Fix: Relax resource URI validation to accept base URL (#1517)

Co-authored-by: Jérémy Ballan <jballan@chapsvision.com>
Co-authored-by: Tarek Mahmoud Sayed <10833894+tarekgh@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -774,15 +774,19 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
     }
 
     /// <summary>
-    /// Verifies that the resource URI in the metadata exactly matches the original request URL as required by the RFC.
+    /// Verifies that the resource URI in the metadata matches the original request URL.
-    /// Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server.
+    /// Accepts either an exact match with the full request URL, or a match with the base URL
+    /// (authority only, path discarded) as allowed by the MCP spec, which derives the authorization
+    /// base URL by discarding the path component from the MCP server URL.
     /// </summary>
     /// <param name="protectedResourceMetadata">The metadata to verify.</param>
     /// <param name="resourceLocation">
     /// The original URL the client used to make the request to the resource server or the root Uri for the resource server
     /// if the metadata was automatically requested from the root well-known location.
     /// </param>
-    /// <returns>True if the resource URI exactly matches the original request URL, otherwise false.</returns>
+    /// <returns>
+    /// True if the resource URI exactly matches the original request URL or its authority-level base URL, otherwise false.
+    /// </returns>
     private static bool VerifyResourceMatch(ProtectedResourceMetadata protectedResourceMetadata, Uri resourceLocation)
     {
         if (protectedResourceMetadata.Resource is null)
@@ -790,14 +794,22 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
             return false;
         }
 
-        // Per RFC: The resource value must be identical to the URL that the client used
-        // to make the request to the resource server. Compare entire URIs, not just the host.
-
         // Normalize the URIs to ensure consistent comparison
         string normalizedMetadataResource = NormalizeUri(protectedResourceMetadata.Resource);
         string normalizedResourceLocation = NormalizeUri(resourceLocation);
 
-        return string.Equals(normalizedMetadataResource, normalizedResourceLocation, StringComparison.OrdinalIgnoreCase);
+        // Accept exact match with the full MCP endpoint URI
+        if (string.Equals(normalizedMetadataResource, normalizedResourceLocation, StringComparison.OrdinalIgnoreCase))
+        {
+            return true;
+        }
+
+        // Per the MCP spec's "Canonical Server URI" section, both the path-specific URI (e.g. https://mcp.example.com/mcp)
+        // and the authority-only URI (e.g. https://mcp.example.com) are valid canonical URIs for identifying an MCP server.
+        // Accept a match with the base URL (authority only, path discarded) to support servers that use the less specific form.
+
+        string normalizedBaseUrl = NormalizeUri(new Uri(resourceLocation.GetLeftPart(UriPartial.Authority)));
+        return string.Equals(normalizedMetadataResource, normalizedBaseUrl, StringComparison.OrdinalIgnoreCase);
     }
 
     /// <summary>
@@ -916,7 +928,8 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
         // https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#protected-resource-metadata-discovery-requirements
         metadata.WwwAuthenticateScope = wwwAuthenticateScope;
 
-        // Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server
+        // Validate that the resource URI in metadata corresponds to the server we're connecting to.
+        // VerifyResourceMatch accepts both an exact URI match and an authority-level (base URL) match per the MCP spec.
         LogValidatingResourceMetadata(resourceUri);
 
         if (!isLegacyFallback && !VerifyResourceMatch(metadata, resourceUri))

tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthTests.cs

@@ -768,7 +768,7 @@ public class AuthTests : OAuthTestBase
         //
         // https://datatracker.ietf.org/doc/html/rfc9728/#section-3.3
         //
-        // CannotAuthenticate_WhenWwwAuthenticateResourceMetadataIsRootPath validates we won't fall back to root in this case.
+        // CanAuthenticate_WhenWwwAuthenticateResourceMetadataIsRootPath validates that a root-level resource is accepted in this case.
         // CanAuthenticate_WithResourceMetadataPathFallbacks validates we will fall back to root when resource_metadata is missing.
         Builder.Services.Configure<AuthenticationOptions>(options => options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme);
         Builder.Services.Configure<McpAuthenticationOptions>(McpAuthenticationDefaults.AuthenticationScheme, options =>
@@ -807,8 +807,14 @@ public class AuthTests : OAuthTestBase
         Assert.Contains("does not match", ex.Message);
     }
 
