Using CNAMEs to bypass ITP has been put to Torch

June 8, 2022

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Browsers are mobilizing to combat the use of DNS CNAME records to bypass anti-tracking tech they have built in to their browsers. November is looking to be a triple whammy of developments in this area.

iOS and Safari

I first wrote about this in September, and now the time is here. On November 5th 2020 Apple released iOS 14.2 and iPad 14.2. With these updates now in play, any device running these new versions will have CNAME Cloaking Mitigation enabled by default. This change was not listed in any patch notes and as of the time of writing Apple has also not updated Webkit’s blog to announce the change.

Safari vs iOS (present state)

An example to show what I am talking about may be in order.

Using Mac OS Safari – Let’s look at Apple.com which based on their Network traffic you can tell is leveraging Adobe’s CNAME solution by calling their subdomain securemetrics.apple.com

If we run a DNS record lookup on this subdomain we can see it points to appleglobal.102.112.2o7.net. The 2o7.net domain is used by Adobe for Adobe Analytics data collection.

How this works is JavaScript on the page would call (in this case) securemetrics.apple.com which forwards the request to appleglobal.102.112.2o7.net. which would proceed to set a s_vi cookie (containing the Adobe Analytics visitor ID) for a period of two years via the response header.

Response Header:

set-cookie: s_vi=[CS]v1|2EAFF145852CC4DD-400009B360002651[CE]; Path=/; Domain=apple.com; Max-Age=63072000; Expires=Sun, 06 Nov 2022 10:07:07 GMT; SameSite=None; Secure

Then if we look at the cookies set by the domain we can see that cookie does indeed last for two years.

domain cookie settings

iOS

Now let’s see how this is different on the recent iOS upgrade.

Using Mobile Safari we repeat the above steps and use a Macbook to see what is going on. So we navigate to apple.com and call securemetrics.apple.com

We can see that indeed, the response header says the s_vi cookie should last for two years. So we move on to check the cookie storage.

screenshot s_vi cookie

We can see indeed that the cookie’s duration is lowered to 7 days. This is a sizable change as many vendors (not just Adobe) use CNAME redirection for a number of reasons (not just tracking). The Webkit feature does not care, however, and only looks at if apple.com is the same as 2o7.net, which it clearly isn’t – and so applies the restrictions.

This means as adoption of iOS 14.2 and iPad 14.2 continue to rise – this will become a common scenario. Since Webkit is the foundation of all browsers on iOS, I strongly suspect that it will apply to all browsers on the platform. I have reached out to Apple and will update if they confirm this is not the case.

In North America iOS accounts for 53% of all mobile web traffic. This should become noticeable in data pulls over the next few weeks for any site with notable iPhone / iPad traffic.

What about Mac OS?

The change is present in Safari builds for the upcoming Mac OS Big Sur. At the time of writing, Apple has an event planned for November 10th, and so details about the release date could be announced at that time. I fully expect it to be present in Safari on the new operating system version.

Brave

Brave announced on November 17th their browser will also combat CNAME cloaking in a different manner. Rather than just lower the duration of cookies set via this method, Brave will block the request entirely.

So what’s this mean?

The door has closed on using CNAMEs to set cookies and expecting them to have long durations. This is a change in web architecture that developers need to now account for.

From the business point of view, any service using CNAMEs for cookie setting will either see that traffic disappear entirely (Brave) or have sizable reductions in lookback windows (iOS/iPadOS/Safari Big Sur).

This includes (but is not limited to) 7 days of lookback window for campaign measurement, possible increase in new users, possible loss of retention identification, and possible re-segmentation of A/B testing cell assignments in scenarios where CNAME was used to set cookies.

What can be done?

It’s going to be interesting.

The clearest answer is transitioning from CNAME usage to use A / AAAA DNS records, which may be easier said than done. Otherwise, you’d need to leverage some sort of server cookie issuance/persistence layer. Both of these can be tricky as I am sure this change is going to blindside a number of vendors.

The best advice I have in scenarios where you were using CNAME for Analytics or Measurement, is to reach out to the vendor(s) involved and see what they say then discuss the answers with your dev team/agency.

Since many brands are now going into code freeze for the shopping season, this is likely to be exceptionally rough on the analytics front as adoption of the upgrade takes hold. The only advice I have for that is to keep this in mind when analyzing or reporting results.

Originally published November 6, 2020. For Cory Underwood’s original post, click here.

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Cory is presently an Analytics Engineer for Search Discovery. Prior to his current role, he had roles in the retail industry as a Technical Lead for Analytics, Optimization, Personalization, and Tag Implementation. In recent years, he’s presented on the topic of browser privacy in a number of podcasts, webinars and conferences. Cory’s blog (cunderwood.dev) focuses on A/B Testing, Personalization, Privacy, and Analytics—his mission is to keep the larger analytics community informed of technical changes imposed by outside forces that could impact their implementations and reporting. You can find him engaging the community on the topics above in the Measure.chat.

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