modelcontextprotocol · mikekistler · May 27, 2026
diff --git a/samples/ProtectedMcpClient/Program.cs b/samples/ProtectedMcpClient/Program.cs
@@ -1,4 +1,5 @@
 using Microsoft.Extensions.Logging;
+using ModelContextProtocol.Authentication;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
 using System.Diagnostics;
@@ -32,7 +33,7 @@
     OAuth = new()
     {
         RedirectUri = new Uri("http://localhost:1179/callback"),
-        AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+        AuthorizationCallbackHandler = HandleAuthorizationUrlAsync,
         DynamicClientRegistration = new()
         {
             ClientName = "ProtectedMcpClient",
@@ -71,8 +72,8 @@
 /// <param name="authorizationUrl">The authorization URL to open in the browser.</param>
 /// <param name="redirectUri">The redirect URI where the authorization code will be sent.</param>
 /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
-/// <returns>The authorization code extracted from the callback, or null if the operation failed.</returns>
-static async Task<string?> HandleAuthorizationUrlAsync(Uri authorizationUrl, Uri redirectUri, CancellationToken cancellationToken)
+/// <returns>The authorization result extracted from the callback, or null if the operation failed.</returns>
+static async Task<AuthorizationResult?> HandleAuthorizationUrlAsync(Uri authorizationUrl, Uri redirectUri, CancellationToken cancellationToken)
 {
     Console.WriteLine("Starting OAuth authorization flow...");
     Console.WriteLine($"Opening browser to: {authorizationUrl}");
@@ -93,6 +94,7 @@
         var context = await listener.GetContextAsync();
         var query = HttpUtility.ParseQueryString(context.Request.Url?.Query ?? string.Empty);
         var code = query["code"];
+        var iss = query["iss"];
         var error = query["error"];
 
         string responseHtml = "<html><body><h1>Authentication complete</h1><p>You can close this window now.</p></body></html>";
@@ -115,7 +117,7 @@
         }
 
         Console.WriteLine("Authorization code received successfully.");
-        return code;
+        return new AuthorizationResult { Code = code, Iss = iss };
     }
     catch (Exception ex)
     {

diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationRedirectDelegate.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationRedirectDelegate.cs
@@ -22,7 +22,11 @@ namespace ModelContextProtocol.Authentication;
 /// <para>
 /// The implementation should handle user interaction to visit the authorization URL and extract
 /// the authorization code from the callback. The authorization code is typically provided as
-/// a query parameter in the redirect URI callback.
+/// a <c>code</c> query parameter in the redirect URI callback.
+/// </para>
+/// <para>
+/// For RFC 9207 issuer validation support, use <see cref="ClientOAuthOptions.AuthorizationCallbackHandler"/>
+/// instead, which allows returning both the authorization code and the <c>iss</c> parameter.
 /// </para>
 /// </remarks>
 public delegate Task<string?> AuthorizationRedirectDelegate(Uri authorizationUri, Uri redirectUri, CancellationToken cancellationToken);
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationResult.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationResult.cs
@@ -0,0 +1,39 @@
+namespace ModelContextProtocol.Authentication;
+
+/// <summary>
+/// Represents the result of an OAuth authorization redirect, containing the authorization code
+/// and optionally the issuer identifier from the authorization response.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The <see cref="Iss"/> property should be populated from the <c>iss</c> query parameter in the
+/// redirect URI when present, as specified by
+/// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+/// This enables the SDK to validate that the authorization response originated from the expected
+/// authorization server, mitigating mix-up attacks.
+/// </para>
+/// </remarks>
+public sealed class AuthorizationResult
+{
+    /// <summary>
+    /// Gets the authorization code returned by the authorization server.
+    /// </summary>
+    public string? Code { get; init; }
+
+    /// <summary>
+    /// Gets the issuer identifier returned in the authorization response per
+    /// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This value should be extracted from the <c>iss</c> query parameter of the redirect URI.
+    /// When present, the SDK validates it against the expected authorization server issuer to
+    /// prevent mix-up attacks.
+    /// </para>
+    /// <para>
+    /// Implementations of <see cref="ClientOAuthOptions.AuthorizationCallbackHandler"/> should populate this property
+    /// whenever the <c>iss</c> parameter is present in the redirect URI callback.
+    /// </para>
+    /// </remarks>
+    public string? Iss { get; init; }
+}
diff --git a/src/ModelContextProtocol.Core/Authentication/AuthorizationServerMetadata.cs b/src/ModelContextProtocol.Core/Authentication/AuthorizationServerMetadata.cs
@@ -72,4 +72,11 @@ internal sealed class AuthorizationServerMetadata
     /// </summary>
     [JsonPropertyName("client_id_metadata_document_supported")]
     public bool ClientIdMetadataDocumentSupported { get; set; }
+
+    /// <summary>
+    /// Indicates whether the authorization server includes the <c>iss</c> parameter in authorization responses
+    /// as defined in <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+    /// </summary>
+    [JsonPropertyName("authorization_response_iss_parameter_supported")]
+    public bool AuthorizationResponseIssParameterSupported { get; set; }
 }
diff --git a/src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs b/src/ModelContextProtocol.Core/Authentication/ClientOAuthOptions.cs
@@ -84,6 +84,24 @@ public sealed class ClientOAuthOptions
     /// </remarks>
     public AuthorizationRedirectDelegate? AuthorizationRedirectDelegate { get; set; }
 
