Implement "iss" support in the server stack and update the token validation logic to support issuers that don't end with a trailing slash

src/OpenIddict.Abstractions/OpenIddictConstants.cs

@@ -439,6 +439,10 @@ public static class OpenIddictConstants
         public static readonly char[] Ampersand = { '&' };
         public static readonly char[] Dash = { '-' };
         public static readonly char[] Dot = { '.' };
+        public static readonly char[] EqualsSign = { '=' };
+        public static readonly char[] Hash = { '#' };
+        public static readonly char[] QuestionMark = { '?' };
+        public static readonly char[] Semicolon = { ';' };
         public static readonly char[] Space = { ' ' };
     }
 

src/OpenIddict.Abstractions/OpenIddictResources.resx

@@ -1587,6 +1587,12 @@ To apply redirection responses, create a class implementing 'IOpenIddictClientHa
   <data name="ID2133" xml:space="preserve">
     <value>The '{0}' claim returned in the specified userinfo response/token doesn't match the expected value.</value>
   </data>
+  <data name="ID2134" xml:space="preserve">
+    <value>Callback URLs cannot contain an "{0}" parameter.</value>
+  </data>
+  <data name="ID2135" xml:space="preserve">
+    <value>The '{0}' parameter must not include a '{1}' component.</value>
+  </data>
   <data name="ID4000" xml:space="preserve">
     <value>The '{0}' parameter shouldn't be null or empty at this point.</value>
   </data>
@@ -2132,6 +2138,9 @@ This may indicate that the hashed entry is corrupted or malformed.</value>
   <data name="ID6180" xml:space="preserve">
     <value>The redirection request was successfully validated.</value>
   </data>
+  <data name="ID6181" xml:space="preserve">
+    <value>The authorization request was rejected because the '{Parameter}' contained a forbidden parameter: {Name}.</value>
+  </data>
   <data name="ID8000" xml:space="preserve">
     <value>https://documentation.openiddict.com/errors/{0}</value>
   </data>

src/OpenIddict.Abstractions/Primitives/OpenIddictResponse.cs

@@ -153,6 +153,15 @@ public class OpenIddictResponse : OpenIddictMessage
         set => SetParameter(OpenIddictConstants.Parameters.IdToken, value);
     }
 
+    /// <summary>
+    /// Gets or sets the "iss" parameter.
+    /// </summary>
+    public string? Iss
+    {
+        get => (string?) GetParameter(OpenIddictConstants.Parameters.Iss);
+        set => SetParameter(OpenIddictConstants.Parameters.Iss, value);
+    }
+
     /// <summary>
     /// Gets or sets the "refresh_token" parameter.
     /// </summary>

src/OpenIddict.Client/OpenIddictClientConfiguration.cs

@@ -65,7 +65,7 @@ public class OpenIddictClientConfiguration : IPostConfigureOptions<OpenIddictCli
                     if (!registration.MetadataAddress.IsAbsoluteUri)
                     {
                         var issuer = registration.Issuer;
-                        if (issuer is null || !issuer.IsAbsoluteUri)
+                        if (issuer is not { IsAbsoluteUri: true })
                         {
                             throw new InvalidOperationException(SR.GetResourceString(SR.ID0136));
                         }

src/OpenIddict.Client/OpenIddictClientHandlers.Protection.cs

@@ -110,8 +110,23 @@ public static partial class OpenIddictClientHandlers
 
                     var parameters = registration!.TokenValidationParameters.Clone();
 
-                    parameters.ValidIssuer ??= configuration.Issuer?.OriginalString;
-                    parameters.ValidateIssuer = !string.IsNullOrEmpty(parameters.ValidIssuer);
+                    parameters.ValidIssuers ??= configuration.Issuer switch
+                    {
+                        null => null,
+
+                        // If the issuer URI doesn't contain any path/query/fragment, allow both http://www.fabrikam.com
+                        // and http://www.fabrikam.com/ (the recommended URI representation) to be considered valid.
+                        // See https://datatracker.ietf.org/doc/html/rfc3986#section-6.2.3 for more information.
+                        { AbsolutePath: "/", Query.Length: 0, Fragment.Length: 0 } issuer => new[]
+                        {
+                            issuer.AbsoluteUri, // Uri.AbsoluteUri is normalized and always contains a trailing slash.
+                            issuer.AbsoluteUri.Substring(0, issuer.AbsoluteUri.Length - 1)
+                        },
+
+                        Uri issuer => new[] { issuer.AbsoluteUri }
+                    };
+
+                    parameters.ValidateIssuer = parameters.ValidIssuers is not null;
 
