## Motivation and Context
The OAuth client checked the scheme of every discovery URL but never checked where it pointed.
`Discovery.secure_url?` returns true for any `https` URL regardless of host, and the `resource_metadata` URL from
a `WWW-Authenticate` challenge is placed first in the candidate list, so a server answering
`401 Bearer resource_metadata="https://169.254.169.254/..."` had the client issue that request during discovery,
before the user approves anything. The same gap applied one hop deeper: a server that serves its own
Protected Resource Metadata can name any host in `authorization_servers`, and the endpoints in the document fetched
from there receive the dynamic client registration and the token request that carries the client credentials.
Two rules now apply to those URLs. Both run before the request is sent, and neither is configurable.
The `resource_metadata` URL must be on the MCP server's own origin. Protected Resource Metadata describes that server,
so a real deployment publishes it there, and requiring it means a challenge cannot aim the first request of the flow at
an unrelated host. RFC 9728 does not itself require this, so the SDK is deliberately stricter than the specification:
the range check below cannot recognize an internal host that is named rather than addressed, and this is the one URL
a server supplies directly.
The PRM `authorization_servers` entry and the `authorization_endpoint`, `token_endpoint`, and `registration_endpoint`
from Authorization Server metadata must not be IP literals in a private, loopback, link-local, or unique-local range,
which is the precaution RFC 9728 Section 7.7 asks clients to take. That check is skipped when the configured MCP server URL
is itself on such an address. Pointing a client at a private network is a deliberate act and its authorization server
usually lives on the same network, so `http://localhost` development and deployments that never leave a corporate network
keep working. Only a server reachable on the public internet is barred from steering the client inward.
A range check is only as good as its reading of the address, so the second rule also refuses hosts written in
the numeric IPv4 grammar `inet_aton` accepts and `IPAddr` rejects: one to four dot-separated parts, each hexadecimal,
octal, or decimal, with the last part filling the remaining bytes. `2130706433`, `127.1`, and `0x7f000001` all reach
127.0.0.1, and `0xa9fea9fe` reaches the cloud metadata address, so reading only the dotted-decimal spelling would refuse
`169.254.169.254` while admitting every alias of it. Those hosts are refused outright rather than decoded and
range-checked, because their value is resolver-dependent: a leading zero reads as octal on one platform and decimal on
another, which is the difference between 8.0.0.1 and 10.0.0.1 for `010.0.0.1`. Choosing a reading here would leave
whichever one was not taken as a way through, and nothing is legitimately addressed in this notation.
Hostnames are not resolved. A lookup here would be a second network request driven by the same untrusted input,
and the address it returned need not be the one the HTTP client connects to a moment later, so the check would read as
a guarantee it cannot make. `MCP::Client::OAuth::Flow` also documents why its Faraday connection has no redirect-following
middleware: every check runs against the URL as written, so a connection that follows a `3xx` on its own would reach hosts
these rules just refused.
## How Has This Been Tested?
- New unit tests in `test/mcp/client/oauth/discovery_test.rb` cover `same_origin?` (case-insensitive scheme and host,
explicit default ports, differing port/host/scheme, subdomains, malformed and hostless input) and `private_network_host?`
(every blocked range, IPv4-mapped IPv6 spellings, bracketed IPv6, `localhost` matched exactly, and public addresses and names).
One test documents that a named internal host reads as public, which is the deliberate limit of the predicate.
- Further unit tests cover the numeric spellings: packed integers, hexadecimal, and short forms that reach a blocked range,
and the ambiguous leading-zero forms whose value depends on the resolver. Removing the rule leaves them addressable,
which is what those tests fail on.
- New tests in `test/mcp/client/oauth/flow_test.rb` and `test/mcp/client/oauth/http_oauth_test.rb` drive both rules through `run!`,
`refresh!`, and the transport. They assert with `assert_not_requested` that no request was issued rather than only that an error
was raised, since nothing downstream can un-send one. One of them names the authorization server `https://0xa9fea9fe`, which
without the rule above is fetched. A further test runs a complete flow with the MCP server and the authorization server on
two loopback ports, which is what the carve-out exists for.
- `rake conformance` passes with all 22 auth scenarios green. Its harness runs the MCP server and the authorization server on
separate loopback ports and publishes PRM on the server's own origin, so it exercises exactly the shapes these rules must not break.
## Breaking Changes
A client whose MCP server is reachable on the public internet no longer follows a `resource_metadata` URL on a different origin,
and no longer contacts an authorization server or OAuth endpoint at a private IP literal or at a host written in the numeric
IPv4 notation above. All were previously allowed. Local development, private-network deployments, and the conformance suite
are unaffected.
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