EFAIL🔒 SSL Verified

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"EFAIL describes vulnerabilities in the end-to-end encryption technologies OpenPGP and S/MIME that leak the plaintext of encrypted emails. Email is a plaintext communication medium whose communication paths are partly protected by TLS (TLS). For people in hostile environments (journalists, political activists, whistleblowers, ...) who depend on the confidentiality of digital communication, this may not be enough. Powerful attackers such as nation state agencies are known to eavesdrop on email communications of a large number of people. To address this, OpenPGP offers end-to-end encryption specifically for sensitive communication in view of these powerful attackers. S/MIME is an alternative standard for email end-to-end encryption that is typically used to secure corporate email communication."

"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."

"There are two different flavors of EFAIL attacks. First, we describe the novel CBC/CFB gadget attacks which abuse vulnerabilities in the specification of OpenPGP and S/MIME to exfiltrate the plaintext. The diagram below describes the idea of CBC gadgets in S/MIME. Because of the specifics of the CBC mode of operation, an attacker can precisely modify plaintext blocks if she knows the plaintext. S/MIME encrypted emails usually start with "Content-type: multipart/signed" so the attacker knows at least one full block of plaintext as shown in (a). She can then form a canonical plaintext block whose content is all zeros as shown in (b). We call the block pair X and C0 a CBC gadget. In step (c), she then repeatedly appends CBC gadgets to inject an image tag into the encrypted plaintext. This creates a single encrypted body part that exfiltrates its own plaintext when the user opens the attacker email. OpenPGP uses the CFB mode of operation, which has very similar cryptographic properties as CBC and allows the same attack using CFB gadgets."

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