Mozilla slams a big Safari privacy feature, calling it ‘a poor trade-off’


Iphone 13 Mini Safari HeroSource: Joseph Keller / iMore

Mozilla, the company behind the popular Firefox web browser, has slammed a Safari privacy feature and dubbed it “a poor trade-off between user privacy” and the functionality needed by web advertisers.

Apple’s Private Click Measurement (PCM) feature was introduced last year and is designed to allow websites and ad companies to keep track of clicks and conversion without giving them unfettered access to all of the data that can also be used to track people from one website to another. But Mozilla says that the way in which it does that causes its own problems that don’t necessarily prevent tracking while making it more difficult for advertisers to get the information they need.

PCM is already available as part of iOS 15, but it needs websites to use its API in order to function. Mozilla explains how PCM is supposed to work in Safari right now:

The goal of PCM is to support conversion measurement through a new in-browser API. Conversion measurement aims to measure how effective advertising is by observing which advertisements lead to sales or other outcomes like page views and sign-ups. PCM measures the most direct kind of conversion, where a user clicks an ad on one site then later takes an action on the advertiser’s site, like buying a product.

The way that PCM works is that the browser records when clicks and conversions occur. When a click is followed by a conversion, the browser creates a report. PCM aims to safeguard privacy by strictly limiting the information that is included in the report and by submitting reports to the websites through an anonymization service after a delay.

As fine as that sounds, an in-depth report claims that the mechanisms used by PCM aren’t sufficient to protect users in the ways Apple is trying to, while making it more difficult for advertisers and removing any incentive for web publishers to use it.

  • Although PCM prevents sites from performing mass tracking, it still allows them to track a small number of users.
    -The measurement capabilities PCM provides are limited relative to the practices that advertisers currently employ, with long delays and too few identifiers for campaigns being the most obvious of the shortcomings.

In fact, Mozilla goes so far as to say that if Firefox included support for PCM in lieu of its own Toral Cookie Protection, it would actually make it less private.

If Firefox implemented PCM, it would enable a new way to perform cross-site tracking. While some sites might choose to use PCM as intended, nothing in the design of PCM prevents sites from using it for tracking.

Mozilla includes various examples of why it believes PCM isn’t all it’s cracked up to be and the longer report is well worth reading.

Finally, Mozilla wraps its blog post up with a damning sentence that sums things up pretty succinctly:

Overall, the design choices in PCM that aim to safeguard privacy provide insufficient privacy protections, but they appear to make the API less useful for measurement.

Apple’s privacy stance has long been a strong one and Safari is one area where it continues to try to ensure advertisers and data brokers are unable to follow us around the internet. Apple would likely argue that preventing ad companies from tracking conversion as well as they would like is a trade-off it is willing to make, but web publishers won’t implement its PCM API if they don’t have to — and that trade-off could well mean that they won’t.





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