Public-key pinning has emerged as an important defense against a variety of attacks, especially man-in-the-middle attacks and the issuance of fraudulent certificates. In the last few years Google, Mozilla and other organizations have discovered several cases of attackers using fraudulent certificates for high-value sites, including Gmail. The function essentially ties a public key, or set of keys, issued by known-good certificate authorities to a given domain. So if a user’s browser encounters a site that’s presenting a certificate that isn’t included in the set of pinned public keys for that domain, it will then reject the connection. The idea is to prevent attackers from using fake certificates in order to intercept secure traffic between a user and the target site.
The first pinset will include all of the sites in the Chromium pinset used by Chrome, along with Mozilla sites and high-value sites such as Facebook. Later versions will add pins for Twitter, a long list of Google domains, Tor, Dropbox and other major sites."