---
title: Microsoft Exchange Server UM Writing Suspicious Files
description: Identifies suspicious files being written by the Microsoft Exchange Server Unified Messaging (UM) service. This activity has been observed exploiting...
url: https://www.elastic.co/docs/reference/security/prebuilt-rules/rules/windows/initial_access_suspicious_ms_exchange_files
products:
  - Elastic Security
---

# Microsoft Exchange Server UM Writing Suspicious Files
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.file-*
- logs-windows.sysmon_operational-*
- endgame-*
- logs-m365_defender.event-*
- logs-sentinel_one_cloud_funnel.*

**Rule Severity**: medium
**Risk Score**: 47
**Runs every**: 
**Searches indices from**: `now-9m`
**Maximum alerts per execution**: 100
**References**:
- [[https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers](https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers)](https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers)
- [[https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities](https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities)](https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities)

**Tags**:
- 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
- Data Source: Sysmon
- Data Source: Microsoft Defender XDR
- Data Source: SentinelOne
- Resources: Investigation Guide
- Noise: Low
- Performance: Normal
- Profile: Recommended
- Threat: Web Shell
- Threat: Vulnerability Exploit
- Rule Type: Event Correlation (EQL)
- Platform: Windows
- Vuln: CVE-2021-26858

**Version**: 315
**Rule authors**:
- Elastic
- Austin Songer

**Rule license**: Elastic License v2

## Setup

This rule is designed for data generated by [Elastic Defend](https://www.elastic.co/security/endpoint-security), which provides native endpoint detection and response, along with event enrichments designed to work with our detection rules.
Setup instructions: [https://ela.st/install-elastic-defend](https://ela.st/install-elastic-defend)

### Additional data sources

This rule also supports the following third-party data sources. For setup instructions, refer to the links below:
- [Microsoft Defender XDR](https://ela.st/m365-defender)
- [SentinelOne Cloud Funnel](https://ela.st/sentinel-one-cloud-funnel)
- [Sysmon Event ID 11 - File Create](https://ela.st/sysmon-event-11-setup)


## Investigation guide


## 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 Query

```eql
file where host.os.type == "windows" and 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&CK
- Tactic:
  - Name: Initial Access
  - Id: TA0001
  - Reference URL: [[https://attack.mitre.org/tactics/TA0001/](https://attack.mitre.org/tactics/TA0001/)](https://attack.mitre.org/tactics/TA0001/)
- Technique:
  - Name: Exploit Public-Facing Application
  - Id: T1190
  - Reference URL: [[https://attack.mitre.org/techniques/T1190/](https://attack.mitre.org/techniques/T1190/)](https://attack.mitre.org/techniques/T1190/)

**Framework:** MITRE ATT&CK
- Tactic:
  - Name: Lateral Movement
  - Id: TA0008
  - Reference URL: [[https://attack.mitre.org/tactics/TA0008/](https://attack.mitre.org/tactics/TA0008/)](https://attack.mitre.org/tactics/TA0008/)
- Technique:
  - Name: Exploitation of Remote Services
  - Id: T1210
  - Reference URL: [[https://attack.mitre.org/techniques/T1210/](https://attack.mitre.org/techniques/T1210/)](https://attack.mitre.org/techniques/T1210/)

**Framework:** MITRE ATT&CK
- Tactic:
  - Name: Persistence
  - Id: TA0003
  - Reference URL: [[https://attack.mitre.org/tactics/TA0003/](https://attack.mitre.org/tactics/TA0003/)](https://attack.mitre.org/tactics/TA0003/)
- Technique:
  - Name: Server Software Component
  - Id: T1505
  - Reference URL: [[https://attack.mitre.org/techniques/T1505/](https://attack.mitre.org/techniques/T1505/)](https://attack.mitre.org/techniques/T1505/)
- Sub Technique:
  - Name: Web Shell
  - Id: T1505.003
  - Reference URL: [[https://attack.mitre.org/techniques/T1505/003/](https://attack.mitre.org/techniques/T1505/003/)](https://attack.mitre.org/techniques/T1505/003/)