[PM-20592] [PM-22737] [PM-22738] Send grant validator (#6151)

**feat**: create `SendGrantValidator` and initial `SendPasswordValidator` for Send access grants  
**feat**: add feature flag to toggle Send grant validation logic  
**feat**: add Send client to Identity and update `ApiClient` to generic `Client`  
**feat**: register Send services in DI pipeline  
**feat**: add claims management support to `ProfileService`  
**feat**: distinguish between invalid grant and invalid request in `SendAccessGrantValidator`

**fix**: update parsing of `send_id` from request  
**fix**: add early return when feature flag is disabled  
**fix**: rename and organize Send access scope and grant type  
**fix**: dotnet format

**test**: add unit and integration tests for `SendGrantValidator`  
**test**: update OpenID configuration and API resource claims

**doc**: move documentation to interfaces and update inline comments  

**chore**: add TODO for future support of `CustomGrantTypes`

perf/MicroBenchmarks/Identity/IdentityServer/StaticClientStoreTests.cs

@@ -20,7 +20,7 @@ public class StaticClientStoreTests
     [Benchmark]
     public Client? TryGetValue()
     {
-        return _store.ApiClients.TryGetValue(ClientId, out var client)
+        return _store.Clients.TryGetValue(ClientId, out var client)
           ? client
           : null;
     }

src/Core/Constants.cs

@@ -191,6 +191,7 @@ public static class FeatureFlagKeys
     public const string UserManagedPrivilegedApps = "pm-18970-user-managed-privileged-apps";
     public const string EnablePMPreloginSettings = "enable-pm-prelogin-settings";
     public const string AppIntents = "app-intents";
+    public const string SendAccess = "pm-19394-send-access-control";
 
     /* Platform Team */
     public const string PersistPopupView = "persist-popup-view";

src/Core/Enums/BitwardenClient.cs

@@ -8,5 +8,6 @@ public static class BitwardenClient
         Desktop = "desktop",
         Mobile = "mobile",
         Cli = "cli",
-        DirectoryConnector = "connector";
+        DirectoryConnector = "connector",
+        Send = "send";
 }

src/Core/Identity/Claims.cs

@@ -39,4 +39,6 @@ public static class Claims
         public const string ManageResetPassword = "manageresetpassword";
         public const string ManageScim = "managescim";
     }
+
+    public const string SendId = "send_id";
 }

src/Core/Identity/IdentityClientType.cs

@@ -5,4 +5,5 @@ public enum IdentityClientType : byte
     User = 0,
     Organization = 1,
     ServiceAccount = 2,
+    Send = 3
 }

src/Core/IdentityServer/ApiScopes.cs

@@ -11,6 +11,7 @@ public static class ApiScopes
     public const string ApiPush = "api.push";
     public const string ApiSecrets = "api.secrets";
     public const string Internal = "internal";
+    public const string ApiSendAccess = "api.send.access";
 
     public static IEnumerable<ApiScope> GetApiScopes()
     {
@@ -23,6 +24,7 @@ public static class ApiScopes
             new(ApiInstallation, "API Installation Access"),
             new(Internal, "Internal Access"),
             new(ApiSecrets, "Secrets Manager Access"),
+            new(ApiSendAccess, "API Send Access"),
         };
     }
 }

src/Core/KeyManagement/KeyManagementServiceCollectionExtensions.cs

@@ -9,6 +9,7 @@ public static class KeyManagementServiceCollectionExtensions
     public static void AddKeyManagementServices(this IServiceCollection services)
     {
         services.AddKeyManagementCommands();
+        services.AddSendPasswordServices();
     }
 
     private static void AddKeyManagementCommands(this IServiceCollection services)

src/Core/KeyManagement/Sends/ISendPasswordHasher.cs

@@ -9,7 +9,6 @@ public interface ISendPasswordHasher
     /// <param name="sendPasswordHash">The send password that is hashed by the server.</param>
     /// <param name="clientPasswordHash">The user provided password hash that has not yet been hashed by the server for comparison.</param>
     /// <returns>true if hashes match false otherwise</returns>
-    /// <exception cref="InvalidOperationException">Thrown if the server password hash or client password hash is null or empty.</exception>
     bool PasswordHashMatches(string sendPasswordHash, string clientPasswordHash);
 
     /// <summary>

src/Core/Settings/GlobalSettings.cs

@@ -89,6 +89,7 @@ public class GlobalSettings : IGlobalSettings
     public virtual IWebPushSettings WebPush { get; set; } = new WebPushSettings();
     public virtual IPhishingDomainSettings PhishingDomain { get; set; } = new PhishingDomainSettings();
 
