Microsoft Exchange Server UM Writing Suspicious Files
editMicrosoft Exchange Server UM Writing Suspicious Files
editIdentifies 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.*
- logs-windows.*
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:
- Elastic
- Host
- Windows
- Threat Detection
- Initial Access
Version: 3
Rule authors:
- Elastic
- Austin Songer
Rule license: Elastic License v2
Investigation guide
edit## Triage and analysis Positive hits can be checked against the established Microsoft [baselines](https://github.com/microsoft/CSS-Exchange/tree/main/Security/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](https://github.com/microsoft/CSS-Exchange/tree/main/Security) ## 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 query
editfile where 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/