Re: What's Up Johnny? -- Covert Content Attacks on Email End-to-End Encryption
This addresses security vulnerabilities in widely used email encryption standards, posing risks to user privacy and data integrity.
The paper demonstrates practical attacks on OpenPGP and S/MIME email encryption by exploiting MIME and HTML features to deceive users into leaking plaintext or signing arbitrary text, with evaluations showing vulnerabilities in 17 of 19 OpenPGP and 21 of 22 S/MIME clients.
We show practical attacks against OpenPGP and S/MIME encryption and digital signatures in the context of email. Instead of targeting the underlying cryptographic primitives, our attacks abuse legitimate features of the MIME standard and HTML, as supported by email clients, to deceive the user regarding the actual message content. We demonstrate how the attacker can unknowingly abuse the user as a decryption oracle by replying to an unsuspicious looking email. Using this technique, the plaintext of hundreds of encrypted emails can be leaked at once. Furthermore, we show how users could be tricked into signing arbitrary text by replying to emails containing CSS conditional rules. An evaluation shows that 17 out of 19 OpenPGP-capable email clients, as well as 21 out of 22 clients supporting S/MIME, are vulnerable to at least one attack. We provide different countermeasures and discuss their advantages and disadvantages.