HTTP Authentication: Dialback Access Authentication

HTTP/1.1 has an extensible authentication mechanism using the "Authorization" header. Basic and Digest Authentication are two authentication schemes for HTTP/1.1. OAuth 1.0 is another. All of these authentication schemes assume that the HTTP server is able to validate the authentication credentials. With distributed publishing and social networking applications, however, the security domain may be separate from the resource domain. In addition, the resource and security domains may have no previously-negotiated relationship. With dialback authentication, an HTTP client can authenticate as a host or Webfinger address without creating a previous relationship. The HTTP server verifies the identity using a dialback endpoint specified by the host or Webfinger address. Because dialback authentication requires one or more additional requests from server to client, its intended use is for bootstrapping longer-term relationships, such as dynamic registration of OAuth clients. It can also be useful for single use requests.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

This spec adds a new Authorization type, "Dialback". The header has the following optional elements: The host authorizing the request. The webfinger account authorizing the request. An opaque value used to confirm the request authorization Exactly one of "host" or "webfinger" is required. "token" is always required.

To validate the token, the server must identify a dialback endpoint for the host or Webfinger address. For a host parameter, the server SHOULD use Web Host Metadata to find the endpoint. It will have the link relation "dialback". For a webfinger parameter, the server SHOULD use Webfinger to find the endpoint with the link relation "dialback".

To confirm, the server makes an HTTP POST request to the dialback endpoint. The request MUST have the Content-Type application/x-www-url-encoded. The HTTP request has the following parameters. The host value provided in the original Authorization header. The webfinger value provided in the original Authorization header. The token provided in the original Authorization header. The URL that the original request was made to. The Date header on the original request. The request MUST include exactly one of "host" or "webfinger". The request MUST include the "token", "url" and "date" parameters. All parameters MUST be encoded in the body of the request. If the token is valid, the endpoint SHOULD return a 200 OK or 204 No Content result. If the token is invalid, the endpoint SHOULD return a 400 Bad Request result.

The client sends an HTTP request with an Authorization header:

The server checks that the Date: header is within an acceptable window (+/- 5 minutes recommended). The server checks that it hasn't seen this (host, url, token, date) tuple before. The server discovers a dialback confirmation endpoint at checkin.example. Its rel type is "dialback". The server posts an HTTP request to confirm the token:

Note: the "\" character is used here to indicate the line wrapping in the request content and is not part of the content itself. checkin.example returns 200 OK for a confirmation, 4xx for confirmation failure, 5xx for server failure.

The client sends an HTTP request with an Authorization header:

The server checks that the Date: header is within an acceptable window (+/- 5 minutes recommended). The server checks that it hasn't seen this (webfinger, url, token, date) tuple before. The server discovers a dialback confirmation endpoint for alice@checkin.example. Its rel type is "dialback". The server posts an HTTP request to confirm the token:

This specification defines a new HTTP authentication scheme, "Dialback", per HTTP/1.1, part 7: Authentication to be registered at http://www.iana.org/assignments/http-authschemes. Authentication Scheme Name: Dialback Reference: (this document)

This specification defines a new link relation type to be registered at http://www.iana.org/assignments/link-relations according to RFC 5988. Relation Name: dialback Description: a dialback token verification endpoint Reference: (this document)

An attacker could capture the Authorization header of a request and replay the header for another payload. To prevent replay attacks, the server MUST NOT accept a request if it has already seen a request with the same host or webfinger, url, token, and date. Servers MAY mitigate storage requirements by rejecting requests with a Date: outside a fixed window. +/- 5 minutes from server time is reasonable.

An attacker could use DNS poisoning techniques to provide links to a false dialback endpoint. Clients supporting dialback SHOULD support TLS for host-meta and Webfinger discovery. HTTP servers SHOULD use the TLS endpoint for host-meta and Webfinger. HTTP servers MAY fall back to the unencrypted equivalent.

An attacker could use DNS poisoning techniques to provide false responses to requests to the dialback verification endpoint. HTTP clients support dialback SHOULD use TLS for dialback endpoints. HTTP servers SHOULD require valid certificates for dialback endpoints.

The dialback endpoint confirms that the host or Webfinger account is responsible for the HTTP request. An attacker could use brute-force methods to determine if the host or Webfinger account has made an HTTP request to a given URL. The dialback endpoint SHOULD NOT verify requests for dates outside a small window around the current time (+/- five minutes). The dialback endpoint SHOULD use large enough tokens to make brute-force attacks impractical.

Dialback authentication does not confirm the contents of the HTTP request. For example, man-in-the-middle attack could replace the contents of a POST request with another payload, which would be verified. Servers SHOULD use TLS to prevent man-in-the-middle attacks.

Dialback authentication lets the HTTP client induce the HTTP server to make additional verification requests. By making requests with a host or webfinger parameter referring to a third party, a malicious client can cause extra HTTP requests to that third party. To avoid denial-of-service attacks, HTTP servers SHOULD cache the results of host-meta and Webfinger requests.