Ruhr-Uni-Bochum

Re: What's up Johnny? – Covert Content Attacks on Email End-to-End Encryption

2019

Konferenz / Medium

Autor*innen

Poddebniak, Damian Müller, Jens Schinzel, Sebastian Brinkmann, Marcus Schwenk, Jörg

Research Hub

Research Hub A: Kryptographie der Zukunft
Research Hub C: Sichere Systeme
Research Hub D: Benutzerfreundlichkeit

Research Challenges

RC 8: Security with Untrusted Components

Abstract

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.

Tags

Cryptographic Protocols
Cryptography
Software Security
Web Security
Usable Security and Privacy