                     // Combine the signing keys registered statically in the token validation parameters
                     // with the signing keys resolved from the OpenID Connect server configuration.

src/OpenIddict.Client/OpenIddictClientHandlers.Userinfo.cs

@@ -129,7 +129,7 @@ public static partial class OpenIddictClientHandlers
                 // Resolve the issuer that will be attached to the claims created by this handler.
                 // Note: at this stage, the optional issuer extracted from the response is assumed
                 // to be valid, as it is guarded against unknown values by the ValidateIssuer handler.
-                var issuer = (string?) context.Response[Claims.Issuer] ?? configuration.Issuer!.OriginalString;
+                var issuer = (string?) context.Response[Claims.Issuer] ?? configuration.Issuer!.AbsoluteUri;
 
                 foreach (var parameter in context.Response.GetParameters())
                 {

src/OpenIddict.Client/OpenIddictClientHandlers.cs

@@ -445,7 +445,8 @@ public static partial class OpenIddictClientHandlers
                 }
 
                 // If the two values don't match, this may indicate a mix-up attack attempt.
-                if (!string.Equals(issuer, context.Issuer.AbsoluteUri, StringComparison.Ordinal))
+                if (!Uri.TryCreate(issuer, UriKind.Absolute, out Uri? uri) ||
+                    !uri.IsWellFormedOriginalString() || uri != configuration.Issuer)
                 {
                     context.Reject(
                         error: Errors.InvalidRequest,

src/OpenIddict.Core/Managers/OpenIddictApplicationManager.cs

@@ -1226,6 +1226,19 @@ public class OpenIddictApplicationManager<TApplication> : IOpenIddictApplication
 
                 break;
             }
+
+            // To prevent issuer fixation attacks where a malicious client would specify an "iss" parameter
+            // in the callback URL, ensure the query - if present - doesn't include an "iss" parameter.
+            if (!string.IsNullOrEmpty(uri.Query) && uri.Query.TrimStart(Separators.QuestionMark[0])
+                .Split(new[] { Separators.Ampersand[0], Separators.Semicolon[0] }, StringSplitOptions.RemoveEmptyEntries)
+                .Select(parameter => parameter.Split(Separators.EqualsSign, StringSplitOptions.RemoveEmptyEntries))
+                .Select(parts => parts[0] is string value ? Uri.UnescapeDataString(value) : null)
+                .Contains(Parameters.Iss, StringComparer.OrdinalIgnoreCase))
+            {
+                yield return new ValidationResult(SR.FormatID2134(Parameters.Iss));
+
+                break;
+            }
         }
     }
 

src/OpenIddict.Server.AspNetCore/OpenIddictServerAspNetCoreHandlers.cs

@@ -240,7 +240,7 @@ public static partial class OpenIddictServerAspNetCoreHandlers
                 .AddFilter<RequireHttpRequest>()
                 .AddFilter<RequireTransportSecurityRequirementEnabled>()
                 .UseSingletonHandler<ValidateTransportSecurityRequirement>()
-                .SetOrder(InferEndpointType.Descriptor.Order + 1_000)
+                .SetOrder(InferIssuerFromHost.Descriptor.Order + 1_000)
                 .SetType(OpenIddictServerHandlerType.BuiltIn)
                 .Build();
 

src/OpenIddict.Server.Owin/OpenIddictServerOwinHandlers.cs

@@ -231,7 +231,7 @@ public static partial class OpenIddictServerOwinHandlers
                 .AddFilter<RequireOwinRequest>()
                 .AddFilter<RequireTransportSecurityRequirementEnabled>()
                 .UseSingletonHandler<ValidateTransportSecurityRequirement>()
-                .SetOrder(InferEndpointType.Descriptor.Order + 1_000)
+                .SetOrder(InferIssuerFromHost.Descriptor.Order + 1_000)
                 .SetType(OpenIddictServerHandlerType.BuiltIn)
                 .Build();
 