+    /// <summary>
+    /// Gets or sets a callback that handles the full OAuth authorization flow, returning both the
+    /// authorization code and the issuer identifier for RFC 9207 validation.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// When set, this handler takes precedence over <see cref="AuthorizationRedirectDelegate"/>.
+    /// It enables the SDK to validate the <c>iss</c> parameter in the authorization response per
+    /// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>, which mitigates
+    /// mix-up attacks.
+    /// </para>
+    /// <para>
+    /// Implementations should extract both the <c>code</c> and <c>iss</c> query parameters from
+    /// the redirect URI callback and return them in an <see cref="AuthorizationResult"/>.
+    /// </para>
+    /// </remarks>
+    public Func<Uri, Uri, CancellationToken, Task<AuthorizationResult?>>? AuthorizationCallbackHandler { get; set; }
+
     /// <summary>
     /// Gets or sets the authorization server selector function.
     /// </summary>

diff --git a/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs b/src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
@@ -32,6 +32,7 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
     private readonly IDictionary<string, string> _additionalAuthorizationParameters;
     private readonly Func<IReadOnlyList<Uri>, Uri?> _authServerSelector;
     private readonly AuthorizationRedirectDelegate _authorizationRedirectDelegate;
+    private readonly Func<Uri, Uri, CancellationToken, Task<AuthorizationResult?>>? _authorizationCallbackHandler;
     private readonly Uri? _clientMetadataDocumentUri;
 
     // _dcrClientName, _dcrClientUri, _dcrInitialAccessToken and _dcrResponseDelegate are used for dynamic client registration (RFC 7591)
@@ -84,6 +85,9 @@ public ClientOAuthProvider(
         // Set up authorization server selection strategy
         _authServerSelector = options.AuthServerSelector ?? DefaultAuthServerSelector;
 
+        // Set up authorization callback handler (new RFC 9207-aware handler takes precedence)
+        _authorizationCallbackHandler = options.AuthorizationCallbackHandler;
+
         // Set up authorization URL handler (use default if not provided)
         _authorizationRedirectDelegate = options.AuthorizationRedirectDelegate ?? DefaultAuthorizationUrlHandler;
 
@@ -370,6 +374,16 @@ private async Task<AuthorizationServerMetadata> GetAuthServerMetadataAsync(Uri a
                 metadata.TokenEndpointAuthMethodsSupported ??= ["client_secret_post"];
                 metadata.CodeChallengeMethodsSupported ??= ["S256"];
 
+                // Validate the issuer in the metadata document per RFC 8414 Section 3.3:
+                // the issuer value MUST be identical to the issuer identifier used to construct
+                // the well-known URL.
+                if (metadata.Issuer is not null &&
+                    !string.Equals(metadata.Issuer.OriginalString, authServerUri.OriginalString, StringComparison.Ordinal))
+                {
+                    ThrowFailedToHandleUnauthorizedResponse(
+                        $"Authorization server metadata issuer '{metadata.Issuer}' does not match the expected issuer '{authServerUri}' (RFC 8414 Section 3.3).");
+                }
+
                 return metadata;
             }
             catch (Exception ex)
@@ -462,14 +476,33 @@ private async Task<string> InitiateAuthorizationCodeFlowAsync(
         var codeChallenge = GenerateCodeChallenge(codeVerifier);
 
         var authUrl = BuildAuthorizationUrl(protectedResourceMetadata, authServerMetadata, codeChallenge);
-        var authCode = await _authorizationRedirectDelegate(authUrl, _redirectUri, cancellationToken).ConfigureAwait(false);
 
