The file is actually compressed and organized in the binary format with the .CHM extension for the Compiled HTML. The file format is frequently utilized for program documentation. The .chm file was introduced as the Microsoft WinHelp successor with the launch of Win 98 and is still available in Win 7. Although the format was created by Microsoft, it’s been reverse-engineered successfully and is now supported in a lot of doc viewer apps.
Help is provided as the binary file with the chm extension. It has a hyperlinked contents’ table, a set of HTML files, and an index file. The file format has actually been reverse-engineered, and the documentation of the file is freely accessible. In ASCII, the file begins with bytes ITSF for the Info-Tech Storage Format. These files support the below-mentioned features:
- Built-in search engine
- Data compression with the LZX compression
- Extended character support, even though it doesn’t support Unicode completely.
- The capability of merging multiple chm help files
Issues With Opening The CHM File From The Internet Or Network:
The cause it takes place is because the chm files downloaded from the Internet, comprising the CHM and ZIP files contained in such zip files, are labeled as coming from the web and so can possibly be malicious, so don’t acquire the browsing rights on your local machine, they cannot access the local Web content, which is precisely what help topics really are. If you glance at the URL of the help topic, you will see something like:
Which actually points at the special MS URL Moniker that, in turn, points your CHM file and the relative path within that help HTML file; try to paste the URL like this into IE, and you will see the help topic appear in the web browser together with the warning almost certainly. Although a URL looks strange, it still associates with the call to your local PC zone, the same as if you had gone to the local file in Internet Explorer, which by default isn’t permitted. Unluckily, unlike the IE, where you’ve the option of simply clicking the security toolbar, a CHM viewer file just refuses to load a page, and you come across an issue.
How To Open .chm Files?
Now let’s see how to open .chm files. The CHM files downloaded from the web or supplied with some 3rd party apps will frequently fail to open correctly on MS Windows (most of the latest versions after windows Vista), as they’re blocked by default by the Win security system. For opening such blocked .chm file, you need first to unblock the file in the properties of your file (first, got to Properties by right-clicking on the file > go to the General tab > click on the Unblock button). Besides such native support file in Win (since Win 98/NT), there’re also numerous Win and non-Win programs that can open a .CHM file and edit, display, or convert the contents inside them.
Is Unblocking Safe?
As an app developer, there is an easy solution to this issue: Always install the Help Files with the Installer. The security warning appears because your Win cannot authenticate the source of the .CHM file. However, if your help file is actually installed as part of the installation, your installation and the file linked with that installation comprising the help file are actually trusted. A fully installed app’s Help File works really okay because it’s trusted by Win.
It is frustrating as all hell that this kind of obtrusive marking is essential, but it is admittedly an essential evil because of MS’s use of the insecure IE engine that can drive the topic viewer of CHM Html Engine. Because the help file is seeing the local content and script is permitted for executing in the CHM files, there is a possibility for the malicious code hiding in your .CHM file, and the precautions are supposed to evade any problems.