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-endpoint.events.*
- 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
References:
Tags:
- Elastic
- Host
- Windows
- Threat Detection
- Initial Access
Version: 100 (version history)
Added (Elastic Stack release): 7.12.0
Last modified (Elastic Stack release): 8.5.0
Rule authors: Elastic, Austin Songer
Rule license: Elastic License v2
Potential false positivesedit
Files generated during installation will generate a lot of noise, so the rule should only be enabled after the fact. This rule was tuned using the following baseline: https://raw.githubusercontent.com/microsoft/CSS-Exchange/main/Security/Baselines/baseline_15.2.792.5.csv from Microsoft. Depending on version, consult https://github.com/microsoft/CSS-Exchange/tree/main/Security/Baselines to help determine normalcy.
Investigation guideedit
## 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)
Rule queryedit
file 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") )
Threat mappingedit
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/
Rule version historyedit
- Version 100 (8.5.0 release)
-
- Formatting only
- Version 5 (8.4.0 release)
-
- Formatting only
- Version 3 (8.2.0 release)
-
- Formatting only
- Version 2 (7.13.0 release)
-
-
Updated query, changed from:
file where event.type == "creation" and process.parent.name : ("UMWorkerProcess.exe", "umservice.exe") and file.extension : ("php", "jsp", "js", "aspx", "asmx", "asax", "cfm", "shtml") and ( file.path : ("C:\\inetpub\\wwwroot\\aspnet_client\\*", "C:\\*\\FrontEnd\\HttpProxy\\owa\\auth\\*") or (file.path : "C:\\*\\FrontEnd\\HttpProxy\\ecp\\auth\\*" and not file.name : "TimeoutLogoff.aspx") )
-