Script Execution via Microsoft HTML Application

[metadata]
creation_date = "2020/09/09"
integration = ["endpoint", "windows", "system", "sentinel_one_cloud_funnel", "m365_defender", "crowdstrike"]
maturity = "production"
updated_date = "2026/04/30"

[rule]
author = ["Elastic"]
description = """
Identifies the execution of scripts via HTML applications using Windows utilities rundll32.exe or mshta.exe. Adversaries
may bypass process and/or signature-based defenses by proxying execution of malicious content with signed binaries.
"""
from = "now-9m"
index = [
    "logs-m365_defender.event-*",
    "logs-sentinel_one_cloud_funnel.*",
    "logs-system.security*",
    "logs-windows.forwarded*",
    "logs-windows.sysmon_operational-*",
    "winlogbeat-*",
    "logs-endpoint.events.process-*",
    "logs-crowdstrike.fdr*",
    "endgame-*"
]
language = "eql"
license = "Elastic License v2"
name = "Script Execution via Microsoft HTML Application"
risk_score = 73
rule_id = "181f6b23-3799-445e-9589-0018328a9e46"
severity = "high"
tags = [
    "Domain: Endpoint",
    "OS: Windows",
    "Use Case: Threat Detection",
    "Tactic: Defense Evasion",
    "Data Source: Windows Security Event Logs",
    "Data Source: Sysmon",
    "Data Source: SentinelOne",
    "Data Source: Microsoft Defender XDR",
    "Data Source: Elastic Defend",
    "Data Source: Crowdstrike",
    "Resources: Investigation Guide",
    "Data Source: Elastic Endgame",
]
timestamp_override = "event.ingested"
type = "eql"

query = '''
process where host.os.type == "windows" and event.type == "start" and
 process.name : ("rundll32.exe", "mshta.exe") and
  (
     (process.command_line :
        (
        "*script*eval(*",
         "*script*GetObject*",
         "*.regread(*",
         "*WScript.Shell*",
         "*.run(*",
         "*).Exec()*",
         "*mshta*http*",
         "*mshtml*RunHTMLApplication*",
         "*mshtml*,#135*",
         "*StrReverse*",
         "*.RegWrite*",
         /* Issue #379 */
         "*window.close(*",
         "* Chr(*"
         )
     and not ?process.parent.executable :
                  ("?:\\Program Files (x86)\\Citrix\\System32\\wfshell.exe",
                   "?:\\Program Files (x86)\\Microsoft Office\\Office*\\MSACCESS.EXE",
                   "?:\\Program Files\\Quokka.Works GTInstaller\\GTInstaller.exe")
     ) or

    (process.name : "mshta.exe" and
     not process.command_line : ("*.hta*", "*.htm*", "-Embedding") and ?process.args_count >=2) or

     /* Execution of HTA file downloaded from the internet */
     (process.name : "mshta.exe" and process.command_line : "*\\Users\\*\\Downloads\\*.hta*") or

     /* Execution of HTA file from archive */
     (process.name : "mshta.exe" and
      process.args : ("?:\\Users\\*\\Temp\\7z*", "?:\\Users\\*\\Temp\\Rar$*", "?:\\Users\\*\\Temp\\Temp?_*", "?:\\Users\\*\\Temp\\BNZ.*"))
   )
'''

note = """## Triage and analysis

### Investigating Script Execution via Microsoft HTML Application

#### Possible investigation steps

- Which HTML-application proxy path did the alert capture?
  - Why: proxy path determines the containment target.
  - Focus: `process.name`, `process.command_line`, and `process.args_count`: ".hta", ".htm", "vbscript:", "javascript:", "GetObject("script:")", "mshtml,RunHTMLApplication", "mshtml,#135", "http", "WScript.Shell", "StrReverse", "window.close(", or "Chr(".
  - Implication: escalate on inline script, remote retrieval, obfuscation, "RunHTMLApplication", archive/temp execution, or non-HTA "mshta.exe"; lower concern only for one stable internal HTA/HTM path with no inline script, remote locator, or obfuscation.

