Published: Mon, May 14, 2018
Technology | By Nina Perez

Security researchers discover critical flaw in PGP encryption that reveals plaintext

Security researchers discover critical flaw in PGP encryption that reveals plaintext

Meanwhile, some vendors are expected to release patches to mitigate EFAIL attacks. Direct Exfiltration affects Apple's macOS and iOS Mail clients, as well as Mozilla's Thunderbird, enabling an attacker to send an email that automatically decodes and shares a victim's encrypted message content in a reply.

Security researchers said Monday they have discovered a critical flaw in the way certain email programs handle a popular encryption technology that safeguards emails from prying eyes.

The second vulnerability partially incorporates the first, and relies on an attacker being able to guess parts of the encrypted communication, which is generally possible due to the nature of the protocol involved.

It is, for example, not enough to deter attacks by "nation state actors, large-scale breaches of email servers, revealing millions of email messages, or attackers compromising email accounts", they explain.

"There is not yet a full fix for the problem, says Sebastian Schinzel, a professor of computer security at Germany's Münster University of Applied Sciences who's part of the research team - together with researchers from Ruhr-University Bochum in Germany and KU Leuven University in Belgium - that has found the flaws, which they've dubbed "#efail".

S/MIME is very similar to PGP except that instead of users defining their own encryption methods and web of trust (how to share their private encryption keys), S/MIME uses predefined encryption standards and public-private keypairs distributed by a trusted authority.

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If you've been using PGP or S/MIME to securely send and receive sensitive emails, you'll want to stop using them right away, as a group of European researchers have found vulnerabilities in both standards. If you're particularly paranoid, you might choose to decrypt emails in applications that are separate from your email program-a step the German researchers recommend.

Prior to the leak, Schnizel stated that there were "no reliable fixes", and recommended that affected users disable breached encryption software. And that person's email client decrypts the email and loads external content, "thus exfiltrating the plaintext to the attacker".

Werner Koch, the principal author of the cryptographic software GNU Privacy Guard, called EFF's warnings about the vulnerability "pretty overblown".

It added, however, that it considered the encryption standards themselves to be safe if correctly implemented and configured. Disabling the client will also prevent the ability for anyone looking over one's shoulder to decrypt past messages. A website has also been set up that advises PGP user to disable HTML renderings in emails sent via PGP as that will close the most prominent way of taking advantage of the vulnerability.

EFF said in a blog post that users should uninstall PGP until the flaw is patched.

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