+    /// <summary>
+    /// Verifies that OAuth authentication succeeds when the protected resource metadata URI
+    /// matches the root server URL, even when the actual MCP endpoint is at a subpath.
+    /// This tests the flexible URI matching behavior where the resource URI can be less specific
+    /// than the actual endpoint being accessed.
+    /// </summary>
     [Fact]
-    public async Task CannotAuthenticate_WhenWwwAuthenticateResourceMetadataIsRootPath()
+    public async Task CanAuthenticate_WhenWwwAuthenticateResourceMetadataIsRootPath()
     {
         const string requestedResourcePath = "/mcp/tools";
 
@@ -839,12 +845,99 @@ public class AuthTests : OAuthTestBase
             },
         }, HttpClient, LoggerFactory);
 
-        var ex = await Assert.ThrowsAsync<McpException>(async () =>
+        await using var client = await McpClient.CreateAsync(
-        {
-            await McpClient.CreateAsync(
             transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+    }
+
+    /// <summary>
+    /// Verifies that OAuth authentication fails when the protected resource metadata URI
+    /// does not match the requested MCP server endpoint. This ensures that clients cannot
+    /// use OAuth tokens intended for one server to access a different server.
+    /// </summary>
+    [Fact]
+    public async Task CannotAuthenticate_WhenResourceMetadataUriDoesNotMatch()
+    {
+        const string requestedResourcePath = "/mcp/tools";
+        const string differentResourceUri = "http://different-server.example.com";
+
+        Builder.Services.Configure<McpAuthenticationOptions>(McpAuthenticationDefaults.AuthenticationScheme, options =>
+        {
+            options.ResourceMetadata = new ProtectedResourceMetadata
+            {
+                Resource = differentResourceUri,
+                AuthorizationServers = { OAuthServerUrl },
+            };
         });
 
+        await using var app = Builder.Build();
+
+        app.MapMcp(requestedResourcePath).RequireAuthorization();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new Uri($"{McpServerUrl}{requestedResourcePath}"),
+            OAuth = new()
+            {
+                ClientId = "demo-client",
+                ClientSecret = "demo-secret",
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+            },
+        }, HttpClient, LoggerFactory);
+
+        // This should fail because the resource URI doesn't match
+        var ex = await Assert.ThrowsAsync<McpException>(() => McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
+
+        Assert.Contains("does not match", ex.Message);
+    }
+
+    /// <summary>
+    /// Verifies that OAuth authentication fails when the protected resource metadata URI is an
+    /// unrelated path on the same host as the requested endpoint (e.g. resource=.../service-a vs
+    /// endpoint .../service-b). This ensures the authority-level fallback only accepts an exact match
+    /// or an authority-only resource, and not arbitrary sibling paths on the same host.
+    /// </summary>
+    [Fact]
+    public async Task CannotAuthenticate_WhenResourceMetadataResourceIsDifferentPathOnSameAuthority()
+    {
+        const string requestedResourcePath = "/service-b";
+        const string differentResourcePath = "/service-a";
+
+        Builder.Services.Configure<McpAuthenticationOptions>(McpAuthenticationDefaults.AuthenticationScheme, options =>
+        {
+            options.ResourceMetadata = new ProtectedResourceMetadata
+            {
+                Resource = $"{McpServerUrl}{differentResourcePath}",
+                AuthorizationServers = { OAuthServerUrl },
+            };
+        });
+
+        await using var app = Builder.Build();
+
+        app.MapMcp(requestedResourcePath).RequireAuthorization();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var transport = new HttpClientTransport(new()
+        {
+            Endpoint = new Uri($"{McpServerUrl}{requestedResourcePath}"),
+            OAuth = new()
+            {
+                ClientId = "demo-client",
+                ClientSecret = "demo-secret",
+                RedirectUri = new Uri("http://localhost:1179/callback"),
+                AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+            },
+        }, HttpClient, LoggerFactory);
+
+        // This should fail because the resource URI is a different path on the same host,
+        // which is neither an exact match nor the authority-only base URL.
+        var ex = await Assert.ThrowsAsync<McpException>(() => McpClient.CreateAsync(
+            transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
+
         Assert.Contains("does not match", ex.Message);
     }