+    public virtual int SendAccessTokenLifetimeInMinutes { get; set; } = 5;
     public virtual bool EnableEmailVerification { get; set; }
     public virtual string KdfDefaultHashKey { get; set; }
     public virtual string PricingUri { get; set; }

src/Identity/IdentityServer/ApiResources.cs

@@ -25,8 +25,12 @@ public class ApiResources
                 Claims.OrganizationCustom,
                 Claims.ProviderAdmin,
                 Claims.ProviderServiceUser,
-                Claims.SecretsManagerAccess,
+                Claims.SecretsManagerAccess
             }),
+            new(ApiScopes.ApiSendAccess, [
+                JwtClaimTypes.Subject,
+                Claims.SendId
+            ]),
             new(ApiScopes.Internal, new[] { JwtClaimTypes.Subject }),
             new(ApiScopes.ApiPush, new[] { JwtClaimTypes.Subject }),
             new(ApiScopes.ApiLicensing, new[] { JwtClaimTypes.Subject }),

src/Identity/IdentityServer/DynamicClientStore.cs

@@ -37,7 +37,7 @@ internal class DynamicClientStore : IClientStore
         if (firstPeriod == -1)
         {
             // No splitter, attempt but don't fail for a static client
-            if (_staticClientStore.ApiClients.TryGetValue(clientId, out var client))
+            if (_staticClientStore.Clients.TryGetValue(clientId, out var client))
             {
                 return Task.FromResult<Client?>(client);
             }

src/Identity/IdentityServer/Enums/CustomGrantTypes.cs

@@ -0,0 +1,11 @@
+namespace Bit.Identity.IdentityServer.Enums;
+
+/// <summary>
+/// A class containing custom grant types used in the Bitwarden IdentityServer implementation
+/// </summary>
+public static class CustomGrantTypes
+{
+    public const string SendAccess = "send_access";
+    // TODO: PM-24471 replace magic string with a constant for webauthn
+    public const string WebAuthn = "webauthn";
+}

src/Identity/IdentityServer/ProfileService.cs

@@ -1,10 +1,8 @@
-// FIXME: Update this file to be null safe and then delete the line below
-#nullable disable
-
-using System.Security.Claims;
+using System.Security.Claims;
 using Bit.Core.AdminConsole.Repositories;
 using Bit.Core.Billing.Services;
 using Bit.Core.Context;
+using Bit.Core.Enums;
 using Bit.Core.Identity;
 using Bit.Core.Repositories;
 using Bit.Core.Services;
@@ -42,8 +40,22 @@ public class ProfileService : IProfileService
     public async Task GetProfileDataAsync(ProfileDataRequestContext context)
     {
         var existingClaims = context.Subject.Claims;
-        var newClaims = new List<Claim>();
 
+        // If the client is a Send client, we do not add any additional claims
+        if (context.Client.ClientId == BitwardenClient.Send)
+        {
+            // preserve all claims that were already on context.Subject
+            // which includes the ones added by the SendAccessGrantValidator
+            context.IssuedClaims.AddRange(existingClaims);
+            return;
+        }
+
+        // Whenever IdentityServer issues a new access token or services a UserInfo request, it calls
+        // GetProfileDataAsync to determine which claims to include in the token or response.
+        // In normal user identity scenarios, we have to look up the user to get their claims and update
+        // the issued claims collection as claim info can have changed since the last time the user logged in or the
+        // last time the token was issued.
+        var newClaims = new List<Claim>();
         var user = await _userService.GetUserByPrincipalAsync(context.Subject);
         if (user != null)
         {
@@ -63,12 +75,16 @@ public class ProfileService : IProfileService
 
         // filter out any of the new claims
         var existingClaimsToKeep = existingClaims
-            .Where(c => !c.Type.StartsWith("org") &&
-                (newClaims.Count == 0 || !newClaims.Any(nc => nc.Type == c.Type)))
-            .ToList();
+            .Where(c =>
+                // Drop any org claims
+                !c.Type.StartsWith("org") &&
+                // If we have no new claims, then keep the existing claims
+                // If we have new claims, then keep the existing claim if it does not match a new claim type
+                (newClaims.Count == 0 || !newClaims.Any(nc => nc.Type == c.Type))
+            ).ToList();
 
         newClaims.AddRange(existingClaimsToKeep);
-        if (newClaims.Any())
+        if (newClaims.Count != 0)
         {
             context.IssuedClaims.AddRange(newClaims);
         }
@@ -76,6 +92,13 @@ public class ProfileService : IProfileService
 
     public async Task IsActiveAsync(IsActiveContext context)
     {
+        // Send Tokens are not refreshed so when the token has expired the user must request a new one via the authentication method assigned to the send.
+        if (context.Client.ClientId == BitwardenClient.Send)
+        {
+            context.IsActive = true;
+            return;
+        }
+
         // We add the security stamp claim to the persisted grant when we issue the refresh token.
         // IdentityServer will add this claim to the subject, and here we evaluate whether the security stamp that
         // was persisted matches the current security stamp of the user. If it does not match, then the user has performed

src/Identity/IdentityServer/RequestValidators/SendAccess/Enums/SendGrantValidatorResultTypes.cs

@@ -0,0 +1,11 @@
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess.Enums;
+
+/// <summary>
+/// These control the results of the SendGrantValidator. <see cref="SendGrantValidator"/>
+/// </summary>
+internal enum SendGrantValidatorResultTypes
+{
+    ValidSendGuid,
+    MissingSendId,
+    InvalidSendId
+}

src/Identity/IdentityServer/RequestValidators/SendAccess/Enums/SendPasswordValidatorResultTypes.cs

@@ -0,0 +1,9 @@
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess.Enums;
+
+/// <summary>
+/// These control the results of the SendPasswordValidator. <see cref="SendPasswordRequestValidator"/>
+/// </summary>
+internal enum SendPasswordValidatorResultTypes
+{
+    RequestPasswordDoesNotMatch
+}

src/Identity/IdentityServer/RequestValidators/SendAccess/ISendPasswordRequestValidator.cs

