SMTP MTA Strict Transport Security 算是 SMTP STARTTLS 裡的 HSTS 機制，而 Google 的人在 RSA Conference 上提出要開始用了：「SMTP STS Coming Soon to Gmail, Other Webmail Providers」。
Elie Bursztein, the head of Google’s anti-abuse research team, said at RSA Conference that SMTP STS will be a major impediment to man-in-the-middle attacks that rely on rogue certificates that are likely forged, stolen or otherwise untrusted. Google, Microsoft, Yahoo and Comcast are expected to adopt the standard this year, a draft of which was submitted to the IETF in March 2016.
補上去後對於 SMTP 的隱私保護就會更好了...
看到「sniffly」這個工具，可以利用 HSTS 資訊檢測逛過哪些網站，程式碼在「diracdeltas/sniffly」這邊可以找到：
Sniffly is an attack that abuses HTTP Strict Transport Security and Content Security Policy to allow arbitrary websites to sniff a user's browsing history. It has been tested in Firefox and Chrome.
測試網站則可以在這邊看到，作者拿 Alexa 上的資料網站來掃，所以熱門網站應該都會被放進去...
主要是利用 HSTS + CSP policy 的 timing attack (有逛過網站而瀏覽器裡有 HSTS 時的 redirect 會比較快，沒有逛過的時候會因為有網路連線而比較慢)：
Sniffly sets a CSP policy that restricts images to HTTP, so image sources are blocked before they are redirected to HTTPS. This is crucial! If the browser completes a request to the HTTPS site, then it will receive the HSTS pin, and the attack will no longer work when the user visits Sniffly.
When an image gets blocked by CSP, its onerror handler is called. In this case, the onerror handler does some fancy tricks to time how long it took for the image to be redirected from HTTP to HTTPS. If this time is on the order of a millisecond, it was an HSTS redirect (no network request was made), which means the user has visited the image's domain before. If it's on the order of 100 milliseconds, then a network request probably occurred, meaning that the user hasn't visited the image's domain.
由於這個技巧，HTTPS Everywhere 必須關閉才會比較準確。
還是在 Zite 上看到的，對最大的一百萬個網站分析與安全有關的 HTTP Header：「Security Headers on the Top 1,000,000 Websites: November 2013 Report」。
數字大致上都有增加，不過對我來說的重點在於有列出所有與安全有關的 HTTP Header...
剛好可以拿來 review 設定...