After you install the Microsoft security update MS05-026 (KB896358), CHM help file topics may not display correctly if the file the help file was received via e-mail, or downloaded from the Internet. Instead, an error message such as ‘Navigation to the webpage was canceled’, ‘Action cancelled’, or ‘DNS error’ is displayed.


Recent Microsoft security updates include changes to the InfoTech protocol, which prevent CHM help files from being displayed correctly if the CHM help file is blocked by Windows Attachment Manager. Even if the security warning is acknowledged, the help file is not displayed correctly. The problem also arises when a blocked CHM help file is opened via the HtmlHelp API.

Note   This problem is not specific to Help Producer. It will affect all Microsoft HTML Help 1.0 systems, regardless of the help authoring tool being used.


Unblock a blocked CHM Help File

  1. In Windows Explorer, right-click the blocked file, and then click Properties.
  2. In the General tab, click Unblock.

Open a blocked CHM Help File

  1. Double-click the help file in Windows Explorer.
  2. If a security warning dialog appears, uncheck the box ‘Always ask before opening this file’.
  3. Click Open to open the help file.

Background Information

When a file is received from an unknown source, for instance via e-mail or Internet download, the Windows Attachment Manager may mark the file as unsafe. Files inside an unsafe zip-archive are also marked as unsafe. If a user attempts to open an unsafe file, a security warning dialog may appear indicating that the file is blocked.

CHM help files are classified as high-risk files, and always marked unsafe when received from an unknown source. If a blocked CHM help file is opened in Explorer a security warning dialog will appear. Other than expected, HTML Help files do not display correctly when ‘Open’ is clicked inside the security warning dialog.

More Information

For more information on the changes made to HTML Help, visit the following Microsoft web pages:

Description: External link Microsoft Security Bulletin MS05-026

Description: External link MS05-026: A vulnerability in HTML Help could allow remote code execution

Applies To

See Also

Miscellaneous Problems | Troubleshooting