Detect Compiled HTML File in Splunk
Adversaries may abuse Compiled HTML files (.chm) to conceal malicious code. CHM files are commonly distributed as part of the Microsoft HTML Help system and are compressed compilations of HTML documents, images, and scripting languages such as VBA, JScript, Java, and ActiveX. CHM content is displayed using underlying components of the Internet Explorer browser loaded by the HTML Help executable program (hh.exe). A custom CHM file containing embedded payloads could be delivered to a victim then triggered by User Execution. CHM execution may also bypass application control on older and/or unpatched systems. Groups known to abuse CHM files include OilRig, Dark Caracal, Silence, APT41, and APT38.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1218 System Binary Proxy Execution
- Sub-technique
- T1218.001 Compiled HTML File
- Canonical reference
- https://attack.mitre.org/techniques/T1218/001/
SPL Detection Query
index=wineventlog (sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1)
(Image="*\\hh.exe" OR ParentImage="*\\hh.exe")
| eval SuspiciousParent=if(match(ParentImage, "(winword|excel|outlook|powerpnt|cmd|wscript|cscript|mshta)\.exe"), 1, 0)
| eval RemoteLoad=if(match(CommandLine, "(http[s]?://|\\\\\\\\[a-zA-Z])"), 1, 0)
| eval SuspiciousChild=if(match(Image, "(cmd|powershell|wscript|cscript|mshta|rundll32|regsvr32|certutil)\.exe") AND ParentImage="*\\hh.exe", 1, 0)
| eval RiskScore=SuspiciousParent + RemoteLoad + SuspiciousChild
| where RiskScore > 0 OR ParentImage="*\\hh.exe"
| table _time, host, User, Image, CommandLine, ParentImage, ParentCommandLine, SuspiciousParent, RemoteLoad, SuspiciousChild, RiskScore
| sort - _timeDetects malicious CHM file execution using Sysmon Event ID 1. Covers hh.exe launched from suspicious parent processes (Office, scripting engines), hh.exe loading remote content via HTTP or UNC paths, and hh.exe spawning known post-exploitation child processes. A composite risk score helps prioritize alerts.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate Windows Help files (.chm) launched by system utilities or software installers
- IT documentation tools that package help content as CHM files and open them via hh.exe
- Software development environments opening SDK or API documentation in CHM format
- Help desk software that renders CHM-based knowledge bases
Other platforms for T1218.001
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1CHM Execution via hh.exe with Embedded Script
Expected signal: Sysmon Event ID 1: Process Create with Image=hh.exe, CommandLine containing 'ms-its:' and '.chm'. Security Event ID 4688 if command line auditing is enabled. Any child process creation (if CHM contains script) will also appear as Sysmon Event ID 1 with ParentImage=hh.exe.
- Test 2CHM File Spawning PowerShell
Expected signal: Sysmon Event ID 1 for hh.exe, then a child Sysmon Event ID 1 for powershell.exe with ParentImage=hh.exe. The suspicious child process relationship is the primary detection indicator. Security Event ID 4688 for both processes if command line auditing is on.
- Test 3CHM File Opened from Email Attachment Location
Expected signal: Sysmon Event ID 11: File Create in the Outlook temp path. Sysmon Event ID 1: hh.exe with the Outlook temp path in its command line. File System: CHM file written to Content.Outlook directory.
References (7)
- https://attack.mitre.org/techniques/T1218/001/
- https://oddvar.moe/2017/08/13/bypassing-device-guard-umci-using-chm-cve-2017-8625/
- https://docs.microsoft.com/previous-versions/windows/desktop/htmlhelp/microsoft-html-help-1-4-sdk
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1218.001/T1218.001.md
- https://www.trendmicro.com/en_us/research.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2017-8625
- https://www.welivesecurity.com/wp-content/uploads/2020/06/ESET_InvisiMole.pdf
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