Privacy Anti-Patterns in Standards

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The Web suffers from large scale, frequent, and often invisible privacy violations. These pervasive privacy problems threaten the Web's ability to serve as a preeminent application platform and information distribution system.

W3C's Privacy Interest Group (PING) has been seeing several patterns in web standards that are harmful to Web Privacy. Though each of these occur in many (many!) standards, we're intentionally not calling out or shaming any specific standards; instead, the goal is to demonstrate frequently recurring problems, to explain why they are harmful to the goal of a privacy-respecting Web, and to provide guidelines for how future standards can be designed to better respect privacy.

1. Strictly Defined Functionality, Loosely-Defined Protections

Many standards strictly describe functionality that browser vendors should implement, but are very loose or vague about the mitigations browsers should deploy to protect users from possible privacy harms.

In extreme cases, standards will have many pages of text describing, in precise language, how new functionality will work, followed by a paragraph or two saying, "implementers can vary from the standard at any time to protect user privacy."

While well meaning, patterns like the above (strictly defined functionality, loosely-defined protections) are harmful to Web privacy for several reasons.

First, though it might seem that weakly/broadly-defined mitigations maximize implementer flexibility in protecting user privacy, the opposite ends up being true; weakly defined mitigations make it extremely difficult to protect user privacy; websites end up assuming the presence of defined functionality, and privacy-oriented implementers end up with unmanageable compatibility problems if they alter the behavior of these privacy-harming, but expected features.

Second, this pattern boils down to a "punt" on dealing with the privacy risks of the new functionality. In some cases this is because future standards are expected to address the current harm (bad!), in other cases because there is no commonly agreed-to solution for dealing with the harm (bad!), and in yet other cases it is because the standards authors believe the benefits of the new functionality outweigh the privacy harms. In all cases, the end result is additional privacy risk to a platform already struggling with tremendous privacy problems.


Standards should define privacy protections as carefully and precisely as they define new functionality. Once functionality hits the Web, it is extremely difficult to change its behavior or roll it back. Protections need to be specific, as mandatory as the defined behavior, and defined from day one to protect Web privacy.

2. Uncommon Use Cases, Global Availability

A second privacy-harming pattern we see in Web standards is the over-availability of new functionality; powerful new functionality aimed at niche use-cases, being made globally available (e.g. third-party code, third-party frames, without permission prompt, user gesture, or user notification, etc). Many of these powerful new browser capabilities end up being beneficial to users on only a small number of websites, but leveraged for passive fingerprinting on a large number of websites.

This pattern of leveraging powerful-but-rarely-used functionality for user identification (instead of its intended, user-serving purposes) can be observed in popular user-tracking (e.g. privacy violating) tools and the changes privacy-focused implementers need to make to keep their users private.

We emphasize that we don't think that this new functionality isn't useful; it is! We only mean to highlight that it's also dangerous to privacy, and should be treated as such.


Websites should not globally be able to access rarely needed functionality, functionality aimed at very specific, uncommon use cases. Instead, websites should be only be allowed to use powerful new features when accessed from privileged positions (e.g. first-party code running in a first party context, but not third-party frames, etc.) or when users have given a clear signal that they desire the additional functionality (e.g. a permission prompt, a user gesture, etc.).

The specific "gating" mechanisms will vary from case to case (and thus is beyond the scope of this blog post), but almost any "gating" is better than global access. Increasing the power of the Web, while improving user privacy, requires keeping rarely needed functionality rarely available.

3. "No Worse Than The Status Quo"

A third privacy-harming anti-pattern in Web standards concerns the bar standards authors use when introducing new privacy-risking functionality. Frequently standards authors start from the position that new standards should not make Web privacy worse than its current (poor) state. As a result, standards are created that replicate existing privacy harms, through new end points or locations. This is extremely harmful for Web privacy.

Even just replicating sensitive, or user identifying, information in new locations makes future mitigation more difficult. Instead of needing to fix or remove one privacy harming feature from the Web (already extremely difficult), browser vendors need to untangle twice as many expected features from the Web. Just replicating existing fingerprinting surface amounts to further technical debt that we in the privacy community need to pay down.


New standards should treat all additional privacy risk as problematic. The appropriate measure for new standards isn't "marginal increase in privacy risk"; it is "could the functionality in the standard be used to harm user privacy." Put differently, when you find you're digging yourself into a hole, the first thing to do is to stop digging. The Web is already rife with difficult privacy problems, new standards should not entrench the problem further.

The Importance of Privacy in Standards

The goal in this post is to highlight the critical role standards play in protecting privacy on the Web, and thus confidence in the Web platform generally. Web standards can either help build a "private by default" platform we can be proud of, or Web standards can normalize privacy-harming functionality and hope that implementers will mutate, violate and deviate from those standards to keep users protected.

The three anti-patterns described play a significant part in how the Web wound up in its current problematic, frequently privacy-harming state. Fixing these anti-patterns is a necessary part of building a humane, privacy respecting Web.

What is the W3C Privacy Interest Group (aka PING)

W3C's Privacy Interest Group has the mission of improving privacy in Web standards so that Web users can be more confident that using the Web won't put their privacy at risk.

PING tries to improve web privacy in several ways, including:

    • reviewing newly proposed standards to remove, modify or restrict privacy threatening features.
    • revising existing standards to improve the privacy-properties of existing functionality.
    • developing and sharing guidelines for standards authors, so that future additions to the Web platform can be designed to be private-by-default.

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