Windows: CVE-2021-26858 Exchange Exploitation

Rule ID

PH_Rule_SIGMA_888

Default Status

Enabled

Description

Detects possible successful exploitation for vulnerability described in CVE-2021-26858 by looking for creation of non-standard files on disk by Exchange Server‚Äö√Ñ√¥s Unified Messaging service which could indicate dropping web shells or other malicious content . This rule is adapted from https://github.com/SigmaHQ/sigma/blob/master/rules/windows/file/file_event/file_event_win_cve_2021_26858_msexchange.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

T1203

Exploitation for Client Execution

Adversaries may exploit software vulnerabilities in client applications to execute code. Vulnerabilities can exist in software due to unsecure coding practices that can lead to unanticipated behavior. Adversaries can take advantage of certain vulnerabilities through targeted exploitation for the purpose of arbitrary code execution. Several types exist: Browser-based Exploitation, Office Applications and Common Third-party Applications.

https://attack.mitre.org/techniques/T1203

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-11-FileCreate" AND procName REGEXP "UMWorkerProcess\.exe$" AND (fileName NOT REGEXP "CacheCleanup\.bin$|\.LOG$|\.cfg$|\.txt$|cleanup\.bin$")

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

fileName,hostName,procName

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

fileName = Filter.fileName,
hostName = Filter.hostName,
procName = Filter.procName

FortiSIEM Rules