content/browser/webauth/client_data_json.md - chromium/src - Git at Google

 # Advice to sites regarding `clientDataJSON`

 When performing a registration or authentication with webauthn, it's critical that sites confirm that the returned [`clientDataJSON`](https://w3c.github.io/webauthn/#dom-authenticatorresponse-clientdatajson) contains the [challenge](https://w3c.github.io/webauthn/#cryptographic-challenges) originally provided. Otherwise old messages can be replayed. Likewise, sites must check the `origin` member to confirm that the action is not being proxied by another site.

 In order to implement this, sites should parse the JSON as a [`CollectedClientData`](https://w3c.github.io/webauthn/#dictdef-collectedclientdata) structure and confirm that the `type`, `challenge`, and `origin` members (at least) are as expected.

 Sites should _not_ implement this by comparing the unparsed value of `clientDataJSON` against a template with the `challenge` value filled in. This would fail when new members are added to `CollectedClientData` in the future as the template would no longer be correct.

 In order to guide sites away from doing this, Chromium will sometimes, randomly insert an extra member into `clientDataJSON` which references this documentation.
	# Advice to sites regarding `clientDataJSON`

	When performing a registration or authentication with webauthn, it's critical that sites confirm that the returned [`clientDataJSON`](https://w3c.github.io/webauthn/#dom-authenticatorresponse-clientdatajson) contains the [challenge](https://w3c.github.io/webauthn/#cryptographic-challenges) originally provided. Otherwise old messages can be replayed. Likewise, sites must check the `origin` member to confirm that the action is not being proxied by another site.

	In order to implement this, sites should parse the JSON as a [`CollectedClientData`](https://w3c.github.io/webauthn/#dictdef-collectedclientdata) structure and confirm that the `type`, `challenge`, and `origin` members (at least) are as expected.

	Sites should _not_ implement this by comparing the unparsed value of `clientDataJSON` against a template with the `challenge` value filled in. This would fail when new members are added to `CollectedClientData` in the future as the template would no longer be correct.

	In order to guide sites away from doing this, Chromium will sometimes, randomly insert an extra member into `clientDataJSON` which references this documentation.