Wandering Thoughts archives


My collection of spam and the spread of SMTP TLS

One of the things that my sinkhole SMTP server does that's new on my workstation is that it supports TLS, unlike my old real mail server there (which dates from a very, very long time ago). This has given me the chance to see how much of my incoming spam is delivered with TLS, which in turn has sparked some thoughts about the spread of SMTP TLS.

The starting point is that a surprising amount of my incoming spam is actually delivered with TLS; right now about 30% of the successful deliveries have used TLS. This is somewhat more striking than it sounds for two reasons; first, the Go TLS code I'm relying on for TLS is incomplete (and thus not all TLS-capable sending MTAs can actually do TLS with it), and second a certain amount of the TLS connection attempts fail because the sending MTA is offering an invalid client certificate.

(I also see a fair number of rejected delivery attempts in my SMTP command log that did negotiate TLS, but the stats there are somewhat tangled and I'm not going to try to summarize them.)

While there are some persistent spammers, most of the incoming email is your typical advance fee fraud and phish spam that's send through various sorts of compromised places. Much of the TLS email I get is this boring sort of spam, somewhat to my surprise. My prejudice is that a fair amount of this spam comes from old and neglected machines, which are exactly the machines that I would expect are least likely to do TLS.

(Some amount of such spam comes from compromised accounts at places like universities, which can and do happen to even modern and well run MTAs. I'm not surprised when they use TLS.)

What this says to me is that support for initiating TLS is fairly widespread in MTAs, even relatively old MTAs, and fairly well used. This is good news (it's now clear that pervasive encryption of traffic on the Internet is a good thing, even casual opportunistic encryption). I suspect that it's happened because common MTAs have enabled client TLS by default and the reason they've been able to do that is that it basically takes no configuration and almost always works.

(It's clear that at least some client MTAs take note when STARTTLS fails and don't try it again even if the server MTA offers it to them, because I see exactly this pattern in my SMTP logs from some clients.)

PS: you might wonder if persistent spammers use TLS when delivering their spam. I haven't done a systematic measurement for various reasons but on anecdotal spot checks it appears that my collection of them basically doesn't use TLS. This is probably unsurprising since TLS does take some extra work and CPU. I suspect that spammers may start switching if TLS becomes something that spam filtering systems use as a trust signal, just as some of them have started advertising DKIM signatures.

spam/SpamAndTLSSpread written at 23:25:16; Add Comment

I want my signed email to work a lot like SSH does

PGP and similar technologies have been in the news lately, and as a result of this I added the Enigmail extension to my testing Thunderbird instance. Dealing with PGP through Enigmail reminded me of why I'm not fond of PGP. I'm aware that people have all sorts of good reasons and that PGP itself has decent reasons for working the way it does, but for me the real strain point is not the interface but fundamentally how PGP wants me to work. Today I want to talk just about signed email, or rather however I want to deal with signed email.

To put it simply, I want people's keys for signed email to mostly work like SSH host keys. For most people the core of using SSH is not about specifically extending trust to specific, carefully validated host keys but instead about noticing if things change. In practical use you accept a host's SSH key the first time you're offered it and then SSH will scream loudly and violently if it ever changes. This is weaker than full verification but is far easier to use, and it complicates the job of an active attacker (especially one that wants to get away with it undetected). Similarly, in casual use of signed email I'm not going to bother carefully verifying keys; I'm instead going to trust that the key I fetched the first time for the Ubuntu or Red Hat or whatever security team is in fact their key. If I suddenly start getting alerts about a key mismatch, then I'm going to worry and start digging. A similar thing applies to personal correspondents; for the most part I'm going to passively acquire their keys from keyservers or other methods and, well, that's it.

(I'd also like this to extend to things like DKIM signatures of email, because frankly it would be really great if my email client noticed that this email is not DKIM-signed when all previous email from a given address had been.)

On the other hand, I don't know how much sense it makes to even think about general MUA interfaces for casual, opportunistic signed email. There is a part of me that thinks signed email is a sexy and easy application (which is why people keep doing it) that actually doesn't have much point most of the time. Humans do terribly at checking authentication, which is why we mostly delegate that to computers, yet casual signed email in MUAs is almost entirely human checked. Quick, are you going to notice that the email announcement of a new update from your vendor's security team is not signed? Are you going to even care if the update system itself insists on signed updates downloaded from secure mirrors?

(My answers are probably not and no, respectively.)

For all that it's nice to think about the problem (and to grumble about the annoyances of PGP), a part of me thinks that opportunistic signed email is not so much the wrong problem as an uninteresting problem that protects almost nothing that will ever be attacked.

(This also ties into the problem of false positives in security. The reality is that for casual message signatures, almost all missing or failed signatures are likely to have entirely innocent explanations. Or at least I think that this is the likely explanation today; perhaps mail gets attacked more often than I think on today's Internet.)

tech/MySignedMailDesire written at 01:42:10; Add Comment

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