diff --git a/services/notification-service/src/main/kotlin/com/otterworks/notification/model/NotificationEvent.kt b/services/notification-service/src/main/kotlin/com/otterworks/notification/model/NotificationEvent.kt
index eeebd8744..5a6a399ae 100644
--- a/services/notification-service/src/main/kotlin/com/otterworks/notification/model/NotificationEvent.kt
+++ b/services/notification-service/src/main/kotlin/com/otterworks/notification/model/NotificationEvent.kt
@@ -92,7 +92,7 @@ data class UnreadCountResponse(
 
 @Serializable
 data class NotificationPreferenceRequest(
-    val userId: String,
+    val userId: String? = null,
     val eventType: String,
     val channels: List<DeliveryChannel>,
 )
diff --git a/services/notification-service/src/main/kotlin/com/otterworks/notification/routes/Routes.kt b/services/notification-service/src/main/kotlin/com/otterworks/notification/routes/Routes.kt
index 2d265b602..38323a054 100644
--- a/services/notification-service/src/main/kotlin/com/otterworks/notification/routes/Routes.kt
+++ b/services/notification-service/src/main/kotlin/com/otterworks/notification/routes/Routes.kt
@@ -152,9 +152,20 @@ fun Application.configureRouting(prometheusRegistry: PrometheusMeterRegistry) {
             }
 
             put {
+                val userId = call.request.headers["X-User-ID"]
+                if (userId.isNullOrBlank()) {
+                    call.respond(HttpStatusCode.Unauthorized, ErrorResponse("X-User-ID header is required"))
+                    return@put
+                }
+
                 val request = call.receive<NotificationPreferenceRequest>()
+                if (request.userId != null && request.userId != userId) {
+                    call.respond(HttpStatusCode.Forbidden, ErrorResponse("Cannot update preferences for another user"))
+                    return@put
+                }
+
                 notificationService.updatePreferences(
-                    userId = request.userId,
+                    userId = userId,
                     eventType = request.eventType,
                     channels = request.channels,
                 )
diff --git a/services/notification-service/src/test/kotlin/com/otterworks/notification/routes/PreferencesRouteTest.kt b/services/notification-service/src/test/kotlin/com/otterworks/notification/routes/PreferencesRouteTest.kt
new file mode 100644
index 000000000..05f810669
--- /dev/null
+++ b/services/notification-service/src/test/kotlin/com/otterworks/notification/routes/PreferencesRouteTest.kt
@@ -0,0 +1,110 @@
+package com.otterworks.notification.routes
+
+import com.otterworks.notification.model.DeliveryChannel
+import com.otterworks.notification.service.NotificationService
+import com.otterworks.notification.websocket.WebSocketManager
+import io.ktor.client.request.header
+import io.ktor.client.request.put
+import io.ktor.client.request.setBody
+import io.ktor.http.ContentType
+import io.ktor.http.HttpStatusCode
+import io.ktor.http.contentType
+import io.ktor.serialization.kotlinx.json.json
+import io.ktor.server.application.install
+import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
+import io.ktor.server.testing.ApplicationTestBuilder
+import io.ktor.server.testing.testApplication
+import io.ktor.server.websocket.WebSockets
+import io.micrometer.prometheus.PrometheusConfig
+import io.micrometer.prometheus.PrometheusMeterRegistry
+import io.mockk.coVerify
+import io.mockk.mockk
+import kotlinx.serialization.json.Json
+import org.koin.core.context.stopKoin
+import org.koin.dsl.module
+import org.koin.ktor.plugin.Koin
+import kotlin.test.AfterTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class PreferencesRouteTest {
+
+    private val notificationService = mockk<NotificationService>(relaxed = true)
+    private val webSocketManager = mockk<WebSocketManager>(relaxed = true)
+
+    @AfterTest
+    fun tearDown() {
+        stopKoin()
+    }
+
+    private fun ApplicationTestBuilder.setupApp() {
+        application {
+            install(ContentNegotiation) { json(Json { ignoreUnknownKeys = true }) }
+            install(WebSockets)
+            install(Koin) {
+                modules(
+                    module {
+                        single { notificationService }
+                        single { webSocketManager }
+                    }
+                )
+            }
+            configureRouting(PrometheusMeterRegistry(PrometheusConfig.DEFAULT))
+        }
+    }
+
+    @Test
+    fun `PUT preferences uses the authenticated X-User-ID, not the body userId`() = testApplication {
+        setupApp()
+
+        val response = client.put("/api/v1/preferences") {
+            header("X-User-ID", "alice")
+            contentType(ContentType.Application.Json)
+            setBody("""{"eventType":"file_shared","channels":["IN_APP"]}""")
+        }
+
+        assertEquals(HttpStatusCode.NoContent, response.status)
+        coVerify(exactly = 1) { notificationService.updatePreferences("alice", "file_shared", listOf(DeliveryChannel.IN_APP)) }
+    }
+
+    @Test
+    fun `PUT preferences rejects a body userId that differs from the caller`() = testApplication {
+        setupApp()
+
+        val response = client.put("/api/v1/preferences") {
+            header("X-User-ID", "attacker")
+            contentType(ContentType.Application.Json)
+            setBody("""{"userId":"victim","eventType":"file_shared","channels":[]}""")
+        }
+
+        assertEquals(HttpStatusCode.Forbidden, response.status)
+        coVerify(exactly = 0) { notificationService.updatePreferences(any(), any(), any()) }
+    }
+
+    @Test
+    fun `PUT preferences accepts a body userId that matches the caller`() = testApplication {
+        setupApp()
+
+        val response = client.put("/api/v1/preferences") {
+            header("X-User-ID", "alice")
+            contentType(ContentType.Application.Json)
+            setBody("""{"userId":"alice","eventType":"comment_added","channels":["EMAIL"]}""")
+        }
+
+        assertEquals(HttpStatusCode.NoContent, response.status)
+        coVerify(exactly = 1) { notificationService.updatePreferences("alice", "comment_added", listOf(DeliveryChannel.EMAIL)) }
+    }
+
+    @Test
+    fun `PUT preferences without an authenticated identity is rejected`() = testApplication {
+        setupApp()
+
+        val response = client.put("/api/v1/preferences") {
+            contentType(ContentType.Application.Json)
+            setBody("""{"userId":"victim","eventType":"file_shared","channels":[]}""")
+        }
+
+        assertEquals(HttpStatusCode.Unauthorized, response.status)
+        coVerify(exactly = 0) { notificationService.updatePreferences(any(), any(), any()) }
+    }
+}