src/OpenIddict.Server/OpenIddictServerHandlers.Authentication.cs

@@ -56,7 +56,8 @@ public static partial class OpenIddictServerHandlers
              */
             AttachRedirectUri.Descriptor,
             InferResponseMode.Descriptor,
-            AttachResponseState.Descriptor);
+            AttachResponseState.Descriptor,
+            AttachIssuer.Descriptor);
 
         /// <summary>
         /// Contains the logic responsible of extracting authorization requests and invoking the corresponding event handlers.
@@ -532,6 +533,27 @@ public static partial class OpenIddictServerHandlers
                     return default;
                 }
 
+                // To prevent issuer fixation attacks where a malicious client would specify an "iss" parameter
+                // in the redirect_uri, ensure the query - if present - doesn't include an "iss" parameter.
+                //
+                // Note: while OAuth 2.0 parameters are case-sentitive, the following check deliberately
+                // uses a case-insensitive comparison to ensure that all variations of "iss" are rejected.
+                if (!string.IsNullOrEmpty(uri.Query) && uri.Query.TrimStart(Separators.QuestionMark[0])
+                    .Split(new[] { Separators.Ampersand[0], Separators.Semicolon[0] }, StringSplitOptions.RemoveEmptyEntries)
+                    .Select(parameter => parameter.Split(Separators.EqualsSign, StringSplitOptions.RemoveEmptyEntries))
+                    .Select(parts => parts[0] is string value ? Uri.UnescapeDataString(value) : null)
+                    .Contains(Parameters.Iss, StringComparer.OrdinalIgnoreCase))
+                {
+                    context.Logger.LogInformation(SR.GetResourceString(SR.ID6181), Parameters.RedirectUri, Parameters.Iss);
+
+                    context.Reject(
+                        error: Errors.InvalidRequest,
+                        description: SR.FormatID2135(Parameters.RedirectUri, Parameters.Iss),
+                        uri: SR.FormatID8000(SR.ID2135));
+
+                    return default;
+                }
+
                 return default;
             }
         }
@@ -1723,8 +1745,11 @@ public static partial class OpenIddictServerHandlers
                     throw new ArgumentNullException(nameof(context));
                 }
 
-                // Attach the request state to the authorization response.
-                if (string.IsNullOrEmpty(context.Response.State))
+                // If the user agent is expected to be redirected to the client application, attach the request
+                // state to the authorization response to help the client mitigate CSRF/session fixation attacks.
+                //
+                // Note: don't override the state if one was already attached to the response instance.
+                if (!string.IsNullOrEmpty(context.RedirectUri) && string.IsNullOrEmpty(context.Response.State))
                 {
                     context.Response.State = context.Request?.State;
                 }
@@ -1732,5 +1757,54 @@ public static partial class OpenIddictServerHandlers
                 return default;
             }
         }
+
+        /// <summary>
+        /// Contains the logic responsible of attaching an "iss" parameter
+        /// containing the address of the authorization server to the response.
+        /// </summary>
+        public class AttachIssuer : IOpenIddictServerHandler<ApplyAuthorizationResponseContext>
+        {
+            /// <summary>
+            /// Gets the default descriptor definition assigned to this handler.
+            /// </summary>
+            public static OpenIddictServerHandlerDescriptor Descriptor { get; }
+                = OpenIddictServerHandlerDescriptor.CreateBuilder<ApplyAuthorizationResponseContext>()
+                    .UseSingletonHandler<AttachIssuer>()
+                    .SetOrder(AttachResponseState.Descriptor.Order + 1_000)
+                    .SetType(OpenIddictServerHandlerType.BuiltIn)
+                    .Build();
+
+            /// <inheritdoc/>
+            public ValueTask HandleAsync(ApplyAuthorizationResponseContext context)
+            {
+                if (context is null)
+                {
+                    throw new ArgumentNullException(nameof(context));
+                }
+
+                // If the user agent is expected to be redirected to the client application, attach the
+                // issuer address to the authorization response to help the client detect mix-up attacks.
+                //
+                // Note: this applies to all authorization responses, whether they represent valid or errored responses.
+                // For more information, see https://datatracker.ietf.org/doc/html/draft-ietf-oauth-iss-auth-resp-05.
+
+                if (!string.IsNullOrEmpty(context.RedirectUri))
+                {
+                    // At this stage, throw an exception if the issuer cannot be retrieved.
+                    if (context.Issuer is not { IsAbsoluteUri: true })
+                    {
+                        throw new InvalidOperationException(SR.GetResourceString(SR.ID0023));
+                    }
+
+                    // Note: don't override the issuer if one was already attached to the response instance.
+                    if (string.IsNullOrEmpty(context.Response.Iss))
+                    {
+                        context.Response.Iss = context.Issuer.AbsoluteUri;
+                    }
+                }
+
+                return default;
+            }
+        }
     }
 }

