Suspicious Microsoft Antimalware Service Execution

[metadata]
creation_date = "2021/07/07"
integration = ["endpoint", "windows", "m365_defender", "crowdstrike"]
maturity = "production"
updated_date = "2026/04/29"

[rule]
author = ["Elastic", "Dennis Perto"]
description = """
Identifies suspicious execution of the Microsoft Antimalware Service Executable (MsMpEng.exe) from non-standard paths or
renamed instances. This may indicate an attempt to evade defenses through DLL side-loading or by masquerading as the
antimalware process.
"""
false_positives = ["Microsoft Antimalware Service Executable installed on non default installation path."]
from = "now-9m"
index = [
    "winlogbeat-*",
    "logs-endpoint.events.process-*",
    "logs-windows.sysmon_operational-*",
    "endgame-*",
    "logs-m365_defender.event-*",
    "logs-crowdstrike.fdr*",
]
language = "eql"
license = "Elastic License v2"
name = "Suspicious Microsoft Antimalware Service Execution"
references = [
    "https://news.sophos.com/en-us/2021/07/04/independence-day-revil-uses-supply-chain-exploit-to-attack-hundreds-of-businesses/",
]
risk_score = 73
rule_id = "053a0387-f3b5-4ba5-8245-8002cca2bd08"
severity = "high"
tags = [
    "Domain: Endpoint",
    "OS: Windows",
    "Use Case: Threat Detection",
    "Tactic: Defense Evasion",
    "Data Source: Elastic Endgame",
    "Data Source: Elastic Defend",
    "Data Source: Sysmon",
    "Data Source: Microsoft Defender XDR",
    "Data Source: Crowdstrike",
    "Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "eql"

query = '''
process where host.os.type == "windows" and event.type == "start" and
(
  (process.pe.original_file_name == "MsMpEng.exe" and not process.name : "MsMpEng.exe") or
  (
    process.name : "MsMpEng.exe" and
    not process.executable : (
            "?:\\ProgramData\\Microsoft\\Windows Defender\\*.exe",
            "?:\\Program Files\\Windows Defender\\*.exe",
            "?:\\Program Files (x86)\\Windows Defender\\*.exe",
            "?:\\Program Files\\Microsoft Security Client\\*.exe",
            "?:\\Program Files (x86)\\Microsoft Security Client\\*.exe",

            /* Crowdstrike specific exclusion as it uses NT Object paths */
            "\\Device\\HarddiskVolume*\\ProgramData\\Microsoft\\Windows Defender\\*.exe",
            "\\Device\\HarddiskVolume*\\Program Files\\Windows Defender\\*.exe",
            "\\Device\\HarddiskVolume*\\Program Files (x86)\\Windows Defender\\*.exe",
            "\\Device\\HarddiskVolume*\\Program Files\\Microsoft Security Client\\*.exe",
            "\\Device\\HarddiskVolume*\\Program Files (x86)\\Microsoft Security Client\\*.exe"
    )
  )
)
'''

