On Sunday night, the 13th of May, a team of nine researchers revealed that they had discovered a critical vulnerability in two of the most widely used email encryption tools; OpenPGP and S/MIME.
OpenPGP (Pretty Good Privacy) is an encryption program that provides cryptographic privacy and authentication for data communication. S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. Both are commonly used by a number of email platforms, including Apple Mail and Microsoft Outlook.
The research team, who are based in Germany, are referring to the flaw in question as EFAIL. They state that "novel attacks built upon a technique we call malleability gadgets to reveal plaintext of encrypted emails." However, not only does the flaw exposes the encrypted emails in plaintext, it can do so even for messages sent in the past, meaning security breaches can be significantly damaging.
There are currently no reliable fixes for the vulnerability. If you use PGP/GPG or S/MIME for very sensitive communication, you should disable it in your email client for now. Also read @EFF’s blog post on this issue: https://t.co/zJh2YHhE5q #efail 2/4
— Sebastian Schinzel (@seecurity) 14 May 2018
This looks... bad. Serious vulnerabilities found in PGP and S/MIME, likely in the protocols themselves rather than in an implementation. Users advised to disable PGP clients. https://t.co/XCpwg414fP
— Martijn Grooten (@martijn_grooten) 14 May 2018
This essentially means, as Sebastian Schinzel plainly puts it, "email is no longer a secure communication method". Schinzel, a professor of Computer Security at Münster University in Germany is one of the nine researchers who discovered the defect and isn't the only one voicing dire security warnings. Many experts and websites have begun sounding the alarm with tips on how to circumvent the vulnerability. The website MacRumors have already compiled guides on how to disable /uninstall the tools for Apple Mail, Mozilla Thunderbird with Enigmail, and Microsoft Outlook with GPG4win.
The research team intended to release more about the details of the vulnerability on Tuesday the 15th of May. however, an embargo which had been placed on the story was lifted so the EFAIL website is now live. For now, this is what they have revealed about the flaw:
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.
However, not everyone is on the same page about the seriousness of the flaw. The principal author of the GNU Privacy Guard, Werner Koch has gone as far to say that the vulnerability is overblown and people are overreacting.
— Kevin Gallagher (@ageis) 14 May 2018
Either way, if you are concerned about the vulnerability of you or your enterprise's email communication, it is suggested that you simply follow the above instructions and disable the afflicted tools. If you are still unsure of your susceptibility to it, visit the efail.de site or read up on the research paper they have also released.