src/OpenIddict.Server/OpenIddictServerHandlers.Discovery.cs

@@ -377,7 +377,7 @@ public static partial class OpenIddictServerHandlers
                     }
 
                     // At this stage, throw an exception if the issuer cannot be retrieved.
-                    if (issuer is null || !issuer.IsAbsoluteUri)
+                    if (issuer is not { IsAbsoluteUri: true })
                     {
                         throw new InvalidOperationException(SR.GetResourceString(SR.ID0023));
                     }
@@ -746,6 +746,11 @@ public static partial class OpenIddictServerHandlers
                 context.Metadata[Metadata.RequestParameterSupported] = false;
                 context.Metadata[Metadata.RequestUriParameterSupported] = false;
 
+                // As of 3.2.0, OpenIddict automatically returns an "iss" parameter containing its own address as
+                // part of authorization responses to help clients mitigate mix-up attacks. For more information,
+                // see https://datatracker.ietf.org/doc/html/draft-ietf-oauth-iss-auth-resp-05.
+                context.Metadata[Metadata.AuthorizationResponseIssParameterSupported] = true;
+
                 return default;
             }
         }

src/OpenIddict.Server/OpenIddictServerHandlers.cs

@@ -2953,7 +2953,7 @@ public static partial class OpenIddictServerHandlers
                 }
 
                 // At this stage, throw an exception if the issuer cannot be retrieved.
-                if (issuer is null || !issuer.IsAbsoluteUri)
+                if (issuer is not { IsAbsoluteUri: true })
                 {
                     throw new InvalidOperationException(SR.GetResourceString(SR.ID0023));
                 }

src/OpenIddict.Validation/OpenIddictValidationConfiguration.cs

@@ -109,7 +109,7 @@ public class OpenIddictValidationConfiguration : IPostConfigureOptions<OpenIddic
                 if (!options.MetadataAddress.IsAbsoluteUri)
                 {
                     var issuer = options.Issuer;
-                    if (issuer is null || !issuer.IsAbsoluteUri)
+                    if (issuer is not { IsAbsoluteUri: true })
                     {
                         throw new InvalidOperationException(SR.GetResourceString(SR.ID0136));
                     }

src/OpenIddict.Validation/OpenIddictValidationHandlers.Introspection.cs

@@ -378,8 +378,8 @@ public static partial class OpenIddictValidationHandlers
                 // Note: at this stage, the optional issuer extracted from the response is assumed
                 // to be valid, as it is guarded against unknown values by the ValidateIssuer handler.
                 var issuer = (string?) context.Response[Claims.Issuer] ??
-                    configuration?.Issuer?.OriginalString ??
-                    context.Issuer?.OriginalString ?? ClaimsIdentity.DefaultIssuer;
+                    configuration?.Issuer?.AbsoluteUri ??
+                    context.Issuer?.AbsoluteUri ?? ClaimsIdentity.DefaultIssuer;
 