- Is the proxy binary a genuine Microsoft LOLBin or a masqueraded executable?
  - Focus: `process.executable`, `process.pe.original_file_name`, `process.hash.sha256`, `process.code_signature.subject_name`, and `process.code_signature.trusted`.
  - Implication: escalate when "mshta.exe" or "rundll32.exe" runs from a user-writable or non-Windows path, has a mismatched original filename, or lacks a trusted Microsoft signature; canonical Microsoft identity lowers masquerade risk but does not clear suspicious arguments.

- What parent and user context explain the launch?
  - Focus: `process.parent.executable`, `process.parent.command_line`, `user.id`, and `host.id`.
  - Implication: escalate when the parent is Office, a browser, archive utility, script host, user-writable executable, or a chain outside the rule's excluded parents; lower concern when the same parent, user cohort, and host cohort repeatedly launch the same recognized HTA workflow.

- What referenced HTA, scriptlet, URL, or path can be recovered?
  - Focus: `process.command_line`, `process.args`, and `process.working_directory`: literal HTA/HTM path, scriptlet URL, remote locator, archive/temp marker, or relative path; recover same-process file and network/DNS events when artifact or remote locator evidence exists. $investigate_0 $investigate_1
  - Implication: escalate when the locator points to Downloads, Temp, archive extraction, external URLs, scriptlet retrieval, alternate data stream-like syntax, or a single-user path; lower concern only for the same stable internal path from the same recognized workflow. Missing file, network, or DNS telemetry is unresolved, not benign.

- Did the proxy spawn follow-on code or payload execution?
  - Focus: child starts on `host.id` where `process.parent.entity_id` equals alert `process.entity_id`; inspect child `process.executable`, `process.command_line`, `process.pe.original_file_name`, `process.code_signature.subject_name`, and `process.code_signature.trusted`. $investigate_2
  - Hint: if `process.entity_id` is absent, use `host.id` + `process.pid` + the tight alert window as a weaker fallback; treat PID reuse as unresolved ambiguity.
  - Implication: escalate when the proxy spawns shells, PowerShell, script hosts, "regsvr32.exe", another "rundll32.exe", unsigned payloads, or tooling unrelated to the parent workflow; lower concern when child activity stays within the same recognized signed application flow.

- If evidence remains suspicious or incomplete, is it isolated?
  - Focus: `user.id`, `host.id`, stable `process.command_line` fragments, child `process.executable`, and child `process.command_line` recovered above. $investigate_3 $investigate_4
  - Implication: broaden scope when the same user, host, command fragment, or child process repeats across alerts or other hosts; keep scope local only when isolated and aligned with one recognized workflow.

- Escalate on proxy script execution, masquerade, suspicious lineage, external/temporary sourcing, follow-on payloads, or recurrence; close only when command intent, binary identity, lineage, locator, child behavior, and scope align with one recognized internal HTA/HTM workflow; preserve artifacts and escalate when evidence is mixed or incomplete.

### False positive analysis

- Legacy internal HTA applications or support tools may use "mshta.exe" when identity, intent, and context align: canonical Microsoft `process.executable`, stable `process.parent.executable`, repeated `process.command_line` for one internal HTA/HTM path, consistent `user.id`/`host.id` cohort, and no child branching into shells or script hosts. If inventory, owner, or change records exist, require alignment with this telemetry; otherwise require recurrence of the same stable process pattern across prior alerts from this rule for the same cohort.
- Vendor installers or deployment tools can trigger archive/temp HTA patterns when parent executable, signer context, command line, and bounded child chain match the same recognized package workflow. Do not close on parent name alone; contradictory locator, child process, user/host scope, or signer evidence stays suspicious. Exceptions should use the minimum confirmed pattern, such as `process.parent.executable` + stable `process.command_line` fragment + `host.id` or `user.id`, not `process.name`.

