What is EFAIL?

EFAIL is a recent attack on PGP en S/MIME email encryption (EFAIL).

EFAIL exploits remote content resolving built into most email clients (like for example images and CSS rules) to get (parts) of a previously encrypted email.

What EFAIL basically does, is that it takes a previously encrypted email (for which the attacker does not have the private key) and embeds this encrypted email into a new email in a special way. The email is then sent to the recipient for decryption. The attacker however designed the email in a way that the decrypted content of the original email gets embedded into the URL for a remote resource (for example an image). If the email client is configured to automatically resolve external content (for example download images), the content of the email gets sent to the remote server as a URL request. If the remote server is under control of the attacker, or if the URL is sent via HTTP, the attacker can access the URL and therefore has access to the plain text content of the original email.