note = """## Triage and analysis

### Investigating Suspicious Microsoft Antimalware Service Execution

#### Possible investigation steps

- Which Defender identity anomaly did the alert capture?
  - Focus: `process.name`, `process.pe.original_file_name`, `process.executable`, `process.code_signature.subject_name`, and `process.code_signature.trusted`.
  - Implication: escalate when `process.pe.original_file_name` is "MsMpEng.exe" under renamed `process.name`, or `process.name` is "MsMpEng.exe" outside Defender/Microsoft Security Client paths, even with trusted Microsoft signing; lower suspicion only when exact path, signer, and name pattern fit controlled packaging, recovery, or malware-analysis copy.
- Does the path, file timing, and parent context look like staged Defender abuse?
  - Why: unusual-path Defender binaries can load same-folder DLLs through search-order behavior, so path and parent context separate masquerading or side-loading from controlled copies.
  - Focus: `process.executable`, `process.Ext.relative_file_creation_time`, `process.Ext.relative_file_name_modify_time`, `process.parent.executable`, and `process.parent.command_line`.
  - Implication: escalate when the binary is fresh, recently renamed, or launched from user-writable, temp, share, archive, agent working, or Windows staging paths by a script, archive tool, RMM agent, or dropper parent; path age and parent context support benign closure only if later side-loading and launcher checks do not contradict them.
- Does the user, token, and session context fit Defender service execution?
  - Focus: `user.id`, `user.name`, `process.Ext.session_info.logon_type`, and `process.Ext.token.integrity_level_name`.
  - Implication: escalate when the process runs under an interactive/domain user, a non-service logon, or a user-level token that does not fit antimalware service startup; SYSTEM or service context lowers only the session concern and does not clear the unusual path by itself.
- If file or library telemetry is available, is there same-directory staging or DLL side-loading evidence?
  - Focus: recover file and library events with `host.id` plus `process.entity_id` when present, or `host.id` plus `process.pid` and a tight alert window; inspect `file.path`, `dll.path`, `dll.name`, `dll.code_signature.trusted`, and `dll.Ext.relative_file_creation_time`. $investigate_0
  - Hint: missing file or library telemetry is unresolved, not benign; prioritize same-folder DLLs whose path, signer, or creation time does not fit the product layout, plus artifacts created before `process.executable` started.
  - Implication: escalate when the unusual Defender copy loads a recent, unsigned/untrusted same-folder DLL or the directory contains newly staged executables, DLLs, scripts, archives, or renamed files; complete recovery with only expected Microsoft components lowers side-loading concern.
- Does the process act as a launcher rather than a passive service component?
  - Focus: child process events where `process.parent.entity_id` matches suspicious `process.entity_id`, repeated starts from `process.executable` on `host.id`, and child `process.name`, `process.executable`, and `process.command_line`.
    - $investigate_1
    - $investigate_2
  - Implication: escalate when it starts shells, PowerShell, certutil, netsh, installers, encryption tooling, or other hands-on-keyboard utilities, or when repeated launches suggest staged execution; no child or repeat behavior lowers launcher concern but does not clear the path anomaly.
- If local findings stay suspicious or unresolved, do related alerts show path reuse or host compromise?
  - Focus: related alerts for `process.executable`, especially unusual-path Defender, masquerading, or side-loading detections.
    - $investigate_3
  - Hint: also review related alerts for `host.id` or `user.id`, especially staging, persistence, credential-access, ransomware, or other masquerading detections.
    - $investigate_4
    - $investigate_5
  - Implication: broaden scope when the same path appears on unrelated hosts or the host has precursor or follow-on alerts; keep the case local only when related alerts show no reuse or follow-on activity and all local evidence is clean.

- Escalate when Defender identity/path evidence plus one meaningful corroborator supports masquerading or DLL side-loading; close only when exact path, signer, parent, session, host/user scope, and optional outside confirmation tie to one controlled workflow with no contradictory telemetry; preserve artifacts and escalate when findings stay mixed or visibility is incomplete.

### False positive analysis

- A non-default Defender installation, controlled security packaging, recovery, or malware-analysis validation can stage Microsoft antimalware binaries outside default paths. Confirm the same workflow by matching exact `process.executable`, `process.hash.sha256` or `process.code_signature.thumbprint_sha256`, Microsoft `process.code_signature.subject_name`, `process.code_signature.trusted`, `process.parent.executable`, `process.parent.command_line`, `user.id`, `host.id`, and session pattern; without outside records, require recurrence across prior rule alerts without side-loading, launcher, or related-alert contradictions.
- Treat production execution from temp, user-writable, share, archive, agent working, or Windows staging paths as an operational anti-pattern unless a controlled workflow proves why the copy exists. Do not close as benign when same-folder DLLs, child tooling, recent rename timing, or unrelated related alerts contradict it.
- Build exceptions only from the minimum confirmed workflow pattern; avoid exceptions on `process.name`, `process.pe.original_file_name`, signer subject alone, or host alone.

