FileHint
JAEN

.chm

High risk

Compiled HTML Help

Microsoft's help file format — HTML plus assets compressed with LZX. Widely used for online help in Windows software since 1998. Still actively distributed in China, Japan, and Korea.

MIME types

  • application/vnd.ms-htmlhelp

Category

document

What opens this file

  • Microsoft HTML Help (built into Windows)
  • xCHM
  • Kchmviewer
  • Sumatra PDF

How to open by OS

Windows

  • HH.EXE (native), Sumatra PDF.

Mac

  • xCHM (Chmox is abandoned).

Linux

  • xCHM, Kchmviewer.

Ios

  • CHM Viewer.

Android

  • EBookDroid, CHM Reader.

Safety notes

  • CHM historically used the Internet Explorer engine, leading to multiple CVEs where opening a hostile CHM executed scripts. Never open CHM from unknown senders.
  • APT campaigns out of Asia have weaponised CHM for malware delivery — treat as suspicious by default.

Common mistakes

  • Unblocking a downloaded CHM via Properties > Unblock defeats the mark-of-the-web protection. Don't do it casually.
  • Hyperlinks inside CHM may open in the embedded IE engine, not your default browser — a known vector for spoofed links.

FAQ

How do I open CHM on Mac or Linux?
xCHM is the usual answer. On Linux, Kchmviewer is another option. Chmox on the Mac stopped working after recent macOS updates, so xCHM is effectively the only reliable choice.

Related extensions

References