diff --git a/dotnet/EcencyApi.Tests/DeviceAuthorizationTests.cs b/dotnet/EcencyApi.Tests/DeviceAuthorizationTests.cs
new file mode 100644
index 00000000..c4b355ff
--- /dev/null
+++ b/dotnet/EcencyApi.Tests/DeviceAuthorizationTests.cs
@@ -0,0 +1,148 @@
+using EcencyApi.Handlers;
+using Xunit;
+
+namespace EcencyApi.Tests;
+
+/// <summary>
+/// /private-api/register-device and /private-api/detail-device act on a push
+/// registration, which belongs to the account the code was issued for.
+/// </summary>
+public class DeviceAuthorizationTests
+{
+    [Theory]
+    [InlineData("good-karma")]
+    // Hive names are lowercase, but the comparison must not hinge on that.
+    [InlineData("Good-Karma")]
+    [InlineData("GOOD-KARMA")]
+    public void TheCodesOwnAccountIsAccepted(string requested)
+    {
+        Assert.True(PrivateApi.IsOwnDeviceRequest("good-karma", requested));
+    }
+
+    [Theory]
+    [InlineData("someone-else")]
+    [InlineData("good-karm")]
+    [InlineData("good-karma2")]
+    [InlineData(" good-karma")]
+    [InlineData("good_karma")]
+    public void AnyOtherAccountIsRefused(string requested)
+    {
+        Assert.False(PrivateApi.IsOwnDeviceRequest("good-karma", requested));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    public void AMissingUsernameIsRefused(string? requested)
+    {
+        Assert.False(PrivateApi.IsOwnDeviceRequest("good-karma", requested));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    public void WithoutAValidatedCodeNothingIsAccepted(string? validated)
+    {
+        Assert.False(PrivateApi.IsOwnDeviceRequest(validated, "good-karma"));
+        Assert.False(PrivateApi.IsOwnDeviceRequest(validated, validated));
+    }
+}
+
+/// <summary>
+/// The device handlers themselves: a code for one account cannot register or read a
+/// device row under another, and nothing reaches upstream when it tries.
+/// </summary>
+[Collection("device-auth")]
+public class DeviceHandlerTests : IDisposable
+{
+    private readonly CurationDeskTestSupport.Recorder _upstream = new();
+
+    public DeviceHandlerTests()
+    {
+        // `code` "as:alice" validates as alice; anything else is invalid.
+        PrivateApi.DeviceValidateCode = body =>
+        {
+            var code = body["code"]?.GetValue<string>();
+            return Task.FromResult(code != null && code.StartsWith("as:", StringComparison.Ordinal) ? code[3..] : null);
+        };
+        PrivateApi.DeviceUpstream = (endpoint, method, payload) =>
+            _upstream.Handle(endpoint, method, Array.Empty<KeyValuePair<string, string>>(), payload);
+    }
+
+    public void Dispose()
+    {
+        PrivateApi.DeviceValidateCode = PrivateApi.ValidateCode;
+        PrivateApi.DeviceUpstream = (endpoint, method, payload) =>
+            EcencyApi.Infrastructure.ApiClient.ApiRequest(endpoint, method, null, payload);
+    }
+
+    private static string Register(string code, string? username) =>
+        username == null
+            ? $$"""{"code":"{{code}}","token":"t1","system":"fcm-ios","allows_notify":1,"notify_types":[1]}"""
+            : $$"""{"code":"{{code}}","username":"{{username}}","token":"t1","system":"fcm-ios","allows_notify":1,"notify_types":[1]}""";
+
+    [Theory]
+    [InlineData("alice")]
+    [InlineData("Alice")]
+    public async Task RegisteringTheCodesOwnAccountIsForwarded(string username)
+    {
+        var ctx = CurationDeskTestSupport.Post("/private-api/register-device", Register("as:alice", username));
+        await PrivateApi.RegisterDevice(ctx);
+
+        Assert.Equal(200, ctx.Response.StatusCode);
+        var call = Assert.Single(_upstream.Calls);
+        Assert.Equal("rgstrmbldvc/", call.Endpoint);
+        Assert.Equal(HttpMethod.Post, call.Method);
+        Assert.Equal(username, call.Payload?["username"]?.GetValue<string>());
+        Assert.Equal("t1", call.Payload?["token"]?.GetValue<string>());
+    }
+
+    [Theory]
+    [InlineData("bob")]
+    [InlineData(null)]
+    public async Task RegisteringAnotherAccountIsRefused(string? username)
+    {
+        var ctx = CurationDeskTestSupport.Post("/private-api/register-device", Register("as:alice", username));
+        await PrivateApi.RegisterDevice(ctx);
+
+        Assert.Equal(403, ctx.Response.StatusCode);
+        Assert.Empty(_upstream.Calls);
+    }
+
+    [Fact]
+    public async Task RegisteringWithoutAValidCodeIsUnauthorized()
+    {
+        var ctx = CurationDeskTestSupport.Post("/private-api/register-device", Register("bogus", "alice"));
+        await PrivateApi.RegisterDevice(ctx);
+
+        Assert.Equal(401, ctx.Response.StatusCode);
+        Assert.Empty(_upstream.Calls);
+    }
+
+    [Fact]
+    public async Task ReadingTheCodesOwnDeviceIsForwarded()
+    {
+        var ctx = CurationDeskTestSupport.Post(
+            "/private-api/detail-device", """{"code":"as:alice","username":"alice","token":"t1"}""");
+        await PrivateApi.DetailDevice(ctx);
+
+        Assert.Equal(200, ctx.Response.StatusCode);
+        var call = Assert.Single(_upstream.Calls);
+        Assert.Equal("mbldvcdtl/alice/t1", call.Endpoint);
+        Assert.Equal(HttpMethod.Get, call.Method);
+    }
+
+    [Fact]
+    public async Task ReadingAnotherAccountsDeviceIsRefused()
+    {
+        var ctx = CurationDeskTestSupport.Post(
+            "/private-api/detail-device", """{"code":"as:alice","username":"bob","token":"t1"}""");
+        await PrivateApi.DetailDevice(ctx);
+
+        Assert.Equal(403, ctx.Response.StatusCode);
+        Assert.Empty(_upstream.Calls);
+    }
+}
+
+[CollectionDefinition("device-auth", DisableParallelization = true)]
+public class DeviceAuthCollection { }
diff --git a/dotnet/EcencyApi/Handlers/PrivateApi.UserData1.cs b/dotnet/EcencyApi/Handlers/PrivateApi.UserData1.cs
index f99969d2..5d8e0c14 100644
--- a/dotnet/EcencyApi/Handlers/PrivateApi.UserData1.cs
+++ b/dotnet/EcencyApi/Handlers/PrivateApi.UserData1.cs
@@ -41,6 +41,22 @@ public static (string? Username, bool FullScope) ResolveNotificationsTarget(
             string.Equals(requestedUser, validatedUsername, StringComparison.OrdinalIgnoreCase));
     }
 
