Chromium extensions are inadvertently revealing a vulnerability that can be exploited for advanced browser fingerprinting, allowing the creation of a unique visitor identifier using the last modified timestamps of extension files in browsers like Google Chrome, Opera, and Microsoft Edge. The exploit involves reading the "Last-Modified" header of extension files, which updates whenever extensions are installed or automatically updated. This timestamp can serve as a strong browser identifier due to its uniqueness, posing a privacy risk by enabling cross-site tracking. The issue arises from a change made in 2022 that reintroduced the "Last-Modified" header for all files loaded from disk, which was initially intended to prevent the exposure of user installation times. The technique is significant because it can serve as a reliable third-party tracker, bypassing efforts by browser vendors to make APIs fingerprint-resistant. While the Google Docs Offline extension, installed by default in Google Chrome, exemplifies this vulnerability, the method is limited to desktop browsers, does not work in incognito mode, and varies across different browser profiles.