@@ -0,0 +1,16 @@
+using Bit.Core.Tools.Models.Data;
+using Duende.IdentityServer.Validation;
+
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+
+public interface ISendPasswordRequestValidator
+{
+    /// <summary>
+    /// Validates the send password hash against the client hashed password.
+    /// If this method fails then it will automatically set the context.Result to an invalid grant result.
+    /// </summary>
+    /// <param name="context">request context</param>
+    /// <param name="resourcePassword">resource password authentication method containing the hash of the Send being retrieved</param>
+    /// <returns>returns the result of the validation; A failed result will be an error a successful will contain the claims and a success</returns>
+    GrantValidationResult ValidateSendPassword(ExtensionGrantValidationContext context, ResourcePassword resourcePassword, Guid sendId);
+}

src/Identity/IdentityServer/RequestValidators/SendAccess/SendAccessGrantValidator.cs

@@ -0,0 +1,150 @@
+using System.Security.Claims;
+using Bit.Core;
+using Bit.Core.Identity;
+using Bit.Core.Services;
+using Bit.Core.Tools.Models.Data;
+using Bit.Core.Tools.SendFeatures.Queries.Interfaces;
+using Bit.Core.Utilities;
+using Bit.Identity.IdentityServer.Enums;
+using Bit.Identity.IdentityServer.RequestValidators.SendAccess.Enums;
+using Duende.IdentityServer.Models;
+using Duende.IdentityServer.Validation;
+
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+
+public class SendAccessGrantValidator(
+    ISendAuthenticationQuery _sendAuthenticationQuery,
+    ISendPasswordRequestValidator _sendPasswordRequestValidator,
+    IFeatureService _featureService)
+: IExtensionGrantValidator
+{
+    string IExtensionGrantValidator.GrantType => CustomGrantTypes.SendAccess;
+
+    private static readonly Dictionary<SendGrantValidatorResultTypes, string>
+    _sendGrantValidatorErrors = new()
+    {
+        { SendGrantValidatorResultTypes.MissingSendId, "send_id is required." },
+        { SendGrantValidatorResultTypes.InvalidSendId, "send_id is invalid." }
+    };
+
+
+    public async Task ValidateAsync(ExtensionGrantValidationContext context)
+    {
+        // Check the feature flag
+        if (!_featureService.IsEnabled(FeatureFlagKeys.SendAccess))
+        {
+            context.Result = new GrantValidationResult(TokenRequestErrors.UnsupportedGrantType);
+            return;
+        }
+
+        var (sendIdGuid, result) = GetRequestSendId(context);
+        if (result != SendGrantValidatorResultTypes.ValidSendGuid)
+        {
+            context.Result = BuildErrorResult(result);
+            return;
+        }
+
+        // Look up send by id
+        var method = await _sendAuthenticationQuery.GetAuthenticationMethod(sendIdGuid);
+
+        switch (method)
+        {
+            case NeverAuthenticate:
+                // null send scenario.
+                // TODO PM-22675: Add send enumeration protection here (primarily benefits self hosted instances).
+                // We should only map to password or email + OTP protected.
+                // If user submits password guess for a falsely protected send, then we will return invalid password.
+                // If user submits email + OTP guess for a falsely protected send, then we will return email sent, do not actually send an email.
+                context.Result = BuildErrorResult(SendGrantValidatorResultTypes.InvalidSendId);
+                return;
+
+            case NotAuthenticated:
+                // automatically issue access token
+                context.Result = BuildBaseSuccessResult(sendIdGuid);
+                return;
+
+            case ResourcePassword rp:
+                // TODO PM-22675: Validate if the password is correct.
+                context.Result = _sendPasswordRequestValidator.ValidateSendPassword(context, rp, sendIdGuid);
+                return;
+            case EmailOtp eo:
+            // TODO PM-22678: We will either send the OTP here or validate it based on if otp exists in the request.
+            // SendOtpToEmail(eo.Emails) or ValidateOtp(eo.Emails);
+            // break;
+
+            default:
+                // shouldn’t ever hit this
+                throw new InvalidOperationException($"Unknown auth method: {method.GetType()}");
+        }
+    }
+
+    /// <summary>
+    /// tries to parse the send_id from the request.
+    /// If it is not present or invalid, sets the correct result error.
+    /// </summary>
+    /// <param name="context">request context</param>
+    /// <returns>a parsed sendId Guid and success result or a Guid.Empty and error type otherwise</returns>
+    private static (Guid, SendGrantValidatorResultTypes) GetRequestSendId(ExtensionGrantValidationContext context)
+    {
+        var request = context.Request.Raw;
+        var sendId = request.Get("send_id");
+
+        // if the sendId is null then the request is the wrong shape and the request is invalid
+        if (sendId == null)
+        {
+            return (Guid.Empty, SendGrantValidatorResultTypes.MissingSendId);
+        }
+        // the send_id is not null so the request is the correct shape, so we will attempt to parse it
+        try
+        {
+            var guidBytes = CoreHelpers.Base64UrlDecode(sendId);
+            var sendGuid = new Guid(guidBytes);
+            // Guid.Empty indicates an invalid send_id return invalid grant
+            if (sendGuid == Guid.Empty)
+            {
+                return (Guid.Empty, SendGrantValidatorResultTypes.InvalidSendId);
+            }
+            return (sendGuid, SendGrantValidatorResultTypes.ValidSendGuid);
+        }
+        catch
+        {
+            return (Guid.Empty, SendGrantValidatorResultTypes.InvalidSendId);
+        }
+    }
+
+    /// <summary>
+    /// Builds an error result for the specified error type.
+    /// </summary>
+    /// <param name="error">The error type.</param>
+    /// <returns>The error result.</returns>
+    private static GrantValidationResult BuildErrorResult(SendGrantValidatorResultTypes error)
+    {
+        return error switch
+        {
+            // Request is the wrong shape
+            SendGrantValidatorResultTypes.MissingSendId => new GrantValidationResult(
+                                TokenRequestErrors.InvalidRequest,
+                                errorDescription: _sendGrantValidatorErrors[SendGrantValidatorResultTypes.MissingSendId]),
+            // Request is correct shape but data is bad
+            SendGrantValidatorResultTypes.InvalidSendId => new GrantValidationResult(
+                                TokenRequestErrors.InvalidGrant,
+                                errorDescription: _sendGrantValidatorErrors[SendGrantValidatorResultTypes.InvalidSendId]),
+            // should never get here
+            _ => new GrantValidationResult(TokenRequestErrors.InvalidRequest)
+        };
+    }
+
+    private static GrantValidationResult BuildBaseSuccessResult(Guid sendId)
+    {
+        var claims = new List<Claim>
+        {
+            new(Claims.SendId, sendId.ToString()),
+            new(Claims.Type, IdentityClientType.Send.ToString())
+        };
+
+        return new GrantValidationResult(
+            subject: sendId.ToString(),
+            authenticationMethod: CustomGrantTypes.SendAccess,
+            claims: claims);
+    }
+}

