A group of security researchers has discovered “critical vulnerabilities” in the common email encryption tools PGP and S/MIME, prompting calls for users to disable email plugins until a fix is developed.

In a paper published Monday, the group outlined a proof-of-concept process for how attackers could exploit weaknesses in how email clients like Apple Mail, iOS Mail, and Mozilla Thunderbird manage HTML in messages.


“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,” the researchers wrote in a paper that dubs the exploits "Efail attacks".

A successful attack could theoretically grant malicious parties access to the entire contents of a target’s inbox. The Electronic Frontier Foundation provided a simplified explanation of the two attack methods:

“The first attack is a “direct exfiltration” attack that is caused by the details of how mail clients choose to display HTML to the user. The attacker crafts a message that includes the old encrypted message. The new message is constructed in such a way that the mail software displays the entire decrypted message—including the captured ciphertext—as unencrypted text. Then the email client’s HTML parser immediately sends or “exfiltrates” the decrypted message to a server that the attacker controls.

The second attack abuses the underspecification of certain details in the OpenPGP standard to exfiltrate email contents to the attacker by modifying a previously captured ciphertext.”

PGP, which stands for “pretty good protection,” and S/MIME have long been standards for end-to-end email encryption. These tools help people like journalists and whistleblowers be reasonably sure that sensitive information is readable only to sender and receiver. But today’s paper has raised debate in the cryptography community about the level of security these tools are able to provide, and what users should expect.

In the short term, the researchers and the Electronic Frontier Foundation (EFF) recommend users disable PGP plugins and use non-email based messaging platforms to decrypt messages until a long-term solution is developed.