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.
More specifically, the vulnerability has been discovered in the PGP or S/MIME software for email encryption. This needs to be done in three emails, with the first one opening the HTML tag, the second one containing the encrypted message, and the third closing the HTML tag.
An attacker could gain access to encrypted emails by monitoring network traffic, compromising email servers or the computers of users, or gaining access to backup servers. In fact, the only clients protected against S/Mime attacks are Claws Mail and Mutt whereas more clients are protected against PGP-targeting attacks.
The primary issue affecting Apple Mail, iOS Mail and Mozilla's Thunderbird client, is a method that uses multipart responses to exploit HTML rendering issues. PGP and S/MIME are said to have flaws that could be exploited to get access to any incoming or outgoing emails on platforms that use either of the two encryption tools. This is not surprising as PGP encryption has so far been considered rock solid.
EFF also advises users to immediately disable and/or uninstall tools that automatically decrypt PGP-encrypted emails, and switch to safer tools until the issue is resolved. According to the researchers, both CFB and CBC enable an attacker to reorder, remove or insert ciphertext blocks, or to perform meaningful plaintext modifications without the encryption key. Ars will have much more coverage of the efail vulnerabilities, and the researchers have more information here.More news: Dozens dead in clashes as United States opens Jerusalem embassy
The attacker creates a new multipart email message and prepares it in a special way.
"The victim's email client decrypts the email and loads any external content, thus exfiltrating the plaintext to the attacker". In the meantime, they are recommending that users stop using OpenPGP and S/MIME for now. In a post-Monday, he said his team wasn't contacted about the flaw and the attack could be mitigated by avoiding HTML emails or using authenticated encryption, which adds a layer of protection to confirm the message hasn't been changed. "Disabling the presentation of incoming HTML emails in your email client will close the most prominent way of attacking Efail". The Efail attacks rely on external communication, and if a user is decrypting emails in a stand-alone application, the risks are somewhat muted.
The team's lead researcher, Sebastian Schinzel, admitted that: "Email is no longer a secure communication medium".
PGP and S/MIME have flaws that could be exploited to decrypt any incoming or outgoing communication. "Therefore, the standards need to be updated, which will take some time".