src/Identity/IdentityServer/RequestValidators/SendAccess/SendPasswordRequestValidator.cs

@@ -0,0 +1,67 @@
+using System.Security.Claims;
+using Bit.Core.Identity;
+using Bit.Core.KeyManagement.Sends;
+using Bit.Core.Tools.Models.Data;
+using Bit.Identity.IdentityServer.Enums;
+using Bit.Identity.IdentityServer.RequestValidators.SendAccess.Enums;
+using Duende.IdentityServer.Models;
+using Duende.IdentityServer.Validation;
+
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+
+public class SendPasswordRequestValidator(ISendPasswordHasher sendPasswordHasher) : ISendPasswordRequestValidator
+{
+    private readonly ISendPasswordHasher _sendPasswordHasher = sendPasswordHasher;
+
+    /// <summary>
+    /// static object that contains the error messages for the SendPasswordRequestValidator.
+    /// </summary>
+    private static Dictionary<SendPasswordValidatorResultTypes, string> _sendPasswordValidatorErrors = new()
+    {
+        { SendPasswordValidatorResultTypes.RequestPasswordDoesNotMatch, "Request Password hash is invalid." }
+    };
+
+    public GrantValidationResult ValidateSendPassword(ExtensionGrantValidationContext context, ResourcePassword resourcePassword, Guid sendId)
+    {
+        var request = context.Request.Raw;
+        var clientHashedPassword = request.Get("password_hash");
+
+        if (string.IsNullOrEmpty(clientHashedPassword))
+        {
+            return new GrantValidationResult(
+                TokenRequestErrors.InvalidRequest,
+                errorDescription: _sendPasswordValidatorErrors[SendPasswordValidatorResultTypes.RequestPasswordDoesNotMatch]);
+        }
+
+        var hashMatches = _sendPasswordHasher.PasswordHashMatches(
+            resourcePassword.Hash, clientHashedPassword);
+
+        if (!hashMatches)
+        {
+            return new GrantValidationResult(
+                TokenRequestErrors.InvalidGrant,
+                errorDescription: _sendPasswordValidatorErrors[SendPasswordValidatorResultTypes.RequestPasswordDoesNotMatch]);
+        }
+
+        return BuildSendPasswordSuccessResult(sendId);
+    }
+
+    /// <summary>
+    /// Builds a successful validation result for the Send password send_access grant.
+    /// </summary>
+    /// <param name="sendId"></param>
+    /// <returns></returns>
+    private static GrantValidationResult BuildSendPasswordSuccessResult(Guid sendId)
+    {
+        var claims = new List<Claim>
+        {
+            new(Claims.SendId, sendId.ToString()),
+            new(Claims.Type, IdentityClientType.Send.ToString())
+        };
+
+        return new GrantValidationResult(
+            subject: sendId.ToString(),
+            authenticationMethod: CustomGrantTypes.SendAccess,
+            claims: claims);
+    }
+}

src/Identity/IdentityServer/StaticClientStore.cs

@@ -1,6 +1,7 @@
 using System.Collections.Frozen;
 using Bit.Core.Enums;
 using Bit.Core.Settings;
+using Bit.Identity.IdentityServer.StaticClients;
 using Duende.IdentityServer.Models;
 
 namespace Bit.Identity.IdentityServer;
@@ -9,16 +10,17 @@ public class StaticClientStore
 {
     public StaticClientStore(GlobalSettings globalSettings)
     {
-        ApiClients = new List<Client>
+        Clients = new List<Client>
         {
             new ApiClient(globalSettings, BitwardenClient.Mobile, 60, 1),
             new ApiClient(globalSettings, BitwardenClient.Web, 7, 1),
             new ApiClient(globalSettings, BitwardenClient.Browser, 30, 1),
             new ApiClient(globalSettings, BitwardenClient.Desktop, 30, 1),
             new ApiClient(globalSettings, BitwardenClient.Cli, 30, 1),
-            new ApiClient(globalSettings, BitwardenClient.DirectoryConnector, 30, 24)
+            new ApiClient(globalSettings, BitwardenClient.DirectoryConnector, 30, 24),
+            SendClientBuilder.Build(globalSettings),
         }.ToFrozenDictionary(c => c.ClientId);
     }
 
