看到「Exploiting custom protocol handlers for cross-browser tracking in Tor, Safari, Chrome and Firefox」這個方式，跨瀏覽器收集 fingerprint 追蹤。
這次用的方式是透過 handler 追：
The scheme flooding vulnerability allows an attacker to determine which applications you have installed. In order to generate a 32-bit cross-browser device identifier, a website can test a list of 32 popular applications and check if each is installed or not. On average, the identification process takes a few seconds and works across desktop Windows, Mac and Linux operating systems.
最近大家比較常使用到的應該就是 Zoom 從網頁把應用程式帶起來的方式：
而要怎麼偵測的部份，用到了不同瀏覽器的 side channel。
Chromium 系列的部份對應的 ticket 在「Issue 1096610: External Protocol handler anti-flood protection is ineffective and flaky」這邊有被提出來。主要用到的方法是，在遇到有 handler 時，連打兩次時會被擋下：
被擋下後再打都會失敗，所以需要一個方式重設 flag，而內建的 Chrome PDF Viewer 剛好可以重設 flag：
The built-in Chrome PDF Viewer is an extension, so every time your browser opens a PDF file it resets the scheme flood protection flag. Opening a PDF file before opening a custom URL makes the exploit functional.
在 Firefox 的 side channel 則是可以透過 same-origin policy 測試當作 side channel，對應的 ticket 在「Scheme flooding technique for reliable cross-browser fingerprinting」這邊：
Every time you navigate to an unknown URL scheme, Firefox will show you an internal page with an error. This internal page has a different origin than any other website, so it is impossible to access it because of the Same-origin policy limitation. On the other hand, a known custom URL scheme will be opened as about:blank, whose origin will be accessible from the current website.
在 Safari 上的問題與 Firefox 一樣，不過沒登入看不到 ticket (也懶的註冊了)：
You are not authorized to access bug #225769. To see this bug, you must first log in to an account with the appropriate permissions.
另外，雖然 Tor Browser 底層是 Firefox，但因為有改變預設值，所以攻擊者也得換方法：
Tor Browser is based on the Firefox source code, so the Same-origin policy trick was used here as well. But because Tor Browser does not show pop-ups, we used the same-origin policy trick with iframe elements instead.
這個方法還蠻暴力的...
