Microsoft Exchange Server UM Writing Suspicious Files

  1[metadata]
  2creation_date = "2021/03/04"
  3integration = ["endpoint", "windows"]
  4maturity = "production"
  5updated_date = "2024/05/21"
  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 = ["winlogbeat-*", "logs-endpoint.events.file-*", "logs-windows.sysmon_operational-*", "endgame-*"]
 26language = "eql"
 27license = "Elastic License v2"
 28name = "Microsoft Exchange Server UM Writing Suspicious Files"
 29note = """## Triage and analysis
 30
 31Positive hits can be checked against the established Microsoft [baselines](https://github.com/microsoft/CSS-Exchange/tree/main/Security/Baselines).
 32
 33Microsoft highly recommends that the best course of action is patching, but this may not protect already compromised systems
 34from existing intrusions. Other tools for detecting and mitigating can be found within their Exchange support
 35[repository](https://github.com/microsoft/CSS-Exchange/tree/main/Security)
 36"""
 37references = [
 38    "https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers",
 39    "https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities",
 40]
 41risk_score = 47
 42rule_id = "6cd1779c-560f-4b68-a8f1-11009b27fe63"
 43setup = """## Setup
 44
 45If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2,
 46events will not define `event.ingested` and default fallback for EQL rules was not added until version 8.2.
 47Hence for this rule to work effectively, users will need to add a custom ingest pipeline to populate
 48`event.ingested` to @timestamp.
 49For more details on adding a custom ingest pipeline refer - https://www.elastic.co/guide/en/fleet/current/data-streams-pipeline-tutorial.html
 50"""
 51severity = "medium"
 52tags = [
 53    "Domain: Endpoint",
 54    "OS: Windows",
 55    "Use Case: Threat Detection",
 56    "Tactic: Initial Access",
 57    "Tactic: Lateral Movement",
 58    "Data Source: Elastic Endgame",
 59    "Use Case: Vulnerability",
 60    "Data Source: Elastic Defend",
 61    "Data Source: Sysmon",
 62]
 63timestamp_override = "event.ingested"
 64type = "eql"
 65
 66query = '''
 67file where host.os.type == "windows" and event.type == "creation" and
 68  process.name : ("UMWorkerProcess.exe", "umservice.exe") and
 69  file.extension : ("php", "jsp", "js", "aspx", "asmx", "asax", "cfm", "shtml") and
 70  (
 71    file.path : "?:\\inetpub\\wwwroot\\aspnet_client\\*" or
 72
 73    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\*" and
 74       not (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\owa\\auth\\version\\*" or
 75            file.name : ("errorFE.aspx", "expiredpassword.aspx", "frowny.aspx", "GetIdToken.htm", "logoff.aspx",
 76                        "logon.aspx", "OutlookCN.aspx", "RedirSuiteServiceProxy.aspx", "signout.aspx"))) or
 77
 78    (file.path : "?:\\*\\Microsoft\\Exchange Server*\\FrontEnd\\HttpProxy\\ecp\\auth\\*" and
 79       not file.name : "TimeoutLogoff.aspx")
 80  )
 81'''
 82
 83
 84[[rule.threat]]
 85framework = "MITRE ATT&CK"
 86[[rule.threat.technique]]
 87id = "T1190"
 88name = "Exploit Public-Facing Application"
 89reference = "https://attack.mitre.org/techniques/T1190/"
 90
 91
 92[rule.threat.tactic]
 93id = "TA0001"
 94name = "Initial Access"
 95reference = "https://attack.mitre.org/tactics/TA0001/"
 96[[rule.threat]]
 97framework = "MITRE ATT&CK"
 98[[rule.threat.technique]]
 99id = "T1210"
100name = "Exploitation of Remote Services"
101reference = "https://attack.mitre.org/techniques/T1210/"
102
103
104[rule.threat.tactic]
105id = "TA0008"
106name = "Lateral Movement"
107reference = "https://attack.mitre.org/tactics/TA0008/"