The Public Key Pinning Extension for HTTP (HPKP) is a security feature that tells a web client to associate a specific cryptographic public key with a certain web server to decrease the risk of MITM attacks with forged certificates.
To ensure the authenticity of a server's public key used in TLS sessions, this public key is wrapped into a X.509 certificate which is usually signed by a certificate authority (CA). Web clients such as browsers trust a lot of these CAs, which can all create certificates for arbitrary domain names. If an attacker is able to compromise a single CA, they can perform MITM attacks on various TLS connections. HPKP can circumvent this threat for the HTTPS protocol by telling the client which public key belongs to a certain web server.
HPKP is a Trust on First Use (TOFU) technique. The first time a web server tells a client via a special HTTP header which public keys belong to it, the client stores this information for a given period of time. When the client visits the server again, it expects at least one certificate in the certificate chain to contain a public key whose fingerprint is already known via HPKP. If the server delivers an unknown public key, the client should present a warning to the user.
Firefox and Chrome disable pin validation for pinned hosts whose validated certificate chain terminates at a user-defined trust anchor (rather than a built-in trust anchor). This means that for users who imported custom root certificates all pinning violations are ignored.
To enable this feature for your site, you need to return the
Public-Key-Pins HTTP header when your site is accessed over HTTPS:
Public-Key-Pins: pin-sha256="base64=="; max-age=expireTime [; includeSubDomains][; report-uri="reportURI"]
Note: The current specification requires including a second pin for a backup key which isn't yet used in production. This allows for changing the server's public key without breaking accessibility for clients that have already noted the pins. This is important for example when the former key gets compromised.
Note: While the example below shows how to set a pin on a server certificate, it is recommended to place the pin on the intermediate certificate of the CA that issued the server certificate, to ease certificates renewals and rotations.
First you need to extract the public key information from your certificate or key file and encode them using Base64.
The following commands will help you extract the Base64 encoded information from a key file, a certificate signing request, or a certificate.
openssl rsa -in my-rsa-key-file.key -outform der -pubout | openssl dgst -sha256 -binary | openssl enc -base64
openssl ec -in my-ecc-key-file.key -outform der -pubout | openssl dgst -sha256 -binary | openssl enc -base64
openssl req -in my-signing-request.csr -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
openssl x509 -in my-certificate.crt -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
The following command will extract the Base64 encoded information for a website.
openssl s_client -servername www.example.com -connect www.example.com:443 | openssl x509 -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
Public-Key-Pins: pin-sha256="cUPcTAZWKaASuYWhhneDttWpY3oBAkE3h2+soZS7sWs="; pin-sha256="M8HztCzM3elUxkcjR2S5P4hhyBNf6lHkmjAHKhpGPWE="; max-age=5184000; includeSubDomains; report-uri="https://www.example.org/hpkp-report"
In this example, pin-sha256="cUPcTAZWKaASuYWhhneDttWpY3oBAkE3h2+soZS7sWs=" pins the server's public key used in production. The second pin declaration pin-sha256="M8HztCzM3elUxkcjR2S5P4hhyBNf6lHkmjAHKhpGPWE=" also pins the backup key. max-age=5184000 tells the client to store this information for two months, which is a reasonable time limit according to the IETF RFC. This key pinning is also valid for all subdomains, which is told by the includeSubDomains declaration. Finally, report-uri="https://www.example.net/hpkp-report" explains where to report pin validation failures.
Instead of using a
Public-Key-Pins header you can also use a
Public-Key-Pins-Report-Only header. This header only sends reports to the
report-uri specified in the header and does still allow browsers to connect to the webserver even if the pinning is violated.
The concrete steps necessary to deliver the HPKP header depend on the web server you use.
Note: These examples use a max-age of two months and include all subdomains. It is advised to verify that this setup will work for your server.
HPKP has the potential to lock out users for a long time if used incorrectly! The use of backup certificates and/or pinning the CA certificate is recommended.
Adding a line similar to the following to your webserver's config will enable HPKP on your Apache. This requires
Header always set Public-Key-Pins "pin-sha256=\"base64+primary==\"; pin-sha256=\"base64+backup==\"; max-age=5184000; includeSubDomains"
Adding the following line and inserting the appropriate
pin-sha256="..." values will enable HPKP on your nginx. This requires the
add_header Public-Key-Pins 'pin-sha256="base64+primary=="; pin-sha256="base64+backup=="; max-age=5184000; includeSubDomains' always;
The following line with your relevant key information (pin-sha256="..." fields) will enable HPKP on lighttpd.
setenv.add-response-header = ( "Public-Key-Pins" => "pin-sha256=\"base64+primary==\"; pin-sha256=\"base64+backup==\"; max-age=5184000; includeSubDomains")
Note: This requires the
mod_setenv server.module loaded which can be included by the following if not already loaded.
server.modules += ( "mod_setenv" )
Add the following line to the Web.config file to send the
<system.webServer> ... <httpProtocol> <customHeaders> <add name="Public-Key-Pins" value="pin-sha256="base64+primary=="; pin-sha256="base64+backup=="; max-age=5184000; includeSubDomains" /> </customHeaders> </httpProtocol> ... </system.webServer>
|RFC 7469, section 2.1: Public-Key-Pins||Public Key Pinning Extension for HTTP|
|Feature||Android||Chrome for Android||Edge mobile||Firefox for Android||IE mobile||Opera Android||iOS Safari|
1. Under consideration for future release.
2. See Bugzilla bug 1091176.
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