Microsoft Exchange Server UM Writing Suspicious Files

[metadata]
creation_date = "2021/03/04"
integration = ["endpoint", "windows", "m365_defender", "sentinel_one_cloud_funnel"]
maturity = "production"
updated_date = "2026/03/24"

[rule]
author = ["Elastic", "Austin Songer"]
description = """
Identifies suspicious files being written by the Microsoft Exchange Server Unified Messaging (UM) service. This activity
has been observed exploiting CVE-2021-26858.
"""
false_positives = [
    """
    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.
    """,
]
from = "now-9m"
index = [
    "winlogbeat-*",
    "logs-endpoint.events.file-*",
    "logs-windows.sysmon_operational-*",
    "endgame-*",
    "logs-m365_defender.event-*",
    "logs-sentinel_one_cloud_funnel.*",
]
language = "eql"
license = "Elastic License v2"
name = "Microsoft Exchange Server UM Writing Suspicious Files"
note = """## 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)
"""
references = [
    "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",
]
risk_score = 47
rule_id = "6cd1779c-560f-4b68-a8f1-11009b27fe63"
severity = "medium"
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 for Endpoint",
    "Data Source: SentinelOne",
    "Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "eql"

query = '''
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")
  )
'''


[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1190"
name = "Exploit Public-Facing Application"
reference = "https://attack.mitre.org/techniques/T1190/"

[rule.threat.tactic]
id = "TA0001"
name = "Initial Access"
reference = "https://attack.mitre.org/tactics/TA0001/"

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1210"
name = "Exploitation of Remote Services"
reference = "https://attack.mitre.org/techniques/T1210/"

[rule.threat.tactic]
id = "TA0008"
name = "Lateral Movement"
reference = "https://attack.mitre.org/tactics/TA0008/"

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1505"
name = "Server Software Component"
reference = "https://attack.mitre.org/techniques/T1505/"

[[rule.threat.technique.subtechnique]]
id = "T1505.003"
name = "Web Shell"
reference = "https://attack.mitre.org/techniques/T1505/003/"

[rule.threat.tactic]
id = "TA0003"
name = "Persistence"
reference = "https://attack.mitre.org/tactics/TA0003/"