Jens Muller - Covert Attacks on Email End-to-End Encryption
Extends EFAIL research with new covert channel attacks that exfiltrate PGP and S/MIME plaintext without triggering known detection mechanisms.
Summary
Covert Email Encryption Attacks
Jens Muller (PhD researcher, Ruhr University Bochum) presents new attacks against email end-to-end encryption, extending the EFAIL research with novel covert channel exfiltration techniques.
EFAIL Recap
The EFAIL vulnerabilities (CVE-2017-17688, CVE-2017-17689) demonstrated that many email clients decrypt PGP/S-MIME messages in an HTML rendering context, allowing adversaries who intercept encrypted emails to craft malicious wrappers that exfiltrate plaintext via CSS background image URLs or HTML form submissions when the victim's client opens the email.
New Covert Channels
Muller identifies additional exfiltration channels not addressed by EFAIL patches: DNS-based exfiltration through MX record lookups triggered during email rendering, and HTTP/2 server push covert channels in email clients with embedded browser renderers.
Mitigation and Detection Gaps
Existing mitigations (disabling remote content loading) fail to address DNS-based channels. The talk provides complete detection guidance and advocates for email client architectural changes that isolate decryption from rendering.
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