Adversaries may abuse Compiled HTML files (.chm) to conceal malicious code. CHM files are commonly distributed as part of the Microsoft HTML Help system. CHM files are compressed compilations of various content such as HTML documents, images, and scripting/web related programming languages such VBA, JScript, Java, and ActiveX. (Citation: Microsoft HTML Help May 2018) CHM content is displayed using underlying components of the Internet Explorer browser (Citation: Microsoft HTML Help ActiveX) loaded by the HTML Help executable program (hh.exe). (Citation: Microsoft HTML Help Executable Program)

A custom CHM file containing embedded payloads could be delivered to a victim then triggered by [User Execution](https://attack.mitre.org/techniques/T1204). CHM execution may also bypass application application control on older and/or unpatched systems that do not account for execution of binaries through hh.exe. (Citation: MsitPros CHM Aug 2017) (Citation: Microsoft CVE-2017-8625 Aug 2017)
Compiled HTML File
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1218/001  external_id: T1218.001     source_name: Microsoft CVE-2017-8625 Aug 2017  description: Microsoft. (2017, August 8). CVE-2017-8625 - Internet Explorer Security Feature Bypass Vulnerability. Retrieved October 3, 2018.  url: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2017-8625     source_name: Microsoft HTML Help May 2018  description: Microsoft. (2018, May 30). Microsoft HTML Help 1.4. Retrieved October 3, 2018.  url: https://docs.microsoft.com/previous-versions/windows/desktop/htmlhelp/microsoft-html-help-1-4-sdk     source_name: Microsoft HTML Help Executable Program  description: Microsoft. (n.d.). About the HTML Help Executable Program. Retrieved October 3, 2018.  url: https://msdn.microsoft.com/windows/desktop/ms524405     source_name: Microsoft HTML Help ActiveX  description: Microsoft. (n.d.). HTML Help ActiveX Control Overview. Retrieved October 3, 2018.  url: https://msdn.microsoft.com/windows/desktop/ms644670     source_name: MsitPros CHM Aug 2017  description: Moe, O. (2017, August 13). Bypassing Device guard UMCI using CHM – CVE-2017-8625. Retrieved October 3, 2018.  url: https://oddvar.moe/2017/08/13/bypassing-device-guard-umci-using-chm-cve-2017-8625/     