Fix: Cannot View CHM Files from a Network Drive in Windows Vista

When you open a .CHM file over a network drive in a Windows XP/2003/Vista computer, the right pane may show the following message instead of displaying the topic/contents.

This is perfectly normal, as Microsoft Security Updates 896358 & 840315 block display of CHM file contents when opened from a network drive. And in Windows Vista, the security update is included by default. Microsoft issues workarounds (registry edits) for this problem in article KB896054. It documents the following methods to enable CHM functionality from a network drive.

1. How to use UrlAllowList to enable specific URLs
2. How to use the MaxAllowedZone value to enable a security zone
3. How to use both UrlAllowList and the MaxAllowedZone value
4. Use NestedProtocolList to enable nested protocols within a URL

Related Tip: For downloaded CHM files which are stored in the local hard drive, right-click the CHM file, and then click Properties. Click Unblock. Double-click the CHM file to view the contents.

HHReg Utility Automates the Task

HHReg (HTML Help Registration Utility) automates the steps in KB896054. It allows you to register your individual .CHM files or the folders containing your .CHM files, so that it does not compromise security. HHReg works fine in Windows Vista as well.

Start HHReg and add the CHM file by clicking Add File. To add a network folder that contains CHM files, use the Add Folder button, Browse and add the UNC path.

You should now be able to view CHM contents from the network share.

The setting will be stored in the following registry keys, in a String value named UrlAllowList:

and

Additional Information

If you want to configure the MaxAllowedZone value to enable a security zone as per Example 2 in KB896054, use the General Security tab in HHReg.