Microsoft Exchange Server UM Writing Suspicious Filesedit

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-windows.*
  • 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



  • 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

Version: 106

Rule authors:

  • Elastic
  • Austin Songer

Rule license: Elastic License v2

Investigation guideedit

## 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

Rule queryedit

file where host.os.type == "windows" and event.type == "creation" and : ("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
   : ("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 : "TimeoutLogoff.aspx")