Efail: Breaking S/MIME and OpenPGP Email Encryption using Exfiltration Channels
Damian Poddebniak, Christian Dresen, Jens Müller, Fabian Ising, Sebastian Schinzel, Simon Friedberger, Juraj Somorovsky, Jörg Schwenk
27th USENIX Security Symposium (USENIX Security 18)
OpenPGP and S/MIME are the two prime standards for providing end-to-end security for emails. We describe novel attacks built upon a technique we call malleability gadgets to reveal the plaintext of encrypted emails. We use CBC/CFB gadgets to inject malicious plaintext snippets into encrypted emails that abuse existing and standard conforming backchannels, for example, in HTML, CSS, or x509 functionality, to exfiltrate the full plaintext after decryption. The attack works for emails even if they were collected long ago, and is triggered as soon as the recipient decrypts a single maliciously crafted email from the attacker.
We devise working attacks for both OpenPGP and S/MIME encryption, and show that exfiltration channels exist for 23 of the 35 tested S/MIME email clients and 10 of the 28 tested OpenPGP email clients. While it is necessary to change the OpenPGP and S/MIME standards to fix these vulnerabilities, some clients had even more severe implementation flaws allowing straightforward exfiltration of the plaintext.[full version]