Every extension uploaded by a developer in Chrome Web Store is approved by Chrome team and are supposedly safe to install in Chrome, but Google can’t control or analyze extensions for malicious behavior hosted on websites other than Chrome web store, as a security measure, latest version of Chrome now doesn’t allows to add third-party extensions from websites.
If you try to install one, then Chrome displays warning on top with the message “Extensions, apps and user scripts only can be added from chrome web store” and extension installation halts, what user need to do now? This is annoying, don’t worry! You can still install extensions on other websites or third-party ones if you think they are trusted and safe.
Update: You’ve to follow developer method (read steps mentioned after the second paragraph in that) to install third-party extension in Chrome
Chrome team encourages developers to offer inline installation for extensions and apps
Chrome team says novice users can easily install malicious extensions from other sites, but from their security point of view Chrome made this change, and this change now reflects in current Chrome stable version, Chrome team also suggesting developers to upload their extension to Web Store and follow inline installation of extensions, that is extensions will be hosted on Chrome Web Store but developer offers the installation link on their site so that user stays on their site, at the same time extension or app will be installed from Chrome Web Store, here user is installing the app without leaving the developer site.
Coming to the point, for e.g I tried to install experimental version of user agent switcher for chrome (this has been done for testing), chrome before initiating installation says “ Extensions, apps and user scripts only can be added from chrome web store” , you can click “OK” to dismiss that message or clicking on learn more takes you to crx warning help page. Here User agent switcher extension is hosted on developer site, this site can be trusted and extension is safe to install, but chrome not allowing me to install it.
Installing Chrome extensions from third-party sites
1.What chrome suggesting you is that download the extension you want to install from other websites and save it your computer.
2. Now open chrome extensions page from tools>extensions
3. Drag and drop the extension from your computer on to the extension page.
4. Click the “add” button after reviewing list of permissions shown in the dialog by Chrome.