Chromium-based web browsers (including Chrome) allow websites to write anything they want into the user's clipboard without getting permission from them.
To test if your browser is affected by that feature, check this website.
If you get the following message in your clipboard, your browser can be abused to push anything into your clipboard:
"Hello, this message is in your clipboard because you visited the website Web Platform News in a browser that allows websites to write to the clipboard without the user’s permission. Sorry for the inconvenience. For more information about this issue, see https://github.com/w3c/clipboard-apis/issues/182."