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Suspicious HTML File Creation

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Suspicious HTML File Creation

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Identifies the execution of a browser process to open an HTML file with high entropy and size. Adversaries may smuggle data and files past content filters by hiding malicious payloads inside of seemingly benign HTML files.

Rule type: eql

Rule indices:

logs-endpoint.events.*

Severity: medium

Risk score: 47

Runs every: 5 minutes

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

Tags:

Elastic
Host
Windows
Threat Detection
Initial Access Added (Elastic Stack release): 8.4.0

Last modified (Elastic Stack release): 8.4.0

Rule authors: Elastic

Rule license: Elastic License v2

Investigation guide

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Rule query

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sequence by user.id with maxspan=5m [file where event.action in
("creation", "rename") and file.extension : ("htm", "html") and
file.path : ("?:\\Users\\*\\Downloads\\*",
"?:\\Users\\*\\Content.Outlook\\*",
"?:\\Users\\*\\AppData\\Local\\Temp\\Temp?_*",
"?:\\Users\\*\\AppData\\Local\\Temp\\7z*",
"?:\\Users\\*\\AppData\\Local\\Temp\\Rar$*") and ((file.Ext.entropy
>= 5 and file.size >= 150000) or file.size >= 1000000)] [process
where event.action == "start" and ( (process.name in
("chrome.exe", "msedge.exe", "brave.exe", "whale.exe", "browser.exe",
"dragon.exe", "vivaldi.exe", "opera.exe") and process.args == "--
single-argument") or (process.name == "iexplore.exe" and
process.args_count == 2) or (process.name in ("firefox.exe",
"waterfox.exe") and process.args == "-url") ) and process.args :
("?:\\Users\\*\\Downloads\\*.htm*",
"?:\\Users\\*\\Content.Outlook\\*.htm*",
"?:\\Users\\*\\AppData\\Local\\Temp\\Temp?_*.htm*",
"?:\\Users\\*\\AppData\\Local\\Temp\\7z*.htm*",
"?:\\Users\\*\\AppData\\Local\\Temp\\Rar$*.htm*")]

Threat mapping

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Framework: MITRE ATT&CK^TM

Tactic:
- Name: Initial Access
- ID: TA0001
- Reference URL: https://attack.mitre.org/tactics/TA0001/
Technique:
- Name: Phishing
- ID: T1566
- Reference URL: https://attack.mitre.org/techniques/T1566/
Tactic:
- Name: Defense Evasion
- ID: TA0005
- Reference URL: https://attack.mitre.org/tactics/TA0005/
Technique:
- Name: Obfuscated Files or Information
- ID: T1027
- Reference URL: https://attack.mitre.org/techniques/T1027/

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