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.  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 application control on older and/or unpatched systems that do not account for execution of binaries through hh.exe.  
Consider using application control to prevent execution of hh.exe if it is not required for a given system or network to prevent potential misuse by adversaries.
|M1021||Restrict Web-Based Content||
Consider blocking download/transfer and execution of potentially uncommon file types known to be used in adversary campaigns, such as CHM files
|ID||Data Source||Data Component|
Monitor and analyze the execution and arguments of hh.exe.  Compare recent invocations of hh.exe with prior history of known good arguments to determine anomalous and potentially adversarial activity (ex: obfuscated and/or malicious commands). Non-standard process execution trees may also indicate suspicious or malicious behavior, such as if hh.exe is the parent process for suspicious processes and activity relating to other adversarial techniques.
Monitor presence and use of CHM files, especially if they are not typically used within an environment.