IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.

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Microsoft Exchange Server UM Writing Suspicious Files

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Microsoft Exchange Server UM Writing Suspicious Files

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Identifies suspicious files being written by the Microsoft Exchange Server Unified Messaging (UM) service. This activity has been observed exploiting CVE-2021-26858.

Rule type: eql

Rule indices:

winlogbeat-*
logs-endpoint.events.file-*
logs-windows.sysmon_operational-*
endgame-*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

Domain: Endpoint
OS: Windows
Use Case: Threat Detection
Tactic: Initial Access
Tactic: Lateral Movement
Data Source: Elastic Endgame
Use Case: Vulnerability
Data Source: Elastic Defend
Data Source: Sysmon

Version: 108

Rule authors:

Elastic
Austin Songer

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Positive hits can be checked against the established Microsoft baselines.

Microsoft highly recommends that the best course of action is patching, but this may not protect already compromised systems from existing intrusions. Other tools for detecting and mitigating can be found within their Exchange support repository

Setup

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Setup

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define event.ingested and default fallback for EQL rules was not added until version 8.2. Hence for this rule to work effectively, users will need to add a custom ingest pipeline to populate event.ingested to @timestamp. For more details on adding a custom ingest pipeline refer - https://www.elastic.co/guide/en/fleet/current/data-streams-pipeline-tutorial.html

Rule query

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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")
  )

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Initial Access
- ID: TA0001
- Reference URL: https://attack.mitre.org/tactics/TA0001/
Technique:
- Name: Exploit Public-Facing Application
- ID: T1190
- Reference URL: https://attack.mitre.org/techniques/T1190/
Tactic:
- Name: Lateral Movement
- ID: TA0008
- Reference URL: https://attack.mitre.org/tactics/TA0008/
Technique:
- Name: Exploitation of Remote Services
- ID: T1210
- Reference URL: https://attack.mitre.org/techniques/T1210/

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