Decade-old Efail flaws can leak plaintext of PGP- and S/MIM-encrypted emails

Enlarge (credit: Sebastian Schinzel)

Unfixed bugs in widely used email programs make it possible for attackers to obtain the plaintext of messages that are encrypted using the PGP and S/MIME standards, researchers said early Monday morning. The attacks assume that an attacker has possession of the encrypted emails and can trick either the original sender or one of the recipients into opening an invisible snippet of the intercepted message in a new email.

The flaws, some of which have existed for more than a decade, are part of a series of vulnerabilities dubbed Efail described by a team of European researchers. The vulnerabilities allow attackers to exfiltrate email plaintexts by embedding the previously obtained ciphertext into unviewable parts of an email and combining it with HTML coding. Earlier on Monday, the researchers and the Electronic Frontier Foundation issued an advisory recommending PGP and S/MIME users disable the encryption in their email clients but had planned to wait until Tuesday to provide technical details of the vulnerabilities. Within hours, the researchers published the paper, which is titled Efail: Breaking S/MIME and OpenPGP Email Encryption using Exfiltration Channels.

The most serious vulnerabilities have resided in Thunderbird, macOS Mail, and Outlook for more than 10 years and remain unfixed at the moment, the researchers said. Flaws in the way the programs handle emails with multiple body parts make it possible to embed invisible snippets of previously obtained encrypted text in new emails. By also including the Web address of an attacker-controlled server, the newly sent emails can cause the programs to send the corresponding plaintext to the server. The surreptitious exfiltration works against both the PGP and S/MIME standards.

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