### Response and remediation

- If confirmed benign:
  - Reverse temporary containment and record the parent workflow, `process.command_line`, extracted locator/path, `user.id`, `host.id`, and recurrence or business records that confirmed the recognized application path. Build exceptions only from the minimum stable workflow pattern, not from "mshta.exe" or "rundll32.exe" alone.
- If suspicious but unconfirmed:
  - Preserve the alert event export, process tree, `process.entity_id`, `process.pid`, `process.command_line`, `process.args`, parent lineage, user/host context, recovered child process events, and literal URLs or paths extracted from the command line before cleanup.
  - Apply reversible containment tied to the findings, such as temporary destination restrictions for remote locators, child-process blocking, or heightened monitoring on the affected `host.id` and `user.id`; use host isolation only when child-process or staging evidence indicates active payload execution and host criticality permits it.
- If confirmed malicious:
  - Isolate the host or contain the affected identity after preserving the process and child-process evidence that established malicious execution. Terminate the proxy and spawned payload processes only after recording their identifiers and command lines.
  - Block confirmed malicious command-line locators, child binaries, domains, and addresses identified during triage, then review other hosts and users for the same command fragment or child-process pattern before eradication.
  - Remove the malicious HTA, scriptlet, archive-extracted payload, or staged child artifacts identified during investigation; remediate the parent document, browser, archive, or script delivery path; investigate credential exposure if follow-on behavior suggests collection or lateral movement.
- Post-incident hardening:
  - Restrict or monitor "mshta.exe" and unusual "rundll32.exe" script-execution paths where the business does not require them.
  - Retain process telemetry for these binaries and record any missing artifact, library, or destination visibility that limited the case so future triage can account for the gap.
"""

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)
- [SentinelOne Cloud Funnel](https://ela.st/sentinel-one-cloud-funnel)
- [Sysmon Event ID 1 - Process Creation](https://ela.st/sysmon-event-1-setup)
- [Windows Process Creation Logs](https://ela.st/audit-process-creation)
"""

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

[transform]

[[transform.investigate]]
label = "File events for the same proxy instance"
description = ""
providers = [
  [
    { 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" }
  ]
]
relativeFrom = "now-1h"
relativeTo = "now"

[[transform.investigate]]
label = "Network and DNS events for the same proxy instance"
description = ""
providers = [
  [
    { excluded = false, field = "event.category", queryType = "phrase", value = "network", 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 = "dns", 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" }
  ]
]
relativeFrom = "now-1h"
relativeTo = "now"

[[transform.investigate]]
label = "Child process events for the same proxy instance"
description = ""
providers = [
  [
    { excluded = false, field = "event.category", queryType = "phrase", value = "process", valueType = "string" },
    { 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" }
  ]
]
relativeFrom = "now-1h"
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"

[[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"

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

[[rule.threat.technique]]
id = "T1218"
name = "System Binary Proxy Execution"
reference = "https://attack.mitre.org/techniques/T1218/"

[[rule.threat.technique.subtechnique]]
id = "T1218.005"
name = "Mshta"
reference = "https://attack.mitre.org/techniques/T1218/005/"

[[rule.threat.technique.subtechnique]]
id = "T1218.011"
name = "Rundll32"
reference = "https://attack.mitre.org/techniques/T1218/011/"

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

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

[[rule.threat.technique]]
id = "T1059"
name = "Command and Scripting Interpreter"
reference = "https://attack.mitre.org/techniques/T1059/"

[[rule.threat.technique.subtechnique]]
id = "T1059.005"
name = "Visual Basic"
reference = "https://attack.mitre.org/techniques/T1059/005/"

[[rule.threat.technique.subtechnique]]
id = "T1059.007"
name = "JavaScript"
reference = "https://attack.mitre.org/techniques/T1059/007/"

[rule.threat.tactic]
id = "TA0002"
name = "Execution"
reference = "https://attack.mitre.org/tactics/TA0002/"