I get the above message when downloading Diablo 2 from the official installer I got from account.blizzard com games#classic-game-accounts. It also says elsewhere on the downloader “This download is not authorized” but I haven’t even been asked for a cd key yet.
What is going on?
Hello Malagant!
Please follow these steps 
I solved this issue by opening “internet options” and restoring the security slider for “Internet” from high back to Medium-high. This is not default but easily overlooked if you ever changed it.
I am having the same issue, there is no resolution by running in admin mode or any other registry edits or deletions.
Thanks dude, that was the problem. For those people who don’t know how to do it, here is the explanation and how to solve it.
It’s a problem with permissions for IE (Internet Explorer), it doesn’t matter if you use other Internet browsers, Windows uses IE to work in some aspects. This is how to change the settings.
1.- Open Internet Explorer
2.- Go to Tools or more tools (it should not be in configuration)
3.-Click on Internet Settings Or can say Internet Options (I don’t know how it appears I have the program in Spanish)
4.-Click on Security
5.-Click on custom level
6.- Click on restore to and select medium (it is the lowest option) and click on the restore button.
7.-Accept and apply now the installer should be working.
Note: Don’t forget when the installation is finished, restore the settings to medium-high again