Windows: Change Default File Association To Executable Via Assoc

Rule ID

PH_Rule_SIGMA_2280

Default Status

Enabled

Description

Detects when a program changes the default file association of any extension to an executable. When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access or by administrators using the built-in assoc utility. Applications can modify the file association for a given file extension to call an arbitrary program when a file with the given extension is opened. . This rule is adapted from https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_cmd_assoc_tamper_exe_file_association.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

T1546.001

Event Triggered Execution: Change Default File Association

Adversaries may establish persistence by executing malicious content triggered by a file type association. When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access The values of the keys listed are commands that are executed when the handler opens the file extension. Adversaries can modify these values to continually execute arbitrary commands.

https://attack.mitre.org/techniques/T1546/001

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 "\\cmd\.exe$" OR srcFileName="Cmd.Exe") AND (command REGEXP ".*assoc .*" AND command REGEXP ".*exefile.*")) AND (command NOT REGEXP ".*\.exe=exefile.*")

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

command,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

command = Filter.command,
hostName = Filter.hostName,
procName = Filter.procName,
srcFileName = Filter.srcFileName

FortiSIEM Rules