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The sad state of SMTP encryption

blog.filippo.io

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Re: The sad state of SMTP encryption

#11

Post by Tepix »

The article clarifies the issues that exist with SMTP encryption nicely.

Regarding the issue with certificates for the servers that the MX points to, I disagree with the author. If the MX for example.com points to mail.example.info, it implies that example.com trusting the handling of its mail to mail.example.info, therefore there is no issue with letting mail.example.info present its own certificate.

The article also suggests that DNSSEC with DANE will solve all issues with SMTP encryption.

However, DNSSEC is a crappy standard. It doesn't do encryption so a surveillant can still collect metadata; it has unsolved issues that facilitate amplification attacks, it's overly complex and has slow adoption. In fact, before DANE arrived on the scene, there was hardly a good reason to deploy it.

If we adopt DNSSEC now we'll be stuck with it (including its lack of privacy) pretty much forever. Instead, I suggest we work on more promising initiatives such as DNSCurve (https://en.wikipedia.org/wiki/DNSCurve)

Re: The sad state of SMTP encryption

#12

Post by fulafel »

snori74 wrote: post #5

In fact (as the article admits), the encryption is fine and fully effective against a passive attacker, the problem is that it's not much use against an active man-in-the-middle. But, that's not something anyone but a NSA or ISP can easily do between mailservers.

There are many other attackers besides NSA and your ISP who are on-path. Anyone on the same network as the sender or receiver will do. There are many servers who are on the same network with compromised boxes (think cohosting, your home network with IoBT devices, typical corporate intranet, etc).

Re: The sad state of SMTP encryption

#13

Post by snori74 »

billpg wrote: post #10
snori74 wrote: post #5

In fact (as the article admits), the encryption is fine and fully effective against a passive attacker, the problem is that it's not much use against an active man-in-the-middle. But, that's not something anyone but a NSA or ISP can easily do between mailservers.

If you can see a DNS request going out and can send a fake response that arrives before the real one, you have enough resources to pull off a MITM.

Yes, but unless you're the ISP that's not going to be possible surely? Mailservers typically don't run over wireless.

Re: The sad state of SMTP encryption

#14

Post by Tepix »

snori74 wrote: post #5

In fact (as the article admits), the encryption is fine and fully effective against a passive attacker, the problem is that it's not much use against an active man-in-the-middle. But, that's not something anyone but a NSA or ISP can easily do between mailservers.

It's a real problem. If you visit countries like Egypt, you will experience STARTSSL-stripping every day.

Re: The sad state of SMTP encryption

#15

Post by mike-cardwell »

Also of relevance:

https://www.mail-archive.com/dane-users@sys4.de/msg00142.htm...

Comcast published TLSA records the other day. So when people like myself who have enabled TLSA in Postfix send email to Comcast users, the SMTP connection is guaranteed to be both encrypted, and the SSL cert validated.

Re: The sad state of SMTP encryption

#16

Post by [deleted] »

snori74 wrote: post #5

In fact (as the article admits), the encryption is fine and fully effective against a passive attacker, the problem is that it's not much use against an active man-in-the-middle. But, that's not something anyone but a NSA or ISP can easily do between mailservers.

[deleted]

Re: The sad state of SMTP encryption

#18

Post by Tepix »

atmosx wrote: post #7
snori74 wrote: post #5

In fact (as the article admits), the encryption is fine and fully effective against a passive attacker, the problem is that it's not much use against an active man-in-the-middle. But, that's not something anyone but a NSA or ISP can easily do between mailservers.

The problem is that most of my emails will end up on Gmail, Yahoo! or Hotmail. Since nearly no one uses PGP, the emails sit on a google, apple, yahoo, microsoft server in plain text for any ads or 'gov' company to see and analyze in bulk. Until we 'solve' THIS problem, there's no point in discussing what happens in between IMHO.

Even if you run your own mailserver, you're not in control unless it's a dedicated server. On a VPS you may be spied upon by your ISP without standing a chance of noticing it. Luckily there are now ISPs that let you rent a dedicated server for 6€ a month (or just host it at home on something like the Raspberry Pi 2 and use a minimal VPS just to tunnel the traffic to it from its non-dialup IP address so email actually works).

Re: The sad state of SMTP encryption

#19

Post by Tepix »

teekert wrote: post #9

It's indeed not a nice situation, I'd love for my mailserver to insert a line into the message subject something like: 'TLS not used' when this is the case (or a plugin for Roundcube (Next?) that colors the subject, how cool would that be?). Just so I know and are aware of suspicious things. Enforcing TLS (and valid certs + strong encryption) is just not very practical yet, although it will definitely not affect mayo…

This should be pretty easy to do with a SIEVE filter rule: If the Received: line inserted by your mail server doesn't contain "using TLS..." you put a prefix into the mail subject.
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