-    public FrozenDictionary<string, Client> ApiClients { get; }
+    public FrozenDictionary<string, Client> Clients { get; }
 }

src/Identity/IdentityServer/StaticClients/SendClientBuilder.cs

@@ -0,0 +1,31 @@
+using Bit.Core.Enums;
+using Bit.Core.IdentityServer;
+using Bit.Core.Settings;
+using Bit.Identity.IdentityServer.Enums;
+using Duende.IdentityServer.Models;
+
+namespace Bit.Identity.IdentityServer.StaticClients;
+public static class SendClientBuilder
+{
+    public static Client Build(GlobalSettings globalSettings)
+    {
+        return new Client()
+        {
+            ClientId = BitwardenClient.Send,
+            AllowedGrantTypes = [CustomGrantTypes.SendAccess],
+            AccessTokenLifetime = 60 * globalSettings.SendAccessTokenLifetimeInMinutes,
+
+            // Do not allow refresh tokens to be issued.
+            AllowOfflineAccess = false,
+
+            // Send is a public anonymous client, so no secret is required (or really possible to use securely).
+            RequireClientSecret = false,
+
+            // Allow web vault to use this client.
+            AllowedCorsOrigins = [globalSettings.BaseServiceUri.Vault],
+
+            // Setup API scopes that the client can request in the scope property of the token request.
+            AllowedScopes = [ApiScopes.ApiSendAccess],
+        };
+    }
+}

src/Identity/Utilities/ServiceCollectionExtensions.cs

@@ -5,6 +5,7 @@ using Bit.Core.Utilities;
 using Bit.Identity.IdentityServer;
 using Bit.Identity.IdentityServer.ClientProviders;
 using Bit.Identity.IdentityServer.RequestValidators;
+using Bit.Identity.IdentityServer.RequestValidators.SendAccess;
 using Bit.SharedWeb.Utilities;
 using Duende.IdentityServer.ResponseHandling;
 using Duende.IdentityServer.Services;
@@ -25,6 +26,7 @@ public static class ServiceCollectionExtensions
         services.AddTransient<IDeviceValidator, DeviceValidator>();
         services.AddTransient<ITwoFactorAuthenticationValidator, TwoFactorAuthenticationValidator>();
         services.AddTransient<ILoginApprovingClientTypes, LoginApprovingClientTypes>();
+        services.AddTransient<ISendPasswordRequestValidator, SendPasswordRequestValidator>();
 
         var issuerUri = new Uri(globalSettings.BaseServiceUri.InternalIdentity);
         var identityServerBuilder = services
@@ -55,7 +57,8 @@ public static class ServiceCollectionExtensions
             .AddResourceOwnerValidator<ResourceOwnerPasswordValidator>()
             .AddClientStore<DynamicClientStore>()
             .AddIdentityServerCertificate(env, globalSettings)
-            .AddExtensionGrantValidator<WebAuthnGrantValidator>();
+            .AddExtensionGrantValidator<WebAuthnGrantValidator>()
+            .AddExtensionGrantValidator<SendAccessGrantValidator>();
 
         if (!globalSettings.SelfHosted)
         {

test/Identity.IntegrationTest/RequestValidation/SendAccessGrantValidatorIntegrationTests.cs

