Restrict OAuth Discovery URL Destinations
## 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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Koichi ITO
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1 changed files with 601 additions and 27 deletions
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@ -437,6 +437,121 @@ module MCP
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refute(Discovery.secure_url?("not a url"))
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end
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def test_same_origin_accepts_matching_scheme_host_and_port
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assert(Discovery.same_origin?("https://srv.example.com/.well-known/x", "https://srv.example.com/mcp"))
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assert(Discovery.same_origin?("https://srv.example.com:8443/a", "https://srv.example.com:8443/b"))
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assert(Discovery.same_origin?("http://localhost:9292/a", "http://localhost:9292/mcp"))
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assert(Discovery.same_origin?("https://srv.example.com/a?q=1#f", "https://srv.example.com/mcp"))
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end
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def test_same_origin_ignores_case_in_scheme_and_host
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assert(Discovery.same_origin?("HTTPS://SRV.Example.COM/a", "https://srv.example.com/mcp"))
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end
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def test_same_origin_treats_an_explicit_default_port_as_the_implicit_one
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assert(Discovery.same_origin?("https://srv.example.com:443/a", "https://srv.example.com/mcp"))
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assert(Discovery.same_origin?("http://srv.example.com:80/a", "http://srv.example.com/mcp"))
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end
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def test_same_origin_rejects_differing_components
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refute(Discovery.same_origin?("https://attacker.example.com/a", "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?("https://169.254.169.254/latest/", "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?("https://srv.example.com:8443/a", "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?("http://srv.example.com/a", "https://srv.example.com/mcp"))
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# A subdomain is a different origin, so a hijacked challenge cannot walk
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# sideways into a sibling host that shares the registrable domain.
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refute(Discovery.same_origin?("https://evil.srv.example.com/a", "https://srv.example.com/mcp"))
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end
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def test_same_origin_rejects_malformed_or_hostless_urls
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refute(Discovery.same_origin?("not a url", "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?("https://srv.example.com/a", "not a url"))
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refute(Discovery.same_origin?("https://", "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?(nil, "https://srv.example.com/mcp"))
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refute(Discovery.same_origin?("/relative/path", "https://srv.example.com/mcp"))
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end
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def test_private_network_host_recognizes_reserved_ipv4_ranges
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assert(Discovery.private_network_host?("0.0.0.0"))
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assert(Discovery.private_network_host?("10.0.0.5"))
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assert(Discovery.private_network_host?("100.64.0.1"))
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assert(Discovery.private_network_host?("127.0.0.1"))
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# The cloud metadata address this check exists for.
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assert(Discovery.private_network_host?("169.254.169.254"))
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assert(Discovery.private_network_host?("172.16.0.1"))
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assert(Discovery.private_network_host?("172.31.255.255"))
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assert(Discovery.private_network_host?("192.168.1.1"))
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end
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def test_private_network_host_recognizes_reserved_ipv6_ranges
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assert(Discovery.private_network_host?("::"))
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assert(Discovery.private_network_host?("::1"))
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assert(Discovery.private_network_host?("[::1]"))
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assert(Discovery.private_network_host?("fd00::1"))
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assert(Discovery.private_network_host?("fe80::1"))
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assert(Discovery.private_network_host?("[fe80::1]"))
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end
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def test_private_network_host_unwraps_ipv4_mapped_ipv6
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# `::ffff:169.254.169.254` reaches the same interface as the IPv4 spelling,
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# so the mapped form must not be a way around the ranges above.
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assert(Discovery.private_network_host?("::ffff:169.254.169.254"))
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assert(Discovery.private_network_host?("[::ffff:10.0.0.5]"))
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assert(Discovery.private_network_host?("::ffff:127.0.0.1"))
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refute(Discovery.private_network_host?("::ffff:93.184.216.34"))
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end
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def test_private_network_host_refuses_non_canonical_ipv4_spellings
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# `inet_aton` accepts forms `IPAddr` rejects, and the resolver reaches the same
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# interfaces through them: `2130706433` and `127.1` are 127.0.0.1, and
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# `0xa9fea9fe` is the cloud metadata address. Checking only the dotted-decimal
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# spelling would refuse `169.254.169.254` and admit every alias of it.
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assert(Discovery.private_network_host?("2130706433"))
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assert(Discovery.private_network_host?("0x7f000001"))
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assert(Discovery.private_network_host?("0xa9fea9fe"))
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assert(Discovery.private_network_host?("127.1"))
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assert(Discovery.private_network_host?("127.0.1"))
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assert(Discovery.private_network_host?("0x7f.0.0.1"))
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assert(Discovery.private_network_host?("0251.0376.0251.0376"))
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end
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def test_private_network_host_refuses_numeric_spellings_whose_value_is_resolver_dependent
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# `010.0.0.1` is 8.0.0.1 read as octal and 10.0.0.1 read as decimal, and platforms
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# disagree on which it is. Both readings are refused rather than one being chosen,
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# since the reading not taken would otherwise be a way through.
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assert(Discovery.private_network_host?("010.0.0.1"))
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assert(Discovery.private_network_host?("0177.0.0.1"))
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# The same applies to numeric spellings that land outside the blocked ranges, or
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# nowhere at all: nothing is addressed this way on purpose.
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assert(Discovery.private_network_host?("0x8080808"))
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assert(Discovery.private_network_host?("256.1.1.1"))
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end
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def test_private_network_host_recognizes_localhost_by_name_only
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assert(Discovery.private_network_host?("localhost"))
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assert(Discovery.private_network_host?("LOCALHOST"))
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refute(Discovery.private_network_host?("foo.localhost"))
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refute(Discovery.private_network_host?("localhost.attacker.com"))
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end
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def test_private_network_host_rejects_public_addresses_and_names
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refute(Discovery.private_network_host?("93.184.216.34"))
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refute(Discovery.private_network_host?("8.8.8.8"))
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refute(Discovery.private_network_host?("172.32.0.1"))
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refute(Discovery.private_network_host?("2606:4700::1111"))
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refute(Discovery.private_network_host?("srv.example.com"))
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refute(Discovery.private_network_host?(nil))
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refute(Discovery.private_network_host?(""))
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end
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def test_private_network_host_does_not_resolve_names
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# Documents the deliberate limit of the predicate: an internal host that is
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# named rather than addressed reads as public here. `same_origin?` is what
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# keeps the `resource_metadata` URL from reaching one.
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refute(Discovery.private_network_host?("vault.corp.internal"))
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end
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def test_client_id_metadata_document_url_accepts_https_with_path
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assert(Discovery.client_id_metadata_document_url?("https://app.example.com/client-metadata.json"))
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assert(Discovery.client_id_metadata_document_url?("https://app.example.com/path/to/cm"))
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