Protocol-relative URLs in email

When you link to an external resource – an image, a javascript file, some css style – from a web page you do so with a URL, usually something like “https://example.com/blahblah.css” or “http://example.com/blahblah.css”.
The world is beginning to go all https, all the time, but until recently good practice was to make a web page available via both http and https.
The problem is that if you try and load a resource from an http URL from a page that was loaded via https it’ll complain about it, and not load the resource.
And if your users web browser is loading the http version of your page because it’s Internet Explorer 6 and doesn’t speak modern SSL then it’ll be unable to load anything over https, including any of your resources.
So whether you choose https or http protocol for loading your page resources it’s going to break for someone.
One common trick to avoid the problem is to use a protocol-relative URL. That looks like “//example.com/blahblah.css”, and it’ll load the resource over the same protocol that the page was loaded over.
While we can safely use “https://…” everywhere now, “//…” URLs are still a common idiom for things like loading things like CSS libraries from public content-distribution networks as well as your own resources.
I was reading long-but-excellent writeup about Stack Overflow’s migration to TLS (hey, I read this sort of stuff for fun) and they point out something I hadn’t considered – mail clients don’t really have any sensible way to use protocol-relative URLs. The mail client loaded the “page” from a mailbox and so has no base document protocol to work from (even if there’s a <base> element in the content it’s not likely to affect a mail client). If the user is reading it via webmail or a mail client that’s using an embedded web browser to render HTML it might work, sometimes, but it’s not going to reliably load that resource in general.
So, if you’re copy-pasting content from your web collateral to reuse in an email, make sure you’re not loading anything external – including images – via a “//…” style URL. Rewrite them to use “https://…”.
 

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Clickthrough forensics

When you click on a link in your mail, where does it go? Are you sure?
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In most bulk mail sent the links in the mail aren’t the same as the page the recipients browser ends up at when they click on it. Instead, the link in the mail goes to a “click tracker” run by the ESP that records that that recipient clicked on this link in this email, then redirects the recipients web browser to the link the mail’s author wanted. That’s how you get the reports on how many unique users clicked through on a campaign.
In the pay-per-click business that’s often still not the final destination, and the users browser may get redirected through several brokers before ending up at the final destination. I walked through some of this a few years ago, including how to follow link redirection by hand.
HTTP Forensics
Evil spammers sometimes deploy countermeasures against that approach, though – having links that will only work once or twice, or redirects that must be followed within a certain time, or javascript within an intermediate page or any of a bunch of other evasions. For those you need something that behaves more like a web browser.
For serious forensics I might use something like wireshark to passively record all the traffic while I interact with a link from inside a sandboxed browser. That’s not terribly user-friendly to use or set up, though, and usually overkill. It’s simpler and usually good enough to use a proxy to record the web traffic from the browser. There are all sorts of web proxies, used for many different things. What they have in common is that you configure a web browser to talk to a proxy and it’ll send all requests to the proxy instead of to the actual website, allowing the proxy to make any changes it wants as it forwards the requests on and the results back.
For investigating what a browser is doing the most useful proxies are those aimed at either web developers debugging web apps or crackers penetration testers compromising web apps. Some examples are Fiddler (Windows), Cellist (OS X, commercial), mitmdump (OS X, linux, Windows with a little work), Charles (anything, commercial) or ZAP (anything).
I’m going to use mitmdump and Firefox. You don’t want to use your main browser for this, as the proxy will record everything you do in that browser while you have it configured – and I want to keep writing this post in Safari as I work.

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Protect your email with TLS

You probably use TLS hundreds of times a day. If you don’t recognize the term, you might know it better by it’s older name, SSL.
TLS is what protects your data in transit whenever you go to Google, or Yahoo or even this blog. The little padlock in your browser address bar tells you that your browser has used the TLS protocol to do two things. First, it’s decided that the server you’re connecting to really is operated by Google, or Yahoo or us – you’re (probably) not having your session intercepted by someone in the middle between you and the webserver, either to read your traffic or modify it en-route. Second, it is encrypting all the traffic between you and the webserver, so that it can’t be passively monitored while in transit. Because of concerns about ubiquitous surveillance many websites – including ours – are moving to use TLS for everything, not just for protecting a login page or a credit card number.
That’s great for the web, but how does it apply to email? One place it’s used is for connections between your mail client and your local mailserver – sending mail to the smarthost via [rfc 4409]SUBMIT[/rfc] and fetching mail using [rfc 2595]IMAP or POP3[/rfc] almost always use TLS. That protects the privacy of your messages between you and your ISP and also protects the username and password you use to authenticate with.
Mail traveling between ISPs didn’t used to be encrypted “on the wire” , but about 15 years ago [rfc 3207]an extension to SMTP was proposed[/rfc] that would allow ISPs to negotiate during each session whether they should encrypt it or not. This extension, often referred to as STARTTLS after the command it uses, allows gradual rollout of encryption of mail traffic between ISPs without requiring any sort of flag day. A mailserver that supports STARTTLS will tell everyone who connects to it “Hey! I support STARTTLS!”. When a smarthost that also supports it connects to that mailserver it will go “Great! I support STARTTLS too! Lets do this!” and convert the plain text SMTP session into an encrypted session protected by TLS.
Fifteen years seems like a long period in Internet time, but non-intrusive protocol changes can take a long time to deploy. Facebook Engineering have done the work to see how that deployment is going with their survey of the current state of SMTP STARTTLS deployment. The results are really quite positive – over three quarters of the mailservers they sent mail to supported STARTTLS, covering nearly 60% of their users. That’s definitely enough to make supporting STARTTLS worthwhile.
More about TLS and encryption tomorrow.

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Our green bar certificate is going away

Later today we’ll be switching from an Extended Validation (“green bar”) SSL certificate to a Domain Validation certificate. This isn’t exactly a planned change but I’m waiting for responses from Comodo before I go into it too much. I’ll share some more details next week.

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