Microsoft Exchange Server UM Spawning Suspicious Processesedit

Identifies suspicious processes being spawned by the Microsoft Exchange Server Unified Messaging (UM) service. This activity has been observed exploiting CVE-2021-26857.

Rule type: eql

Rule indices:

  • winlogbeat-*
  • logs-windows.*

Severity: medium

Risk score: 47

Runs every: 5 minutes

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

Maximum alerts per execution: 100



  • Elastic
  • Host
  • Windows
  • Threat Detection
  • Initial Access

Version: 2 (version history)

Added (Elastic Stack release): 7.12.0

Last modified (Elastic Stack release): 8.2.0

Rule authors: Elastic, Austin Songer

Rule license: Elastic License v2

Potential false positivesedit

Legitimate processes may be spawned from the Microsoft Exchange Server Unified Messaging (UM) service. If known processes are causing false positives, they can be exempted from the rule.

Investigation guideedit

## Config

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 8.2, so you will need to add a custom pipeline to populate `event.ingested` to @timestamp for this rule to work.

Rule queryedit

process where event.type == "start" and :
("UMService.exe", "UMWorkerProcess.exe") and not :
("werfault.exe", "wermgr.exe")

Threat mappingedit


Rule version historyedit

Version 2 (8.2.0 release)
  • Formatting only