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Let's consider a simple scenario 1 user copies a text from a website 2 user pastes it into a msword document 3.1 the text has a transparent img in it, when the user pastes it into msword, msword loads the img from network, deanonimizing the user. or 3.2 the text has a transparent swf in it, when the user copies it, the clipboard logger application understands it is a html tryes to render, in order to do it it loads MSIE engine which loads flash plugin which executes swf which collects and sends sensitive data.

TorBrowser must sanitize the info transferred to/from clipboard removing all the content available from the network and all the active content (scripts, swfs, applets, etc).