Safari bug allows websites to track browsing activity and unique identifiers

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Researchers have discovered a bug in Apple’s Safari browser that allows websites to track a user’s browsing activities on other sites.

The bug, discovered by browser fingerprinting service FingerprintJS, also exposes a user’s unique identifier for certain websites to other sites they visit.

The flaw, found in Apple’s WebKit browser engine, affects Safari 15 on macOS and all browsers on iOS and iPadOS 15. It resides in WebKit’s implementation of the indexed database API, commonly known as IndexedDB , a JavaScript API that browsers use to access a database of objects, and it frequently stores data generated when interacting with a web application. This includes a user’s unique identifier for interacting with web applications, such as their Google ID.

When implemented correctly, IndexedDB follows the co-origin principle. This ensures that information stored from a webpage is only available to webpages in the same domain. It prevents overly curious web pages from accessing another domain’s stored information, which could include sensitive user or session data.

FingerprintJS found that WebKit’s IndexedDB implementation violates the same-origin principle, instead making stored information available to websites in other domains.

FingerprintJS called the bug a privacy breach. “It allows arbitrary websites to learn which websites the user visits in different tabs or windows,” the company said in its analysis of the bug. “This is possible because database names are usually unique and website-specific.”

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The company has found some websites using user-specific IndexedDB data, such as ID numbers in their IndexedDB database names, which makes it easy for any other website to find the ID of a user on other sites. Using this identifier to search user assets (such as profile pictures) could lead to the identification of the user, the company warned. Google websites store ID numbers this way, allowing other sites to harvest Google IDs using the bug.

The bug affects all browsers on iOS 15 because Apple mandates the use of WebKit on this platform in its developer guidelines. Section 2.5.6 states that “applications that browse the web must use the appropriate WebKit framework and WebKit Javascript”.

FingerprintJS said it notified Apple of this bug on November 28, but Apple did not fix it. Apple engineers started creating a fix on Sunday, February 17, the day FingerprintJS released the bug details.

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