Depending on how a website is configured, it may be able to read or modify Clipboard events that happen on it. Say, you highlight text on the site to copy it to the clipboard of your operating system to paste it into another application.
A script could record what you are copying, or, to take it to another extreme, modify the contents. Some services may for instance add copyright information to the copied text automatically.
A website could also use these events to block you from copying any contents, again by manipulating the event so that nothing is returned.
Other possibilities include preventing paste operations on the site, so that you cannot paste data into form fields but have to type it instead.
Besides being annoying, as you may get a different result than expected, it does have privacy implications as well as sites can log what users paste -- into text areas only I assume -- as well.
Firefox has a preference for that, to stop that behavior in its tracks. There may be a downside however to it which needs to be mentioned.
If you are working with rich text editors on the Internet, some may require that those events remain enabled for functionality. A basic example is an editor that processes the text that you are pasting in it to remove all formatting information from it.
To block sites from reading or modifying Clipboard contents in Firefox, do the following:
- Type about:config into Firefox's address bar and hit the enter key on the keyboard.
- Confirm you will be careful if a warning message pops up.
- Use the search up to to find the dom.event.clipboardevents.enabled preference.
- The default value of the preference is true, which means that sites can intercept Clipboard events.
- Double-click the preference to change its value to false.
- Once set, it will block sites from intercepting these events.
Tip: I suggest you monitor your online activities to see if any online editor or site behaves in a different way after you have disabled the feature.
There is unfortunately no option to enable it for individual websites.
Anyway, I highly suggest you set the value of the preference to false to block websites from manipulating Clipboard events in the web browser.