https://blog.hboeck.de/archives/893-efail-Outdated-Crypto-Standards-are-to-blame.html

I have a lot of thoughts about the recently published efail vulnerability, so I thought I'd start to writeup some of them. I'd like to skip all the public outrage about the disclosure process for now, as I mainly wanted to get into the technical issues, explain what I think went wrong and how things can become more secure in the future. I read lots of wrong statements that "it's only the mail clients" and the underlying crypto standards are fine, so I'll start by explaining why I believe the OpenPGP and S/MIME standards are broken and why we still see these kinds of bugs in 2018. I plan to do a second writeup that will be titled "efail: HTML mails are to blame".

I assume most will have heard of efail by now, but the quick version is this: By combining a weakness in cryptographic modes along with HTML emails a team of researchers was able to figure out a variety of ways in which mail clients can be tricked into exfiltrating the content of encrypted e-mails. Not all of the attack scenarios involve crypto, but those that do exploit a property of encryption modes that is called malleability. It means that under certain circumstances you can do controlled changes of the content of an encrypted message.