Accessing .CHM Help Files from a Network Drive

Recent security patches from Microsoft have made it impossible to access .CHM files that have been installed to a network drive without resorting to patching the registry. In order to access help files on a network drive you must specifically allow each directory that you want to be able to access or allow everything on the local intranet.

To allow .CHM files to be displayed from particular directories, you need to create the following key, if it does not already exist:


Under this key you need to create a string value named UrlAllowList. You must modify this value to look like this:


where x: is the drive letter and \path is the directory which contains the help file(s).

If you are using UNC paths you must modify the value to look like this:


where server is the host name or IP address of the server which is sharing the directory and path is the directory which contains the help file(s).

You can keep adding additional directories to UrlAllowList by separating them with semi-colons.

To allow .CHM files to be displayed from all shares in the local intranet, you must create two keys if they don’t already exist:


In both of they keys you must create a DWORD value named MaxAllowedZone.   You must then modify both of these values to 1.

Note:  If you are running 64 bit Windows you will also have to make the same entries under HKEY_LOCAL_MACHINE\Software\WOW6432Node to enable HTML help for 32 bit applications.

Leave a Reply