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
Jens Müller is a PhD student at the Chair for Network and Data Security, Ruhr University Bochum, Germany. His research interests are legacy protocols and data formats, for which he loves to investigate what could possibly go wrong in a modern world. He has experience as a speaker on international security conferences (BlackHat, IEEE S&P, OWASP) and as a freelancer in network penetration testing and security auditing. Besides breaking thinks, he develops free open source software, for example, tools related to network printer exploit^H^H^H^H^H^H^H, um, «debugging».