15.05.2018 16:05

#efail #fail

Aktuell gehen Berichte um (Twitter, ars technica, EFF, ...), die vor einem Sicherheitsproblem mit verschlüsselten Mails berichten. Die EFF geht soweit, eine Deinstallation diverser Tools zu empfehlen.

Während ich diesen Blogpost schreibe, gingen die Researcher mit ihren Ergebnissen online: https://efail.de/ Yay! Eine Vuln mit coolem Namen und Logo.

Hier die wichtigsten Punkte:

  • Das Problem ist nicht die Verschlüsselung, sondern liegt im automatischen Entschlüsseln in Verbindung mit aktivem Inhalt (also HTML).

  • Es werden morgen zwei verschiedene Angriffsvarianten vorgestellt werden.

  • Eine davon ist nicht trivial ausnutzbar.

  • Es betrifft nicht nur PGP, sondern auch S/MIME.

  • Durch Updates der Mail-Software lässt sich das Problem leicht aus der Welt schaffen.

Unsere Empfehlungen:

  • PGP Plugins aktuell halten

  • Optimalerweise Mails in der Plaintext-Version anzeigen lassen, wenn HTML, dann wenigstens "remote content" (also nachgeladene Bilder) abdrehen.

  • Aktuell ist die größere Gefahr eine Kurzschlussreaktion wegen Drucks von den Medien und/oder dem Management.

Quellen:

Ein Entwickler von Enigmail (PGP-Plugin für Mozilla-Produkte wie Thunderbird) schreibt:

We saw a preview of that paper. It's under embargo so it would be inappropriate for us to comment on it until it's released. It was also inappropriate for the EFF to comment on it. You can expect us to have an official statement on it once the paper is published.

I will say this is a tempest in a teapot. Patrick, Werner, and I have all seen it. We are not in the least bit worried. We wish the EFF had reached out to us before running with an alarmist article.

tl;dr: as always, please use the latest Enigmail version, and do so with confidence.

Die GPG-Enwickler schreiben:

The topic of that paper is that HTML is used as a back channel to create an oracle for modified encrypted mails. It is long known that HTML mails and in particular external links like are evil if the MUA actually honors them (which many meanwhile seem to do again; see all these newsletters). Due to broken MIME parsers a bunch of MUAs seem to concatenate decrypted HTML mime parts which makes it easy to plant such HTML snippets.

There are two ways to mitigate this attack

- Don't use HTML mails. Or if you really need to read them use a proper MIME parser and disallow any access to external links.

- Use authenticated encryption.

[...]

Update 2018/05/15

An Official Statement on New Claimed Vulnerabilities by the GnuPG and Gpg4Win teams

1. This paper is misnamed. 2. This attack targets buggy email clients. 3. The authors made a list of buggy email clients.

Enigmail Update

Thereare two different attacks outlined in the Efail paper. One targets OpenPGP directly, and GnuPG has had mitigations against it for almost twenty years. Reports saying that GnuPG is vulnerable are wrong.

The other one targets buggy MIME parsing by email clients. Enigmail previously had some susceptibility to it, but as of Enigmail 2.0 we've closed up all the leaks on our side of things. There is still a small bit of attack surface in Thunderbird. The code to fix that has been checked into Thunderbird and will be part of the next Thunderbird release.

Aaron Kaplan zu Mitigation am Mac

Luckily there is a good tool for this: Little Snitch (and as far as I know, this is the only tool of its kind on OS X). With Little Snitch, which acts as an outgoing firewall, I am able to protect and filter the communication flows.

Autor: Otmar Lendl