### Response and remediation

- If confirmed benign, reverse any temporary containment and document the exact workflow: executable path, Microsoft signer or hash, parent process, session context, user/host scope, and any controlled packaging, recovery, or lab record that corroborated the telemetry. Create an exception only after the same narrow workflow pattern is stable across prior alerts.
- If suspicious but unconfirmed, preserve the alert details, process tree, command line, binary copy and hash, parent context, directory listing, same-folder DLLs, and related-alert timeline before containment. Apply reversible containment first, such as execution prevention on the suspicious path or temporary host isolation when active launcher behavior or side-loading creates continuing risk and the host role can tolerate interruption.
- If confirmed malicious, preserve process and artifact evidence first, including the suspicious Defender copy, same-folder DLLs, support files, launcher context, and related-alert timeline. Then isolate the host or apply an equivalent endpoint containment control, terminate only the suspicious non-default-path or renamed Defender instance, quarantine the suspicious executable and supporting files, remove launcher or persistence artifacts found during scoping, and restore the legitimate security product from known-good media if the masquerading copy replaced or shadowed a trusted component.
- After containment, restrict execution from user-writable, temporary, share, archive, and agent working directories where feasible, retain process/file/library telemetry that affected this case, and document the confirmed benign workflow or malicious artifact set for future triage.
"""

setup = """## 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

### Additional data sources

This rule also supports the following third-party data sources. For setup instructions, refer to the links below:

- [CrowdStrike](https://ela.st/crowdstrike-integration)
- [Microsoft Defender XDR](https://ela.st/m365-defender)
- [Sysmon Event ID 1 - Process Creation](https://ela.st/sysmon-event-1-setup)
"""

[rule.investigation_fields]
field_names = [
    "@timestamp",
    "host.name",
    "host.id",
    "user.name",
    "user.id",
    "process.entity_id",
    "process.pid",
    "process.name",
    "process.executable",
    "process.command_line",
    "process.pe.original_file_name",
    "process.parent.executable",
    "process.parent.command_line",
    "process.code_signature.subject_name",
    "process.code_signature.trusted",
]

[transform]

[[transform.investigate]]
label = "File and library events for the suspicious process"
description = ""
providers = [
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "process.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" },
    { excluded = false, field = "event.category", queryType = "phrase", value = "file", valueType = "string" }
  ],
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "process.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" },
    { excluded = false, field = "event.category", queryType = "phrase", value = "library", valueType = "string" }
  ]
]
relativeFrom = "now-1h"
relativeTo = "now"

[[transform.investigate]]
label = "Child process events for the suspicious process"
description = ""
providers = [
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "process.parent.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" },
    { excluded = false, field = "event.category", queryType = "phrase", value = "process", valueType = "string" }
  ]
]
relativeFrom = "now-1h"
relativeTo = "now"

[[transform.investigate]]
label = "Process events for the suspicious executable path"
description = ""
providers = [
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "process.executable", queryType = "phrase", value = "{{process.executable}}", valueType = "string" },
    { excluded = false, field = "event.category", queryType = "phrase", value = "process", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[transform.investigate]]
label = "Alerts associated with the suspicious executable path"
description = ""
providers = [
  [
    { excluded = false, field = "event.kind", queryType = "phrase", value = "signal", valueType = "string" },
    { excluded = false, field = "process.executable", queryType = "phrase", value = "{{process.executable}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[transform.investigate]]
label = "Alerts associated with the host"
description = ""
providers = [
  [
    { excluded = false, field = "event.kind", queryType = "phrase", value = "signal", valueType = "string" },
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[transform.investigate]]
label = "Alerts associated with the user"
description = ""
providers = [
  [
    { excluded = false, field = "event.kind", queryType = "phrase", value = "signal", valueType = "string" },
    { excluded = false, field = "user.id", queryType = "phrase", value = "{{user.id}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

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

[[rule.threat.technique]]
id = "T1036"
name = "Masquerading"
reference = "https://attack.mitre.org/techniques/T1036/"

[[rule.threat.technique.subtechnique]]
id = "T1036.003"
name = "Rename Legitimate Utilities"
reference = "https://attack.mitre.org/techniques/T1036/003/"

[[rule.threat.technique.subtechnique]]
id = "T1036.005"
name = "Match Legitimate Resource Name or Location"
reference = "https://attack.mitre.org/techniques/T1036/005/"

[[rule.threat.technique]]
id = "T1574"
name = "Hijack Execution Flow"
reference = "https://attack.mitre.org/techniques/T1574/"

[[rule.threat.technique.subtechnique]]
id = "T1574.001"
name = "DLL"
reference = "https://attack.mitre.org/techniques/T1574/001/"

[rule.threat.tactic]
id = "TA0005"
name = "Defense Evasion"
reference = "https://attack.mitre.org/tactics/TA0005/"