When a message was forwarded inline, edited as new, or had an attachment dragged into a compose window, a specially crafted link attachment (a .desktop file on Linux, or a .url or .lnk file on Windows) was replaced with the content of the file or URL it pointed to when the message was saved or sent. This could disclose local files, or content from the user's network, to the message recipients or to the mail provider.
This bug only affects Thunderbird on Linux and Windows. Other operating systems are unaffected. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.