@@ -0,0 +1,271 @@
+using Bit.Core;
+using Bit.Core.Enums;
+using Bit.Core.IdentityServer;
+using Bit.Core.Services;
+using Bit.Core.Tools.Models.Data;
+using Bit.Core.Tools.SendFeatures.Queries.Interfaces;
+using Bit.Core.Utilities;
+using Bit.Identity.IdentityServer.Enums;
+using Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+using Bit.IntegrationTestCommon.Factories;
+using Duende.IdentityServer.Validation;
+using NSubstitute;
+using Xunit;
+
+namespace Bit.Identity.IntegrationTest.RequestValidation;
+
+// in order to test the default case for the authentication method, we need to create a custom one so we can ensure the
+// method throws as expected.
+internal record AnUnknownAuthenticationMethod : SendAuthenticationMethod { }
+
+public class SendAccessGrantValidatorIntegrationTests(IdentityApplicationFactory factory) : IClassFixture<IdentityApplicationFactory>
+{
+    private readonly IdentityApplicationFactory _factory = factory;
+
+    [Fact]
+    public async Task SendAccessGrant_FeatureFlagDisabled_ReturnsUnsupportedGrantType()
+    {
+        // Arrange
+        var sendId = Guid.NewGuid();
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                // Mock feature service to return false
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(false);
+                services.AddSingleton(featureService);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId);
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("unsupported_grant_type", content);
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_ValidNotAuthenticatedSend_ReturnsAccessToken()
+    {
+        // Arrange
+        var sendId = Guid.NewGuid();
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                // Mock feature service to return true
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+
+                // Mock send authentication query
+                var sendAuthQuery = Substitute.For<ISendAuthenticationQuery>();
+                sendAuthQuery.GetAuthenticationMethod(sendId).Returns(new NotAuthenticated());
+                services.AddSingleton(sendAuthQuery);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId);
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        Assert.True(response.IsSuccessStatusCode);
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("access_token", content);
+        Assert.Contains("bearer", content.ToLower());
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_MissingSendId_ReturnsInvalidRequest()
+    {
+        // Arrange
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+            });
+        }).CreateClient();
+
+        var requestBody = new FormUrlEncodedContent([
+            new KeyValuePair<string, string>("grant_type", CustomGrantTypes.SendAccess),
+            new KeyValuePair<string, string>("client_id", BitwardenClient.Send)
+        ]);
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("invalid_request", content);
+        Assert.Contains("send_id is required", content);
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_EmptySendGuid_ReturnsInvalidGrant()
+    {
+        // Arrange
+        var sendId = Guid.Empty;
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId);
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("invalid_grant", content);
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_NeverAuthenticateSend_ReturnsInvalidGrant()
+    {
+        // Arrange
+        var sendId = Guid.NewGuid();
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+
+                var sendAuthQuery = Substitute.For<ISendAuthenticationQuery>();
+                sendAuthQuery.GetAuthenticationMethod(sendId).Returns(new NeverAuthenticate());
+                services.AddSingleton(sendAuthQuery);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId);
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("invalid_grant", content);
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_UnknownAuthenticationMethod_ThrowsInvalidOperation()
+    {
+        // Arrange
+        var sendId = Guid.NewGuid();
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+
+                var sendAuthQuery = Substitute.For<ISendAuthenticationQuery>();
+                sendAuthQuery.GetAuthenticationMethod(sendId).Returns(new AnUnknownAuthenticationMethod());
+                services.AddSingleton(sendAuthQuery);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId);
+
+        // Act
+        var error = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        // We want to parse the response and ensure we get the correct error from the server
+        var content = await error.Content.ReadAsStringAsync();
+        Assert.Contains("invalid_grant", content);
+    }
+
+    [Fact]
+    public async Task SendAccessGrant_PasswordProtectedSend_CallsPasswordValidator()
+    {
+        // Arrange
+        var sendId = Guid.NewGuid();
+        var resourcePassword = new ResourcePassword("test-password-hash");
+        var client = _factory.WithWebHostBuilder(builder =>
+        {
+            builder.ConfigureServices(services =>
+            {
+                var featureService = Substitute.For<IFeatureService>();
+                featureService.IsEnabled(FeatureFlagKeys.SendAccess).Returns(true);
+                services.AddSingleton(featureService);
+
+                var sendAuthQuery = Substitute.For<ISendAuthenticationQuery>();
+                sendAuthQuery.GetAuthenticationMethod(sendId).Returns(resourcePassword);
+                services.AddSingleton(sendAuthQuery);
+
+                // Mock password validator to return success
+                var passwordValidator = Substitute.For<ISendPasswordRequestValidator>();
+                passwordValidator.ValidateSendPassword(
+                    Arg.Any<ExtensionGrantValidationContext>(),
+                    Arg.Any<ResourcePassword>(),
+                    Arg.Any<Guid>())
+                    .Returns(new GrantValidationResult(
+                        subject: sendId.ToString(),
+                        authenticationMethod: CustomGrantTypes.SendAccess));
+                services.AddSingleton(passwordValidator);
+            });
+        }).CreateClient();
+
+        var requestBody = CreateTokenRequestBody(sendId, "password123");
+
+        // Act
+        var response = await client.PostAsync("/connect/token", requestBody);
+
+        // Assert
+        Assert.True(response.IsSuccessStatusCode);
+        var content = await response.Content.ReadAsStringAsync();
+        Assert.Contains("access_token", content);
+        Assert.Contains("Bearer", content);
+    }
+
+    private static FormUrlEncodedContent CreateTokenRequestBody(
+        Guid sendId,
+        string password = null,
+        string sendEmail = null,
+        string emailOtp = null)
+    {
+        var sendIdBase64 = CoreHelpers.Base64UrlEncode(sendId.ToByteArray());
+        var parameters = new List<KeyValuePair<string, string>>
+        {
+            new("grant_type", CustomGrantTypes.SendAccess),
+            new("client_id", BitwardenClient.Send ),
+            new("scope", ApiScopes.ApiSendAccess),
+            new("deviceType", ((int)DeviceType.FirefoxBrowser).ToString()),
+            new("send_id", sendIdBase64)
+        };
+
+        if (!string.IsNullOrEmpty(password))
+        {
+            parameters.Add(new("password_hash", password));
+        }
+
+        if (!string.IsNullOrEmpty(emailOtp) && !string.IsNullOrEmpty(sendEmail))
+        {
+            parameters.AddRange(
+            [
+                new KeyValuePair<string, string>("email", sendEmail),
+                new KeyValuePair<string, string>("email_otp", emailOtp)
+            ]);
+        }
+
+        return new FormUrlEncodedContent(parameters);
+    }
+}

