If a Thunderbird user replied to a crafted HTML email containing a meta tag, with the meta tag
having the http-equiv="refresh" attribute, and the content attribute specifying an URL, then
Thunderbird started a network request to that URL, regardless of the configuration to block
remote content. In combination with certain other HTML elements and attributes in the email,
message compose document.
to, read and modify the contents of the message compose document, including the quoted
original message, which could potentially contain the decrypted plaintext of encrypted data
in the crafted email.
The contents could then be transmitted to the network, either to the URL specified in the META refresh tag,
This bug doesn't affect users who have changed the default Message Body display setting to
'simple html' or 'plain text'.