Chrome’s key pinning feature is a strong form of web site authentication that requires a web server’s certificate chain not only to be valid and to chain to a known-good trust anchor, but also that at least one of the public keys in the certificate chain is known to be valid for the particular site the user is visiting. This is a good defense against the risk that any trust anchor can authenticate any web site, even if not intended by the site owner: if an otherwise-valid chain does not include a known pinned key (“pin”), Chrome will reject it because it was not issued in accordance with the site operator’s expectations.

Chrome does not perform pin validation when the certificate chain chains up to a private trust anchor. A key result of this policy is that private trust anchors can be used to proxy (or MITM) connections, even to pinned sites. “Data loss prevention” appliances, firewalls, content filters, and malware can use this feature to defeat the protections of key pinning.

We deem this acceptable because the proxy or MITM can only be effective if the client machine has already been configured to trust the proxy’s issuing certificate — that is, the client is already under the control of the person who controls the proxy (e.g. the enterprise’s IT administrator). If the client does not trust the private trust anchor, the proxy’s attempt to mediate the connection will fail as it should.

4.7 Pre-loaded/HPKPの併用

仕様では実装依存になっていますが、 Chrome は HPKPを先にチェックするようになっています。

Public-Key-Pins:max-age=5184000; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho="; pin-sha256="k2v657xBsOVe1PQRwOsHsw3bsGT2VzIqz5K+59sNQws="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="IQBnNBEiFuhj+8x6X8XLgh01V9Ic5/V3IRQLNFFc7v4="; pin-sha256="iie1VXtL7HzAMF+/PVPR9xzT80kQxdZeJ+zduCB3uj0="; pin-sha256="LvRiGEjRqfzurezaWuj8Wie2gyHMrW5Q06LspMnox7A="; includeSubDomains

// If mHostname isn't set, we're not verifying in the context of a TLS

// handshake, so don't verify HPKP in those cases.

A DNS client system is configured with an out-of-band key-pinned

privacy profile from a network service, using a pin set containing

two pins. Represented in HTTP Public Key Pinning (HPKP) [RFC7469]

style, the pins are:

It has very low adoption, and although it provides security against certificate mis-issuance, it also creates risks of denial of service and hostile pinning.

To defend against certificate misissuance, web developers should use the Expect-CT header, including its reporting function. Expect-CT is safer than HPKP due to the flexibility it gives site operators to recover from configuration errors, and due to the built-in support offered by a number of certificate authorities.

We expect to remove this in Chrome 69.