test/Identity.IntegrationTest/openid-configuration.json

@@ -15,6 +15,7 @@
     "api.installation",
     "internal",
     "api.secrets",
+    "api.send.access",
     "offline_access"
   ],
   "claims_supported": [
@@ -33,6 +34,7 @@
     "providerserviceuser",
     "accesssecretsmanager",
     "sub",
+    "send_id",
     "organization"
   ],
   "grant_types_supported": [
@@ -43,7 +45,8 @@
     "password",
     "urn:ietf:params:oauth:grant-type:device_code",
     "urn:openid:params:grant-type:ciba",
-    "webauthn"
+    "webauthn",
+    "send_access"
   ],
   "response_types_supported": [
     "code",

test/Identity.Test/IdentityServer/SendAccessGrantValidatorTests.cs

@@ -0,0 +1,333 @@
+using System.Collections.Specialized;
+using Bit.Core;
+using Bit.Core.Enums;
+using Bit.Core.Identity;
+using Bit.Core.IdentityServer;
+using Bit.Core.Services;
+using Bit.Core.Tools.Models.Data;
+using Bit.Core.Tools.SendFeatures.Queries.Interfaces;
+using Bit.Core.Utilities;
+using Bit.Identity.IdentityServer.Enums;
+using Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+using Bit.Test.Common.AutoFixture;
+using Bit.Test.Common.AutoFixture.Attributes;
+using Duende.IdentityServer.Extensions;
+using Duende.IdentityServer.Validation;
+using IdentityModel;
+using NSubstitute;
+using Xunit;
+
+namespace Bit.Identity.Test.IdentityServer;
+
+[SutProviderCustomize]
+public class SendAccessGrantValidatorTests
+{
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_FeatureFlagDisabled_ReturnsUnsupportedGrantType(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider)
+    {
+        // Arrange
+        sutProvider.GetDependency<IFeatureService>()
+            .IsEnabled(FeatureFlagKeys.SendAccess)
+            .Returns(false);
+
+        var context = new ExtensionGrantValidationContext
+        {
+            Request = tokenRequest
+        };
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.True(context.Result.IsError);
+        Assert.Equal(OidcConstants.TokenErrors.UnsupportedGrantType, context.Result.Error);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_MissingSendId_ReturnsInvalidRequest(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider)
+    {
+        // Arrange
+        sutProvider.GetDependency<IFeatureService>()
+            .IsEnabled(FeatureFlagKeys.SendAccess)
+            .Returns(true);
+
+        var context = new ExtensionGrantValidationContext
+        {
+            Request = tokenRequest
+        };
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.Equal(OidcConstants.TokenErrors.InvalidRequest, context.Result.Error);
+        Assert.Equal("send_id is required.", context.Result.ErrorDescription);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_InvalidSendId_ReturnsInvalidGrant(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider)
+    {
+        // Arrange
+        sutProvider.GetDependency<IFeatureService>()
+            .IsEnabled(FeatureFlagKeys.SendAccess)
+            .Returns(true);
+
+        var context = new ExtensionGrantValidationContext();
+
+        tokenRequest.GrantType = CustomGrantTypes.SendAccess;
+        tokenRequest.Raw = CreateTokenRequestBody(Guid.Empty);
+
+        // To preserve the CreateTokenRequestBody method for more general usage we over write the sendId
+        tokenRequest.Raw.Set("send_id", "invalid-guid-format");
+        context.Request = tokenRequest;
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.Equal(OidcConstants.TokenErrors.InvalidGrant, context.Result.Error);
+        Assert.Equal("send_id is invalid.", context.Result.ErrorDescription);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_EmptyGuidSendId_ReturnsInvalidGrant(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            Guid.Empty, // Empty Guid as sendId
+            tokenRequest);
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.Equal(OidcConstants.TokenErrors.InvalidGrant, context.Result.Error);
+        Assert.Equal("send_id is invalid.", context.Result.ErrorDescription);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_NeverAuthenticateMethod_ReturnsInvalidGrant(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            sendId,
+            tokenRequest);
+
+        sutProvider.GetDependency<ISendAuthenticationQuery>()
+            .GetAuthenticationMethod(sendId)
+            .Returns(new NeverAuthenticate());
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.Equal(OidcConstants.TokenErrors.InvalidGrant, context.Result.Error);
+        Assert.Equal("send_id is invalid.", context.Result.ErrorDescription);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_NotAuthenticatedMethod_ReturnsSuccess(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            sendId,
+            tokenRequest);
+
+        sutProvider.GetDependency<ISendAuthenticationQuery>()
+            .GetAuthenticationMethod(sendId)
+            .Returns(new NotAuthenticated());
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.False(context.Result.IsError);
+        // get the claims principal from the result
+        var subject = context.Result.Subject;
+        Assert.NotNull(subject);
+        Assert.Equal(sendId.ToString(), subject.GetSubjectId());
+        Assert.Equal(CustomGrantTypes.SendAccess, subject.GetAuthenticationMethod());
+        // get the claims from the subject
+        var claims = subject.Claims.ToList();
+        Assert.NotEmpty(claims);
+        Assert.Contains(claims, c => c.Type == Claims.SendId && c.Value == sendId.ToString());
+        Assert.Contains(claims, c => c.Type == Claims.Type && c.Value == IdentityClientType.Send.ToString());
