Elastic Security Detection Rules

Microsoft Exchange Server UM Writing Suspicious Files

Last updated 2 months ago on 2025-03-20
Created 4 years ago on 2021-03-04

About

Identifies suspicious files being written by the Microsoft Exchange Server Unified Messaging (UM) service. This activity has been observed exploiting CVE-2021-26858.
Tags
Domain: EndpointOS: WindowsUse Case: Threat DetectionTactic: Initial AccessTactic: Lateral MovementData Source: Elastic EndgameUse Case: VulnerabilityData Source: Elastic DefendData Source: SysmonData Source: Microsoft Defender for EndpointData Source: SentinelOneLanguage: eql
Severity
medium
Risk Score
47
MITRE ATT&CK™

Initial Access (TA0001)(opens in a new tab or window)

↳ Exploit Public-Facing Application (T1190)(opens in a new tab or window)

Lateral Movement (TA0008)(opens in a new tab or window)

↳ Exploitation of Remote Services (T1210)(opens in a new tab or window)

False Positive Examples
Files generated during installation will generate a lot of noise, so the rule should only be enabled after the fact. This rule was tuned using the following baseline: https://raw.githubusercontent.com/microsoft/CSS-Exchange/main/Security/Baselines/baseline_15.2.792.5.csv from Microsoft. Depending on version, consult https://github.com/microsoft/CSS-Exchange/tree/main/Security/Baselines to help determine normalcy.
License
Elastic License v2(opens in a new tab or window)

Definition

Rule Type
Event Correlation Rule
Integration Pack
Prebuilt Security Detection Rules
Index Patterns
winlogbeat-*logs-endpoint.events.file-*logs-windows.sysmon_operational-*endgame-*logs-m365_defender.event-*logs-sentinel_one_cloud_funnel.*
Related Integrations

endpoint(opens in a new tab or window)

windows(opens in a new tab or window)

m365_defender(opens in a new tab or window)

sentinel_one_cloud_funnel(opens in a new tab or window)

Query
file where host.os.type == "windows" and event.type == "creation" and
  process.name : ("UMWorkerProcess.exe", "umservice.exe") and
  file.extension : ("php", "jsp", "js", "aspx", "asmx", "asax", "cfm", "shtml") and
  (
    file.path : "?:\\inetpub\\wwwroot\\aspnet_client\\*" or

    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\*" and
       not (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\version\\*" or
            file.name : ("errorFE.aspx", "expiredpassword.aspx", "frowny.aspx", "GetIdToken.htm", "logoff.aspx",
                        "logon.aspx", "OutlookCN.aspx", "RedirSuiteServiceProxy.aspx", "signout.aspx"))) or

    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\ecp\\auth\\*" and
       not file.name : "TimeoutLogoff.aspx")
  )

Install detection rules in Elastic Security

Detect Microsoft Exchange Server UM Writing Suspicious Files in the Elastic Security detection engine by installing this rule into your Elastic Stack.

To setup this rule, check out the installation guide for Prebuilt Security Detection Rules(opens in a new tab or window).