Microsoft Exchange Server UM Writing Suspicious Files

Elastic Stack Serverless Security

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-*
• logs-m365_defender.event-*
• logs-sentinel_one_cloud_funnel.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

• https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers
• https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities

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
• Data Source: Microsoft Defender for Endpoint
• Data Source: SentinelOne
• Resources: Investigation Guide

Version: 309

Rule authors:

• Elastic
• Austin Songer

Rule license: Elastic License v2

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

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&CKTM

• 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/