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I have read many times that you shouldn't use additional add-ons on Tor Browser because it makes your browser easier to identify (more susceptible to fingerprinting).

But what if every Tor Browser Bundle installation came with uBlock Origin, uMatrix, and Decentraleyes by default instead of just individual people installing add-ons? Wouldn't that also make fingerprinting harder if so many people have the same configuration?

Or what am I missing?

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You are correct, if everyone had these plugins then everyone would look the same.

However, let me share a couple of quotes:

From the FAQ:

Some people have suggested we include ad-blocking software or anti-tracking software with Tor Browser. Right now, we do not think that's such a good idea. Tor Browser aims to provide sufficient privacy that additional add-ons to stop ads and trackers are not necessary. Using add-ons like these may cause some sites to break, which we don't want to do. Additionally, maintaining a list of "bad" sites that should be black-listed provides another opportunity to uniquely fingerprint users.

From the The Design and Implementation of the Tor Browser document:

No filters Site-specific or filter-based addons such as AdBlock Plus, Request Policy, Ghostery, Priv3, and Sharemenot are to be avoided. We believe that these addons do not add any real privacy to a proper implementation of the above privacy requirements, and that development efforts should be focused on general solutions that prevent tracking by all third parties, rather than a list of specific URLs or hosts.

Implementing filter-based blocking directly into the browser, such as done with Firefox' Tracking Protection, does not alleviate the concerns mentioned in the previous paragraph. There is still just a list containing specific URLs and hosts which, in this case, are assembled by Disconnect and adapted by Mozilla.

Trying to resort to filter methods based on machine learning does not solve the problem either: they don't provide a general solution to the tracking problem as they are working probabilistically. Even with a precision rate at 99% and a false positive rate at 0.1% trackers would be missed and sites would be wrongly blocked.

Filter-based solutions in general can also introduce strange breakage and cause usability nightmares. For instance, there is a trend to observe that websites start detecting filer extensions and block access to content on them. Coping with this fallout easily leads to just whitelisting the affected domains, hoping that this helps, defeating the purpose of the filter in the first place. Filters will also fail to do their job if an adversary simply registers a new domain or creates a new URL path. Worse still, the unique filter sets that each user creates or installs will provide a wealth of fingerprinting targets.

As a general matter, we are also generally opposed to shipping an always-on Ad blocker with Tor Browser. We feel that this would damage our credibility in terms of demonstrating that we are providing privacy through a sound design alone, as well as damage the acceptance of Tor users by sites that support themselves through advertising revenue.

Users are free to install these addons if they wish, but doing so is not recommended, as it will alter the browser request fingerprint.

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