                 foreach (var parameter in context.Response.GetParameters())
                 {

src/OpenIddict.Validation/OpenIddictValidationHandlers.Protection.cs

@@ -70,8 +70,24 @@ public static partial class OpenIddictValidationHandlers
                 // Clone the token validation parameters and set the issuer using the value found in the
                 // OpenID Connect server configuration (that can be static or retrieved using discovery).
                 var parameters = context.Options.TokenValidationParameters.Clone();
-                parameters.ValidIssuer ??= configuration.Issuer?.OriginalString;
-                parameters.ValidateIssuer = !string.IsNullOrEmpty(parameters.ValidIssuer);
+
+                parameters.ValidIssuers ??= configuration.Issuer switch
+                {
+                    null => null,
+
+                    // If the issuer URI doesn't contain any path/query/fragment, allow both http://www.fabrikam.com
+                    // and http://www.fabrikam.com/ (the recommended URI representation) to be considered valid.
+                    // See https://datatracker.ietf.org/doc/html/rfc3986#section-6.2.3 for more information.
+                    { AbsolutePath: "/", Query.Length: 0, Fragment.Length: 0 } issuer => new[]
+                    {
+                        issuer.AbsoluteUri, // Uri.AbsoluteUri is normalized and always contains a trailing slash.
+                        issuer.AbsoluteUri.Substring(0, issuer.AbsoluteUri.Length - 1)
+                    },
+
+                    Uri issuer => new[] { issuer.AbsoluteUri }
+                };
+
+                parameters.ValidateIssuer = parameters.ValidIssuers is not null;
 
                 // Combine the signing keys registered statically in the token validation parameters
                 // with the signing keys resolved from the OpenID Connect server configuration.

test/OpenIddict.Abstractions.Tests/Primitives/OpenIddictResponseTests.cs

@@ -71,6 +71,13 @@ public class OpenIddictResponseTests
                 /* value: */ new OpenIddictParameter("802A3E3E-DCCA-4EFC-89FA-7D82FE8C27E4")
             };
 
+            yield return new object[]
+            {
+                /* property: */ nameof(OpenIddictResponse.Iss),
+                /* name: */ Parameters.Iss,
+                /* value: */ new OpenIddictParameter("802A3E3E-DCCA-4EFC-89FA-7D82FE8C27E4")
+            };
+
             yield return new object[]
             {
                 /* property: */ nameof(OpenIddictResponse.RefreshToken),

test/OpenIddict.Server.IntegrationTests/OpenIddictServerIntegrationTests.Authentication.cs

@@ -241,6 +241,41 @@ public abstract partial class OpenIddictServerIntegrationTests
         Assert.Equal(SR.FormatID8000(message), response.ErrorUri);
     }
 