+    /// <summary>
+    /// Whether a device request may act on `requestedUsername`. A push registration
+    /// belongs to the account the code was issued for, so the device endpoints only
+    /// accept that account. Same case rule as ResolveNotificationsTarget.
+    /// </summary>
+    public static bool IsOwnDeviceRequest(string? validatedUsername, string? requestedUsername) =>
+        !string.IsNullOrEmpty(validatedUsername)
+        && string.Equals(requestedUsername, validatedUsername, StringComparison.OrdinalIgnoreCase);
+
+    /// <summary>Signed-code validation for the device routes, replaceable for tests (no chain RPC).</summary>
+    internal static Func<JsonObject, Task<string?>> DeviceValidateCode = ValidateCode;
+
+    /// <summary>The device routes' upstream call, replaceable so tests can observe it.</summary>
+    internal static Func<string, HttpMethod, JsonNode?, Task<UpstreamResponse>> DeviceUpstream =
+        (endpoint, method, payload) => ApiClient.ApiRequest(endpoint, method, null, payload);
+
     /// <summary>Header enotify reads the shared secret from.</summary>
     public const string EnotifyInternalTokenHeader = "X-Ecency-Internal-Token";
 
@@ -193,37 +209,49 @@ public static async Task MarkNotifications(HttpContext ctx)
     public static async Task RegisterDevice(HttpContext ctx)
     {
         var body = await ctx.ReadBody();
-        var authedUsername = await ValidateCode(body);
+        var authedUsername = await DeviceValidateCode(body);
         if (string.IsNullOrEmpty(authedUsername))
         {
             await ctx.SendText(401, "Unauthorized");
             return;
         }
 
-        // Payload takes the fields from the request body, not the authed user.
+        if (!IsOwnDeviceRequest(authedUsername, body.Str("username")))
+        {
+            await ctx.SendText(403, "Forbidden");
+            return;
+        }
+
+        // Payload takes the fields from the request body; username matches the authed user.
         var data = new JsonObject();
         UserData1Helpers.CopyIfPresent(data, body, "username");
         UserData1Helpers.CopyIfPresent(data, body, "token");
         UserData1Helpers.CopyIfPresent(data, body, "system");
         UserData1Helpers.CopyIfPresent(data, body, "allows_notify");
         UserData1Helpers.CopyIfPresent(data, body, "notify_types");
-        await Upstream.Pipe(ApiClient.ApiRequest("rgstrmbldvc/", HttpMethod.Post, null, data), ctx);
+        await Upstream.Pipe(DeviceUpstream("rgstrmbldvc/", HttpMethod.Post, data), ctx);
     }
 
     // POST ^/private-api/detail-device$
     public static async Task DetailDevice(HttpContext ctx)
     {
         var body = await ctx.ReadBody();
-        var authedUsername = await ValidateCode(body);
+        var authedUsername = await DeviceValidateCode(body);
         if (string.IsNullOrEmpty(authedUsername))
         {
             await ctx.SendText(401, "Unauthorized");
             return;
         }
 
+        if (!IsOwnDeviceRequest(authedUsername, body.Str("username")))
+        {
+            await ctx.SendText(403, "Forbidden");
+            return;
+        }
+
         var username = UserData1Helpers.TemplateOf(body, "username");
         var token = UserData1Helpers.TemplateOf(body, "token");
-        await Upstream.Pipe(ApiClient.ApiRequest($"mbldvcdtl/{username}/{token}", HttpMethod.Get), ctx);
+        await Upstream.Pipe(DeviceUpstream($"mbldvcdtl/{username}/{token}", HttpMethod.Get, null), ctx);
     }
 
     // POST ^/private-api/images$