Compiled HTML File
Where this stands
One published detection covers this technique. Every one is unverified — no rule on Siemphony has been executed against real telemetry.
What the technique is
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.
Read it on attack.mitre.orgWhat MITRE says you would watch
- AN0968
Execution of hh.exe to open a.chm file followed by suspicious child processes or script engine invocation (VBScript, JScript, mshta, powershell). Behavior includes loading a CHM file from untrusted locations, or immediately spawning commands indicative of payload execution.
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