+    [Theory]
+    [InlineData("http://www.fabrikam.com/path?iss")]
+    [InlineData("http://www.fabrikam.com/path?iss=value")]
+    [InlineData("http://www.fabrikam.com/path?;iss")]
+    [InlineData("http://www.fabrikam.com/path?;iss=value")]
+    [InlineData("http://www.fabrikam.com/path?&iss")]
+    [InlineData("http://www.fabrikam.com/path?&iss=value")]
+    [InlineData("http://www.fabrikam.com/path?state;iss")]
+    [InlineData("http://www.fabrikam.com/path?state;iss=value")]
+    [InlineData("http://www.fabrikam.com/path?state&iss")]
+    [InlineData("http://www.fabrikam.com/path?state&iss=value")]
+    [InlineData("http://www.fabrikam.com/path?state=abc;iss")]
+    [InlineData("http://www.fabrikam.com/path?state=abc;iss=value")]
+    [InlineData("http://www.fabrikam.com/path?state=abc&iss")]
+    [InlineData("http://www.fabrikam.com/path?state=abc&iss=value")]
+    public async Task ValidateAuthorizationRequest_RedirectUriWithIssuerParameterCausesAnError(string address)
+    {
+        // Arrange
+        await using var server = await CreateServerAsync(options => options.EnableDegradedMode());
+        await using var client = await server.CreateClientAsync();
+
+        // Act
+        var response = await client.PostAsync("/connect/authorize", new OpenIddictRequest
+        {
+            ClientId = "Fabrikam",
+            RedirectUri = address,
+            Scope = Scopes.OpenId
+        });
+
+        // Assert
+        Assert.Equal(Errors.InvalidRequest, response.Error);
+        Assert.Equal(SR.FormatID2135(Parameters.RedirectUri, Parameters.Iss), response.ErrorDescription);
+        Assert.Equal(SR.FormatID8000(SR.ID2135), response.ErrorUri);
+    }
+
     [Fact]
     public async Task ValidateAuthorizationRequest_MissingResponseTypeCausesAnError()
     {
@@ -2335,4 +2370,86 @@ public abstract partial class OpenIddictServerIntegrationTests
         // Assert
         Assert.Equal("custom_state", response.State);
     }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("http://www.fabrikam.com/")]
+    public async Task ApplyAuthorizationResponse_SetsIssuerParameter(string issuer)
+    {
+        // Arrange
+        await using var server = await CreateServerAsync(options =>
+        {
+            options.EnableDegradedMode();
+
+            if (!string.IsNullOrEmpty(issuer))
+            {
+                options.SetIssuer(new Uri(issuer, UriKind.Absolute));
+            }
+
+            options.AddEventHandler<HandleAuthorizationRequestContext>(builder =>
+                builder.UseInlineHandler(context =>
+                {
+                    context.Principal = new ClaimsPrincipal(new ClaimsIdentity("Bearer"))
+                        .SetClaim(Claims.Subject, "Bob le Magnifique");
+
+                    return default;
+                }));
+        });
+
+        await using var client = await server.CreateClientAsync();
+
+        // Act
+        var response = await client.PostAsync("/connect/authorize", new OpenIddictRequest
+        {
+            ClientId = "Fabrikam",
+            RedirectUri = "http://www.fabrikam.com/path",
+            ResponseType = ResponseTypes.Code,
+            State = "af0ifjsldkj"
+        });
+
+        // Assert
+        Assert.Equal(issuer is not null ? issuer : "http://localhost/", response.Iss);
+    }
+
+    [Fact]
+    public async Task ApplyAuthorizationResponse_DoesNotOverrideIssuerSetByApplicationCode()
+    {
+        // Arrange
+        await using var server = await CreateServerAsync(options =>
+        {
+            options.EnableDegradedMode();
+            options.SetIssuer(new Uri("http://www.fabrikam.com/", UriKind.Absolute));
+
+            options.AddEventHandler<HandleAuthorizationRequestContext>(builder =>
+                builder.UseInlineHandler(context =>
+                {
+                    context.Principal = new ClaimsPrincipal(new ClaimsIdentity("Bearer"))
+                        .SetClaim(Claims.Subject, "Bob le Magnifique");
+
+                    return default;
+                }));
+
+            options.AddEventHandler<ApplyAuthorizationResponseContext>(builder =>
+                builder.UseInlineHandler(context =>
+                {
+                    context.Response.Iss = "http://www.contoso.com/";
+
+                    return default;
+                }));
+        });
+
+        await using var client = await server.CreateClientAsync();
+
+        // Act
+        var response = await client.PostAsync("/connect/authorize", new OpenIddictRequest
+        {
+            ClientId = "Fabrikam",
+            RedirectUri = "http://www.fabrikam.com/path",
+            ResponseType = ResponseTypes.Code,
+            State = "af0ifjsldkj"
+        });
+
+        // Assert
+        Assert.Equal("http://www.contoso.com/", response.Iss);
+    }
 }

test/OpenIddict.Server.IntegrationTests/OpenIddictServerIntegrationTests.cs

@@ -1201,7 +1201,12 @@ public abstract partial class OpenIddictServerIntegrationTests
         });
 
         // Assert
-        Assert.Equal(0, response.Count);
+        Assert.Null(response.AccessToken);
+        Assert.Null(response.Code);
+        Assert.Null(response.DeviceCode);
+        Assert.Null(response.IdToken);
+        Assert.Null(response.RefreshToken);
+        Assert.Null(response.UserCode);
     }
 
     [Theory]