Draft paper linked from efail.de.
TL;DR: the vulnerability is in some popular email client softwares combined with an extension simplifying use of GnuPG within said software, and bundled in popular distributions of GnuPG v2, thus often used.
The issue is that un-validated deciphered ciphertext is made available to the email client, which acts upon it (e.g. following HTML links), giving a decryption oracle.
Disabling GnuPG functionality in the email client and getting back to manual use of GnuPG should be fine (if warnings emitted by PGP are acted upon; feeding what's piped out to a vulnerable software could be bad). If I used GnuPG within my email client (I don't), I would stop doing that until the extension allowing it is improved to suppress deciphered plaintext that did not pass the Modification Detection Code test.
From an official statement by the GnuPG and Gpg4Win teams:
- This paper is misnamed. It's not an attack on OpenPGP. It's an attack on broken email clients that ignore GnuPG's warnings and do silly things after being warned. 2. This attack targets buggy email clients. Correct use of the MDC completely prevents this attack. GnuPG has had MDC support since the summer of 2000.