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_ResourcePasswordMethod_CallsPasswordValidator(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId,
+        ResourcePassword resourcePassword,
+        GrantValidationResult expectedResult)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            sendId,
+            tokenRequest);
+
+        sutProvider.GetDependency<ISendAuthenticationQuery>()
+            .GetAuthenticationMethod(sendId)
+            .Returns(resourcePassword);
+
+        sutProvider.GetDependency<ISendPasswordRequestValidator>()
+            .ValidateSendPassword(context, resourcePassword, sendId)
+            .Returns(expectedResult);
+
+        // Act
+        await sutProvider.Sut.ValidateAsync(context);
+
+        // Assert
+        Assert.Equal(expectedResult, context.Result);
+        sutProvider.GetDependency<ISendPasswordRequestValidator>()
+            .Received(1)
+            .ValidateSendPassword(context, resourcePassword, sendId);
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_EmailOtpMethod_NotImplemented_ThrowsError(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId,
+        EmailOtp emailOtp)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            sendId,
+            tokenRequest);
+
+
+        sutProvider.GetDependency<ISendAuthenticationQuery>()
+            .GetAuthenticationMethod(sendId)
+            .Returns(emailOtp);
+
+        // Act
+        // Assert
+        // Currently the EmailOtp case doesn't set a result, so it should be null
+        await Assert.ThrowsAsync<InvalidOperationException>(async () => await sutProvider.Sut.ValidateAsync(context));
+    }
+
+    [Theory, BitAutoData]
+    public async Task ValidateAsync_UnknownAuthMethod_ThrowsInvalidOperationException(
+        [AutoFixture.ValidatedTokenRequest] ValidatedTokenRequest tokenRequest,
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId)
+    {
+        // Arrange
+        var context = SetupTokenRequest(
+            sutProvider,
+            sendId,
+            tokenRequest);
+
+        // Create a mock authentication method that's not handled
+        var unknownMethod = Substitute.For<SendAuthenticationMethod>();
+        sutProvider.GetDependency<ISendAuthenticationQuery>()
+            .GetAuthenticationMethod(sendId)
+            .Returns(unknownMethod);
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => sutProvider.Sut.ValidateAsync(context));
+
+        Assert.StartsWith("Unknown auth method:", exception.Message);
+    }
+
+    [Fact]
+    public void GrantType_ReturnsCorrectType()
+    {
+        // Arrange & Act
+        var validator = new SendAccessGrantValidator(null!, null!, null!);
+
+        // Assert
+        Assert.Equal(CustomGrantTypes.SendAccess, ((IExtensionGrantValidator)validator).GrantType);
+    }
+
+    /// <summary>
+    /// Mutator method fo the SutProvider and the Context to set up a valid request
+    /// </summary>
+    /// <param name="sutProvider">current sut provider</param>
+    /// <param name="context">test context</param>
+    /// <param name="sendId">the send id</param>
+    /// <param name="request">the token request</param>
+    private static ExtensionGrantValidationContext SetupTokenRequest(
+        SutProvider<SendAccessGrantValidator> sutProvider,
+        Guid sendId,
+        ValidatedTokenRequest request)
+    {
+        sutProvider.GetDependency<IFeatureService>()
+            .IsEnabled(FeatureFlagKeys.SendAccess)
+            .Returns(true);
+
+        var context = new ExtensionGrantValidationContext();
+
+        request.GrantType = CustomGrantTypes.SendAccess;
+        request.Raw = CreateTokenRequestBody(sendId);
+        context.Request = request;
+
+        return context;
+    }
+
+    private static NameValueCollection CreateTokenRequestBody(
+        Guid sendId,
+        string passwordHash = null,
+        string sendEmail = null,
+        string otpCode = null)
+    {
+        var sendIdBase64 = CoreHelpers.Base64UrlEncode(sendId.ToByteArray());
+
+        var rawRequestParameters = new NameValueCollection
+        {
+            { "grant_type", CustomGrantTypes.SendAccess },
+            { "client_id", BitwardenClient.Send },
+            { "scope", ApiScopes.ApiSendAccess },
+            { "deviceType", ((int)DeviceType.FirefoxBrowser).ToString() },
+            { "send_id", sendIdBase64 }
+        };
+
+        if (passwordHash != null)
+        {
+            rawRequestParameters.Add("password_hash", passwordHash);
+        }
+
+        if (sendEmail != null)
+        {
+            rawRequestParameters.Add("send_email", sendEmail);
+        }
+
+        if (otpCode != null && sendEmail != null)
+        {
+            rawRequestParameters.Add("otp_code", otpCode);
+        }
+
+        return rawRequestParameters;
+    }
+
+    // we need a list of sendAuthentication methods to test against since we cannot create new objects in the BitAutoData
+    public static Dictionary<string, SendAuthenticationMethod> SendAuthenticationMethods => new()
+    {
+        { "NeverAuthenticate", new NeverAuthenticate() },       // Send doesn't exist or is deleted
+        { "NotAuthenticated", new NotAuthenticated() },         // Public send, no auth needed
+        // TODO: PM-22675 - {"ResourcePassword", new ResourcePassword("clientHashedPassword")};  // Password protected send
+        // TODO: PM-22678 - {"EmailOtp", new EmailOtp(["emailOtp@test.dev"]};    // Email + OTP protected send
+    };
+}