-        if (string.IsNullOrEmpty(authCode))
+        string? authorizationCode;
+        string? iss = null;
+
+        if (_authorizationCallbackHandler is not null)
         {
-            ThrowFailedToHandleUnauthorizedResponse($"The {nameof(AuthorizationRedirectDelegate)} returned a null or empty authorization code.");
+            var authResult = await _authorizationCallbackHandler(authUrl, _redirectUri, cancellationToken).ConfigureAwait(false);
+            if (authResult is null || string.IsNullOrEmpty(authResult.Code))
+            {
+                ThrowFailedToHandleUnauthorizedResponse($"The {nameof(ClientOAuthOptions.AuthorizationCallbackHandler)} returned a null or empty authorization code.");
+            }
+
+            authorizationCode = authResult!.Code!;
+            iss = authResult.Iss;
+        }
+        else
+        {
+            authorizationCode = await _authorizationRedirectDelegate(authUrl, _redirectUri, cancellationToken).ConfigureAwait(false);
+            if (string.IsNullOrEmpty(authorizationCode))
+            {
+                ThrowFailedToHandleUnauthorizedResponse($"The {nameof(AuthorizationRedirectDelegate)} returned a null or empty authorization code.");
+            }
         }
 
-        return await ExchangeCodeForTokenAsync(protectedResourceMetadata, authServerMetadata, authCode!, codeVerifier, cancellationToken).ConfigureAwait(false);
+        ValidateIssuerResponse(iss, authServerMetadata);
+
+        return await ExchangeCodeForTokenAsync(protectedResourceMetadata, authServerMetadata, authorizationCode!, codeVerifier, cancellationToken).ConfigureAwait(false);
     }
 
     private Uri BuildAuthorizationUrl(
@@ -773,6 +806,47 @@ private async Task PerformDynamicClientRegistrationAsync(
         return scope + " " + OfflineAccess;
     }
 
+    /// <summary>
+    /// Validates the <c>iss</c> parameter from an authorization response per
+    /// <see href="https://datatracker.ietf.org/doc/html/rfc9207">RFC 9207</see>.
+    /// </summary>
+    /// <param name="iss">The issuer identifier received in the authorization response, or null if absent.</param>
+    /// <param name="authServerMetadata">The authorization server metadata containing the expected issuer.</param>
+    private void ValidateIssuerResponse(string? iss, AuthorizationServerMetadata authServerMetadata)
+    {
+        var expectedIssuer = authServerMetadata.Issuer?.OriginalString;
+
+        if (authServerMetadata.AuthorizationResponseIssParameterSupported)
+        {
+            // Server advertises iss support: iss MUST be present and match.
+            if (string.IsNullOrEmpty(iss))
+            {
+                ThrowFailedToHandleUnauthorizedResponse(
+                    "Authorization server advertises RFC 9207 iss parameter support but none was received in the authorization response.");
+            }
+
+            // Use exact string comparison per RFC 9207 / RFC 3986 §6.2.1.
+            if (!string.Equals(iss, expectedIssuer, StringComparison.Ordinal))
+            {
+                ThrowFailedToHandleUnauthorizedResponse(
+                    $"Authorization response issuer '{iss}' does not match expected issuer '{expectedIssuer}'.");
+            }
+        }
+        else
+        {
+            // Server does not advertise iss support: if iss is present, still validate it.
+            if (!string.IsNullOrEmpty(iss))
+            {
+                if (!string.Equals(iss, expectedIssuer, StringComparison.Ordinal))
+                {
+                    ThrowFailedToHandleUnauthorizedResponse(
+                        $"Authorization response issuer '{iss}' does not match expected issuer '{expectedIssuer}'.");
+                }
+            }
+            // If iss is absent and not advertised, proceed normally.
+        }
+    }
+
     /// <summary>
     /// Verifies that the resource URI in the metadata exactly matches the original request URL as required by the RFC.
     /// Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server.

diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthEventTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/OAuth/AuthEventTests.cs
@@ -48,7 +48,7 @@ public async Task CanAuthenticate_WithResourceMetadataFromEvent()
                     ClientId = "demo-client",
                     ClientSecret = "demo-secret",
                     RedirectUri = new Uri("http://localhost:1179/callback"),
-                    AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                    AuthorizationCallbackHandler = HandleAuthorizationUrlAsync,
                 },
             },
             HttpClient,
@@ -76,7 +76,7 @@ public async Task CanAuthenticate_WithDynamicClientRegistration_FromEvent()
                 OAuth = new ClientOAuthOptions()
                 {
                     RedirectUri = new Uri("http://localhost:1179/callback"),
-                    AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
+                    AuthorizationCallbackHandler = HandleAuthorizationUrlAsync,
                     Scopes = ["mcp:tools"],
                     DynamicClientRegistration = new()
                     {