The Stylish add-on and the linked userstyles.org repository for website styles changed ownership twice in the past years. The original owner sold Stylish back in October 2016 and the new owners of Stylish sold it again to the current owner SimilarWeb.
Stylish is a handy extension that you can use to apply custom CSS to websites. You may use it to change colors, remove elements, or add elements to sites to adjust them to your liking. You can remove advertisement, the comment section on YouTube, or turn Google Search's white background into something more eye pleasing.
The switch to the new owner of Stylish had massive privacy implications. SimilarWeb is known for its analytics offerings and it appears that the company collects browsing data from Stylish.
Users who install Stylish are automatically opted-in to sending anonymous data to Stylish. Stylish does include an option to opt-out of that in the extension options.
Robert Heaton analyzed Stylish's data collecting recently and discovered that the extension sends a user's complete browsing history back to Stylish servers. The data is linked to a unique identifier so that all of a user's browsing history can be linked together.
It is even worse for users who have an account on userstyles.org, a property owned by SimilarWeb, as Similarweb could link accounts to the browsing history.
Even if that is not the case, it is problematic even if SimilarWeb claims that it collects anonymized data only. One of the cases where this is problematic is when sites add information to the URL directly. Heaton mentions URLs that contain profile names, tokens, and URLs that use obscurity to protect data from third-parties.
Stylish makes a number of connections to api.userstyles.org whenever you connect to web resources. While you could think that this is done to return existing userstyles for these web resources, Stylish does transmit more information than it needs to for that functionality.
Heaton discovered that Stylish was transmitting obfuscated data to the userstyles address. He managed to decrypt it to find out that Stylish was submitting all browsing data to company servers. In other words, Stylish submits the full URL of any site you open in the browser the extension is installed in and Google search results as well.
From the Stylish desktop browser extension:
Standard web server log information (i.e., web request) as well as data sent in response to that request, such as URL used, Internet Protocol address (trimmed and hashed for anonymization), TabID, HTTP referrer, and user agent; and
Search engine results page data (keyword, order/index of results, links of results, title, description, and ads displayed).
From the Stylish mobile app:
Standard web server log information (i.e., web request) as well as data sent in response to that request, such as URL used, Internet Protocol address (trimmed and hashed for anonymization), HTTP referrer, and user agent;
Search engine results page data (keyword, order/index of results, links of results, title, description, and ads displayed);
Device ID (anonymized and/or de-identified using irreversible encryption and/or hashing);
Browser type, operating system and Mobile Network Code;
Device model name, device screen size and whether the device is rooted;
All web connections; and
Information regarding installed applications and their use (names, app IDs, versions of installed apps, installation and update dates, whether they are system apps, which apps are used, duration of use, whether the apps are on the home page);
If you use Stylish, at the very least disable the collecting of data in the extension settings.
We reviewed Stylus in 2017 which is a fork of Stylish that does not include the analytics component. You can install the extension and use it to load userstyles.
You may also use Chrome's overrides tool to make permanent changes directly to websites.
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