Microsoft Exchange Server UM Writing Suspicious Files

  1[metadata]
  2creation_date = "2021/03/04"
  3integration = ["endpoint", "windows", "m365_defender", "sentinel_one_cloud_funnel"]
  4maturity = "production"
  5updated_date = "2025/03/20"
  6
  7[rule]
  8author = ["Elastic", "Austin Songer"]
  9description = """
 10Identifies suspicious files being written by the Microsoft Exchange Server Unified Messaging (UM) service. This activity
 11has been observed exploiting CVE-2021-26858.
 12"""
 13false_positives = [
 14    """
 15    Files generated during installation will generate a lot of noise, so the rule should only be enabled after the fact.
 16    """,
 17    """
 18    This rule was tuned using the following baseline:
 19    https://raw.githubusercontent.com/microsoft/CSS-Exchange/main/Security/Baselines/baseline_15.2.792.5.csv from
 20    Microsoft. Depending on version, consult https://github.com/microsoft/CSS-Exchange/tree/main/Security/Baselines to
 21    help determine normalcy.
 22    """,
 23]
 24from = "now-9m"
 25index = [
 26    "winlogbeat-*",
 27    "logs-endpoint.events.file-*",
 28    "logs-windows.sysmon_operational-*",
 29    "endgame-*",
 30    "logs-m365_defender.event-*",
 31    "logs-sentinel_one_cloud_funnel.*",
 32]
 33language = "eql"
 34license = "Elastic License v2"
 35name = "Microsoft Exchange Server UM Writing Suspicious Files"
 36note = """## Triage and analysis
 37
 38Positive hits can be checked against the established Microsoft [baselines](https://github.com/microsoft/CSS-Exchange/tree/main/Security/Baselines).
 39
 40Microsoft highly recommends that the best course of action is patching, but this may not protect already compromised systems
 41from existing intrusions. Other tools for detecting and mitigating can be found within their Exchange support
 42[repository](https://github.com/microsoft/CSS-Exchange/tree/main/Security)
 43"""
 44references = [
 45    "https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers",
 46    "https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities",
 47]
 48risk_score = 47
 49rule_id = "6cd1779c-560f-4b68-a8f1-11009b27fe63"
 50severity = "medium"
 51tags = [
 52    "Domain: Endpoint",
 53    "OS: Windows",
 54    "Use Case: Threat Detection",
 55    "Tactic: Initial Access",
 56    "Tactic: Lateral Movement",
 57    "Data Source: Elastic Endgame",
 58    "Use Case: Vulnerability",
 59    "Data Source: Elastic Defend",
 60    "Data Source: Sysmon",
 61    "Data Source: Microsoft Defender for Endpoint",
 62    "Data Source: SentinelOne",
 63    "Resources: Investigation Guide",
 64]
 65timestamp_override = "event.ingested"
 66type = "eql"
 67
 68query = '''
 69file where host.os.type == "windows" and event.type == "creation" and
 70  process.name : ("UMWorkerProcess.exe", "umservice.exe") and
 71  file.extension : ("php", "jsp", "js", "aspx", "asmx", "asax", "cfm", "shtml") and
 72  (
 73    file.path : "?:\\inetpub\\wwwroot\\aspnet_client\\*" or
 74
 75    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\*" and
 76       not (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\version\\*" or
 77            file.name : ("errorFE.aspx", "expiredpassword.aspx", "frowny.aspx", "GetIdToken.htm", "logoff.aspx",
 78                        "logon.aspx", "OutlookCN.aspx", "RedirSuiteServiceProxy.aspx", "signout.aspx"))) or
 79
 80    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\ecp\\auth\\*" and
 81       not file.name : "TimeoutLogoff.aspx")
 82  )
 83'''
 84
 85
 86[[rule.threat]]
 87framework = "MITRE ATT&CK"
 88[[rule.threat.technique]]
 89id = "T1190"
 90name = "Exploit Public-Facing Application"
 91reference = "https://attack.mitre.org/techniques/T1190/"
 92
 93
 94[rule.threat.tactic]
 95id = "TA0001"
 96name = "Initial Access"
 97reference = "https://attack.mitre.org/tactics/TA0001/"
 98[[rule.threat]]
 99framework = "MITRE ATT&CK"
100[[rule.threat.technique]]
101id = "T1210"
102name = "Exploitation of Remote Services"
103reference = "https://attack.mitre.org/techniques/T1210/"
104
105
106[rule.threat.tactic]
107id = "TA0008"
108name = "Lateral Movement"
109reference = "https://attack.mitre.org/tactics/TA0008/"