eFail: Critical Flaws in PGP and S/MIME Tools Can Reveal Encrypted Emails
A team of European security researchers has released a warning about a set of critical vulnerabilities discovered in PGP and S/Mime encryption tools that could reveal your encrypted emails in plaintext. What's worse? The vulnerabilities also impact encrypted emails you sent in the past.
PGP, or Pretty Good Privacy, is an open source end-to-end encryption standard used to encrypt emails in a way that no one, not even the company, government, or cyber criminals, can spy on your communication. S/MIME, Secure/Multipurpose Internet Mail Extensions, is an asymmetric cryptography-based technology that allows users to send digitally signed and encrypted emails.
The EFAIL attacks exploit vulnerabilities in the OpenPGP and S/MIME standards to reveal the plaintext of encrypted emails. In a nutshell, EFAIL abuses active content of HTML emails, for example externally loaded images or styles, to exfiltrate plaintext through requested URLs. To create these exfiltration channels, the attacker first needs access to the encrypted emails, for example, by eavesdropping on network traffic, compromising email accounts, email servers, backup systems or client computers. The emails could even have been collected years ago.
The attacker changes an encrypted email in a particular way and sends this changed encrypted email to the victim. The victim's email client decrypts the email and loads any external content, thus exfiltrating the plaintext to the attacker. Being able to intercept and modify e-mails in transit is the sort of thing the NSA can do, but is hard for the average hacker. The vulnerability isn't with PGP or S/MIME itself, but in the way they interact with modern e-mail programs. Vulnerability disclosure and associated (mis)communication for eFail created a lot of heated debates
within the security community.