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CCCamp19 and further EFail mitigations

Posted by Bjarni RĂșnar on August 31, 2019

Hello world!

As I write this, I have mostly recovered from the amazing Chaos Communication Camp in Germany. I attended the camp mostly as a holiday, but there were of course quite a few Mailpile related discussions.

The most interesting of those related to EFail. I spent a couple of afternoons sitting in the shade with researchers and developers who work on OpenPGP related things. We discussed EFail.

For those of you who haven't heard of EFail, it was one of the most serious vulnerabilities the OpenPGP community has ever had to deal with: by combining flaws in e-mail clients with flaws in legacy PGP (and S/MIME) implementations, EFail described multiple ways an attacker could turn a vulnerable e-mail client into a decryption oracle and steal the cleartext of previously secured communications. Cryptosystem flaws don't get much worse than that.

This was such a serious issue that the EFF recommended people disable PGP entirely, at least for a little while. This triggered a rather emotional backlash from the PGP community, and unfortunately a lot of misinformation and misunderstandings were published. Some of which still have not been corrected.

As a community, we're still coming to terms with some of the implications. Those of us who aren't in denial (which is disturbingly common) are still mulling over ways to secure our tools and defend against similar flaws in the future.

I have written before about Mailpile and EFail: there were a few issues that needed fixing, but overall Mailpile weathered EFail relatively well. Exfiltrating cleartext from Mailpile was possible, but it was not fully automated and required social engineering.

The social engineering aspects are still quite serious, and some are easier to exploit than others. The most trivial EFail exploit is to send someone a message they're likely to reply to, with the ciphertext you want to exfiltrate appended to the end after a long boring boilerplate signature or quoted message. If the mail client decrypts, and the recipient replies without reading and pruning their response... hey presto, you've exfiltratrated the secret message.

One of the outcomes of these discussions at camp, were some concrete recommendations on how Mailpile could make such social engineering less likely to succeed. The guiding principle I ended up with, was:

If the user is probably not going to see the content, do not decrypt.

On the plane home from Germany, I implemented this strategy. So for the first time, Mailpile will deliberately decline to decrypt parts of incoming e-mail, if the message structure is such that it might might provide cover for EFail social engineering attacks.

The change wasn't huge, but the security impact is significant. We welcome any and all feedback: the code is here. This patch is already in the nightly packages and will hit the release branch next time I update it - which should be soon, we've got quite a few important fixes queued up by now.

Many thanks to Sebastian and Vincent for helping me figure out how to improve Mailpile on this front. And thanks to CCC for providing venues for these conversations to take place.

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