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: 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: 101
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](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")
)
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/