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For its part, the privacy community is insisting that this vulnerability is overblown and that people are overreacting. Werner Koch, principal author of GNU Privacy Guard, writes that the two ways to mitigate this attack are to simply not use HTML emails, and to use authenticated encryption, something noted in the paper.

“They figured out mail clients which don’t properly check for decryption errors and also follow links in HTML mails. So the vulnerability is in the mail clients and not in the protocols. In fact OpenPGP is immune if used correctly while S/MIME has no deployed mitigation,” GNU Privacy Guard said on Twitter.

Here are some strategies to prevent EFAIL attacks:

Short term: No decryption in email client. The best way to prevent EFAIL attacks is to only decrypt S/MIME or PGP emails in a separate application outside of your email client. Start by removing your S/MIME and PGP private keys from your email client, then decrypt incoming encrypted emails by copy&pasting the ciphertext into a separate application that does the decryption for you. That way, the email clients cannot open exfiltration channels. This is currently the safest option with the downside that the process gets more involved.

Short term: Disable HTML rendering. The EFAIL attacks abuse active content, mostly in the form of HTML images, styles, etc. Disabling the presentation of incoming HTML emails in your email client will close the most prominent way of attacking EFAIL. Note that there are other possible backchannels in email clients which are not related to HTML but these are more difficult to exploit.

I don't send HTML emails. Am I safe?
No. The attacker can change encrypted text/only emails to HTML emails. You need to disable viewing HTML email to increase protection from EFAIL attacks.

I have disabled HTML in my email client. Am I safe now?
Depends. S/MIME or PGP encrypted emails are encrypted with the public keys of all recipients and the sender. The attacker can thus perform the EFAIL attacks if only one of the participants is vulnerable. In order to prevent the EFAIL attacks, all participants must use secure email clients

I have encrypted data using OpenPGP or S/MIME and I won't decrypt it in the email context. Am I safe?
For now yes. There may be edge cases though that we hadn't looked into. For example, if you encrypted a directory with sensitive files, an attacker could change these encrypted files to contain false information or even malware. If a victim decrypts the directory and opens any of the files, malware or even just an HTML file could be used to exfiltrate plaintext or even compromise the system.

Our advice, which mirrors that of the researchers, is to immediately disable and/or uninstall tools that automatically decrypt PGP-encrypted email.