HTTP Public Key Pinning (HPKP) is an obsolete
Internet security mechanism delivered via an
HTTP header which allows
HTTPS websites to resist
impersonation by attackers using misissued or otherwise fraudulent
digital certificates.
A server uses it to deliver to the
client (e.g.
web browser) a set of hashes of
public keys that must appear in the certificate chain of future connections to the same
domain name.
For example, attackers might compromise a
certificate authority, and then mis-issue certificates for a
web origin. To combat this risk, the HTTPS web server serves a list of “pinned” public key hashes valid for a given time; on subsequent connections, during that validity time, clients expect the server to use one or more of those public keys in its certificate chain. If it does not, an error message is shown, which cannot be (easily) bypassed by the user.
The technique does not pin certificates, but
public key hashes. This means that one can use the
key pair to get a certificate from any certificate authority, when one has access to the private key. Also the user can pin public keys of
root or
intermediate certificates (created by certificate authorities), restricting site to certificates issued by the said certificate authority.
Due to HPKP mechanism complexity and possibility of accidental misuse, browsers deprecated and removed HPKP support in favor of
Certificate Transparency and its
Expect-CT
header.
Certificate Authority Authorization can also be used to restrict which certificate authorities can issue certificates for a particular domain, thus reducing chance of certificate issue errors.
Mechanism
The server communicates the HPKP policy to the user agent via an
HTTP response header field named
Public-Key-Pins
(or
Public-Key-Pins-Report-Only
for reporting-only purposes).
The HPKP policy specifies
hashes of the subject public key info of one of the certificates in the website's authentic X.509
public key certificate
In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a public key. The certificate includes information about the key, information about the ...
chain (and at least one backup key) in
pin-sha256
directives, and a period of time during which the user agent shall enforce public key pinning in
max-age
directive, optional
includeSubDomains
directive to include all subdomains (of the domain that sent the header) in pinning policy and optional
report-uri
directive with URL where to send pinning violation reports. At least one of the public keys of the certificates in the certificate chain needs to match a pinned public key in order for the chain to be considered valid by the user agent.
At the time of publishing, only allowed the
SHA-256 hash algorithm.
Appendix A. of RFC 7469mentions some tools and required arguments that can be used to produce hashes for HPKP policies.)
A website operator can choose to either pin the
root certificate public key of a particular root certificate authority, allowing only that certificate authority (and all intermediate authorities signed by its key) to issue valid certificates for the website's domain, and/or to pin the key(s) of one or more intermediate issuing certificates, or to pin the end-entity public key.
At least one backup key must be pinned, in case the current pinned key needs to be replaced. The HPKP is not valid without this backup key (a backup key is defined as a public key not present in the current certificate chain).
HPKP is standardized in
RFC
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Computing
* Request for Comments, a memorandum on Internet standards
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* Rhye's and Fall of Civilization, a modification for Sid Meier's Civ ...
7469.
It expands on static
certificate pinning, which hardcodes public key hashes of well-known websites or services within web browsers and applications.
Most browsers disable pinning for
certificate chains with private
root certificates to enable various corporate
content inspection scanners and web debugging tools (such as
mitmproxy or
Fiddler).
The RFC 7469 standard recommends disabling pinning violation reports for "user-defined" root certificates, where it is "acceptable" for the browser to disable pin validation.
Reporting
If the user agent performs pin validation and fails to find a valid SPKI fingerprint in the served certificate chain, it will POST a JSON formatted
violation report to the host specified in the
report-uri directive containing details of the violation. This URI may be served via
HTTP or
HTTPS; however, the user agent cannot send HPKP violation reports to an HTTPS URI in the same domain as the domain for which it is reporting the violation. Hosts may either use HTTP for the
report-uri
, use an alternative domain, or use a reporting service.
Some browsers also support the
Public-Key-Pins-Report-Only
, which only triggers this reporting while not showing an error to the user.
Criticism and decline
During its peak adaptation, HPKP was reported to be used by 3,500 of top 1 million internet sites, a figure that declined to 650 around the end of 2019.
Criticism and concern revolved around malicious or human error scenarios known as HPKP Suicide and RansomPKP.
In such scenarios, a website owner would have their ability to publish new contents to their domain severely hampered by either losing access to their own keys or having new keys announced by a malicious attacker.
Browser support and deprecation
See also
*
Certificate authority compromise
*
Certificate Transparency
*
HTTP Strict Transport Security
*
List of HTTP header fields
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*
DNS Certification Authority Authorization
DNS Certification Authority Authorization (CAA) is an Internet security policy mechanism that allows domain name holders to indicate to certificate authorities whether they are authorized to issue digital certificates for a particular domain n ...
Public Key Pinning Extension for HTTP (HPKP)on
MDN Web Docs
References
{{SSL/TLS
Hypertext Transfer Protocol headers
Web security exploits
Transport Layer Security