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fix(auth): allow same-origin IPv6 ULA issuers #1393

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fix(auth): allow same-origin IPv6 ULA issuers #1393

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80 changes: 75 additions & 5 deletions crates/rmcp/src/transport/auth.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2868,11 +2868,11 @@ impl AuthorizationManager {
},
};

if !Self::is_allowed_metadata_url(
&self.base_url,
&candidate_url,
self.allow_private_metadata_addresses,
) {
// A resource at an IPv6 unique-local address may name its own origin as issuer.
let allow_private = self.allow_private_metadata_addresses
|| (matches!(candidate_url.host(), Some(Host::Ipv6(ip)) if ip.is_unique_local())
&& Self::is_same_origin(&self.base_url, &candidate_url));
if !Self::is_allowed_metadata_url(&self.base_url, &candidate_url, allow_private) {
Comment on lines +2871 to +2875

@DaleSeo DaleSeo Nov 5, 2026 •
edited
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#1393 (review) reads the ULA-only scope as a safety feature. But a same-origin candidate is already configured by the user and already has an active connection, so it doesn’t make any new address reachable, regardless of the IP class.

resolve_resource_metadata_url and the resolver already trust the same origin. Could discovery use that rule too, so a resource at https://10.0.0.1/mcp can name itself as the issuer?

Suggested change
// A resource at an IPv6 unique-local address may name its own origin as issuer.
let allow_private = self.allow_private_metadata_addresses
|| (matches!(candidate_url.host(), Some(Host::Ipv6(ip)) if ip.is_unique_local())
&& Self::is_same_origin(&self.base_url, &candidate_url));
if !Self::is_allowed_metadata_url(&self.base_url, &candidate_url, allow_private) {
if !Self::is_same_origin(&self.base_url, &candidate_url)
&& !Self::is_allowed_metadata_url(
&self.base_url,
&candidate_url,
self.allow_private_metadata_addresses,
)
{

warn!("rejecting authorization server metadata URL `{candidate_url}`");
continue;
}
Expand Down Expand Up @@ -6791,6 +6791,76 @@ mod tests {
);
}

#[rstest]
#[case::ula_default_port("https://[fc00::1]:443", "https://[fc00::1]", false, true)]
#[case::ula_custom_port("https://[fd00::1]:8443", "https://[fd00::1]:8443", false, true)]
#[case::other_host("https://[fd00::1]", "https://[fd00::2]", false, false)]
#[case::other_port("https://[fd00::1]", "https://[fd00::1]:8443", false, false)]
#[case::other_scheme("https://[fd00::1]", "http://[fd00::1]", false, false)]
#[case::named_resource("https://mcp.example.com", "https://[fd00::1]", false, false)]
#[case::private_ipv4("https://10.0.0.1", "https://10.0.0.1", false, false)]
#[case::cgnat("https://100.64.0.1", "https://100.64.0.1", false, false)]
#[case::mapped_ipv4("https://[::ffff:a00:1]", "https://[::ffff:a00:1]", false, false)]
#[case::nat64("https://[64:ff9b::a00:1]", "https://[64:ff9b::a00:1]", false, false)]
#[case::link_local("https://[fe80::1]", "https://[fe80::1]", false, false)]
#[case::explicit_opt_in("https://mcp.example.com", "https://[fd00::1]", true, true)]
#[tokio::test]
async fn private_issuer_discovery(
#[case] resource_origin: &str,
#[case] candidate: &str,
#[case] opt_in: bool,
#[case] candidate_allowed: bool,
) {
let resource = Url::parse(resource_origin).unwrap().join("/mcp").unwrap();
let metadata_url = "https://metadata.example.com/prm";
let fallback = "https://auth.example.com";
let expected = if candidate_allowed {
candidate
} else {
fallback
};
let client = RecordingOAuthHttpClient::with_responses(vec![
challenge_response(&format!(r#"Bearer resource_metadata="{metadata_url}""#)),
http_response(
200,
serde_json::json!({
"resource": resource.as_str(),
"authorization_servers": [candidate, fallback],
}),
),
http_response(
200,
serde_json::json!({
"issuer": expected,
"authorization_endpoint": format!("{expected}/authorize"),
"token_endpoint": format!("{expected}/token"),
}),
),
]);
let mut manager = AuthorizationManager::new_with_oauth_http_client(
resource.as_str(),
Arc::new(client.clone()),
)
.await
.unwrap();
manager.allow_private_metadata_addresses = opt_in;

let metadata = manager.resolve_metadata().await.unwrap().metadata;
assert_eq!(metadata.issuer.as_deref(), Some(expected));
assert_eq!(
client
.requests()
.into_iter()
.map(|r| r.uri)
.collect::<Vec<_>>(),
[
resource.to_string(),
metadata_url.to_string(),
format!("{expected}/.well-known/oauth-authorization-server")
],
);
}

#[tokio::test]
async fn allows_loopback_authorization_server_when_resource_is_loopback() {
let challenge = oauth2::http::Response::builder()
Expand Down
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