002

MITRE ATT&CK® Techniques

T1072

Software Deployment Tools

Adversaries may gain access to and use third-party software suites installed within an enterprise network, such as administration, monitoring, and deployment systems, to move laterally through the network. Third-party applications and software deployment systems may be in use in the network environment for administration purposes (e.g., SCCM, VNC, HBSS, Altiris, etc.).

https://attack.mitre.org/techniques/T1072

Impacts

Server

Data Source

Windows Sysmon via FortiSIEM Agent

Detection

Correlation

Remediation Guidance

No remediation guidance specified

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 (procName REGEXP "\\csi\.exe$|\\rcsi\.exe$" OR srcFileName IN ("csi.exe","rcsi.exe")) AND company="Microsoft Corporation"

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

company,hostName,procName,srcFileName

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

company = Filter.company,
hostName = Filter.hostName,
procName = Filter.procName,
srcFileName = Filter.srcFileName

FortiSIEM Rules