Is your email being sent in the clear? While PGP and S/MIME provide end-to-end encrypted mail, most users have yet to adopt these practices, and for users who have, these tools leave metadata, such as the subject, sender, and recipient, visible everywhere along a message’s path. SMTP—the ubiquitous mail transport protocol—has evolved over the years to add encryption and authentication, both of which take place behind the scenes and help guard against surveillance and spam. While these features are being increasingly deployed, our research shows that they are almost always configured in vulnerable ways—the details of which are hidden from the users sending and receiving mail. Even more disturbingly, these vulnerabilities are being widely exploited in the wild: in seven countries, more than 20% of inbound Gmail messages are downgraded to cleartext by network-based attacks. In the most severe case, 96% of messages sent from Tunisia to Gmail are downgraded to cleartext. In this talk, I’ll introduce the commonly used SMTP security extensions—including STARTTLS, SPF, DKIM, and DMARC—and describe the current state of mail security on the Internet. I'll describe several commonly occurring attacks our recent research has found and discuss how mail operators can configure their servers to secure email transport. Finally, I'll discuss several weaknesses in the protocols we're using and recent proposals for helping secure email transport.
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