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

TL;DR

This paper reveals practical vulnerabilities in email encryption standards like OpenPGP and S/MIME, exploiting MIME and HTML features to deceive users and leak encrypted email contents, with extensive client vulnerability analysis.

Contribution

It introduces novel attack techniques exploiting MIME and HTML features to compromise email encryption security and evaluates their effectiveness across multiple email clients.

Findings

Most email clients are vulnerable to at least one attack.

Hundreds of encrypted emails can be decrypted through user deception.

Countermeasures have been proposed and analyzed.

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.