diff --git a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/HostValidation.kt b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/HostValidation.kt
index 88b56bc1..ef010fe6 100644
--- a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/HostValidation.kt
+++ b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/HostValidation.kt
@@ -19,6 +19,16 @@ import io.modelcontextprotocol.kotlin.sdk.types.RPCError
  */
 internal val LOCALHOST_ALLOWED_HOSTS: List<String> = listOf("localhost", "127.0.0.1", "[::1]")
 
+/**
+ * Default list of `Origin` values allowed for localhost DNS rebinding protection.
+ *
+ * Mirrors [LOCALHOST_ALLOWED_HOSTS] but carries a scheme so the values parse as URLs:
+ * [extractOriginHost] rejects schemeless input. Comparison is hostname-only and
+ * scheme-agnostic, so these entries also match `https://` origins on the same host.
+ */
+internal val LOCALHOST_ALLOWED_ORIGINS: List<String> =
+    listOf("http://localhost", "http://127.0.0.1", "http://[::1]")
+
 /**
  * Characters that are valid in a URL but must not appear in an HTTP `Host` header.
  * Rejecting them prevents the parser from accepting malformed values
diff --git a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt
index d6191dff..7e47d765 100644
--- a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt
+++ b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt
@@ -41,7 +41,9 @@ private val logger = KotlinLogging.logger {}
  * @param allowedHosts hostnames allowed in the `Host` header. Defaults to `localhost`, `127.0.0.1`, `[::1]`.
  * @param allowedOrigins origins allowed in the `Origin` header, compared by hostname only
  *      (scheme and port are ignored). Requests without an `Origin` header are allowed.
- *      Pass `null` to skip origin validation.
+ *      When `null` while the localhost host defaults are in effect (no custom `allowedHosts`),
+ *      the `Origin` header is validated against `localhost`, `127.0.0.1`, `[::1]`.
+ *      With custom `allowedHosts`, `null` skips origin validation.
  * @param block factory block with access to the [ServerSSESession]
  *      that creates and returns the [Server] to handle the connection.
  * @throws IllegalStateException if the [SSE] plugin is not installed.
@@ -71,7 +73,9 @@ public fun Route.mcp(
  * @param allowedHosts hostnames allowed in the `Host` header. Defaults to `localhost`, `127.0.0.1`, `[::1]`.
  * @param allowedOrigins origins allowed in the `Origin` header, compared by hostname only
  *      (scheme and port are ignored). Requests without an `Origin` header are allowed.
- *      Pass `null` to skip origin validation.
+ *      When `null` while the localhost host defaults are in effect (no custom `allowedHosts`),
+ *      the `Origin` header is validated against `localhost`, `127.0.0.1`, `[::1]`.
+ *      With custom `allowedHosts`, `null` skips origin validation.
  * @param block factory block with access to the [ServerSSESession]
  *      that creates and returns the [Server] to handle the connection.
  * @throws IllegalStateException if the [SSE] plugin is not installed.
@@ -119,7 +123,9 @@ public fun Route.mcp(
  * @param allowedHosts hostnames allowed in the `Host` header. Defaults to `localhost`, `127.0.0.1`, `[::1]`.
  * @param allowedOrigins origins allowed in the `Origin` header, compared by hostname only
  *      (scheme and port are ignored). Requests without an `Origin` header are allowed.
- *      Pass `null` to skip origin validation.
+ *      When `null` while the localhost host defaults are in effect (no custom `allowedHosts`),
+ *      the `Origin` header is validated against `localhost`, `127.0.0.1`, `[::1]`.
+ *      With custom `allowedHosts`, `null` skips origin validation.
  * @param block factory block with access to the [ServerSSESession]
  *      that creates and returns the [Server] to handle the connection.
  */
@@ -205,7 +211,9 @@ private fun Application.mcpStreamableHttp(
  *          If `null` and DNS rebinding protection is enabled, defaults to `localhost`, `127.0.0.1`, `[::1]`.
  * @param allowedOrigins A list of allowed `Origin` header values, compared by hostname only
  *          (scheme and port are ignored). Requests without an `Origin` header are allowed.
- *          If `null`, origin validation is disabled.
+ *          When `null` while the localhost host defaults are in effect (no custom `allowedHosts`),
+ *          the `Origin` header is validated against `localhost`, `127.0.0.1`, `[::1]`.
+ *          With custom `allowedHosts`, `null` skips origin validation.
  * @param eventStore An optional [EventStore] instance to enable resumable event stream functionality.
  *          Allows storing and replaying events.
  * @param block factory block with access to the [RoutingContext] (for reading request headers)
@@ -451,6 +459,9 @@ private fun Route.installDnsRebindingProtection(enabled: Boolean, hosts: List<St
     if (!enabled) return
     install(DnsRebindingProtection) {
         allowedHosts = hosts ?: LOCALHOST_ALLOWED_HOSTS
-        origins?.let { allowedOrigins = it }
+        // Secure-by-default: when relying on the localhost host defaults, validate the Origin
+        // header against localhost too, so a request with a valid Host but a hostile Origin
+        // (e.g. a DNS-rebinding page) is rejected. Callers with custom hosts opt in explicitly.
+        allowedOrigins = origins ?: LOCALHOST_ALLOWED_ORIGINS.takeIf { hosts == null }
     }
 }
diff --git a/kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/DnsRebindingProtectionTest.kt b/kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/DnsRebindingProtectionTest.kt
index 96c44efb..ea38219e 100644
--- a/kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/DnsRebindingProtectionTest.kt
+++ b/kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/DnsRebindingProtectionTest.kt
@@ -3,6 +3,7 @@ package io.modelcontextprotocol.kotlin.sdk.server
 import io.kotest.assertions.ktor.client.shouldHaveStatus
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.shouldNotBe
 import io.kotest.matchers.string.shouldContain
 import io.kotest.matchers.string.shouldNotContain
 import io.ktor.client.request.header
@@ -347,6 +348,51 @@ class DnsRebindingProtectionTest {
         response.shouldHaveStatus(HttpStatusCode.Forbidden)
     }
 
+    // -- default Origin validation (secure-by-default for localhost) --
+
+    @Test
+    fun `mcpStreamableHttp rejects hostile Origin by default`() = testApplication {
+        application {
+            mcpStreamableHttp { testServer() }
+        }
+
+        val response = client.post("/mcp") {
+            header(HttpHeaders.Host, "localhost")
+            header(HttpHeaders.Origin, "http://evil.com")
+            contentType(ContentType.Application.Json)
+        }
+        response.shouldHaveStatus(HttpStatusCode.Forbidden)
+        response.bodyAsText() shouldContain "Invalid Origin host: evil.com"
+    }
+
+    @Test
+    fun `mcpStreamableHttp allows localhost Origin by default`() = testApplication {
+        application {
+            mcpStreamableHttp { testServer() }
+        }
+
+        val response = client.post("/mcp") {
+            header(HttpHeaders.Host, "localhost")
+            header(HttpHeaders.Origin, "http://localhost:5173")
+            contentType(ContentType.Application.Json)
+        }
+        response.status shouldNotBe HttpStatusCode.Forbidden
+    }
+
+    @Test
+    fun `mcpStreamableHttp with custom allowedHosts does not auto-validate Origin`() = testApplication {
+        application {
+            mcpStreamableHttp(allowedHosts = listOf("myapp.com")) { testServer() }
+        }
+
+        val response = client.post("/mcp") {
+            header(HttpHeaders.Host, "myapp.com")
+            header(HttpHeaders.Origin, "http://evil.com")
+            contentType(ContentType.Application.Json)
+        }
+        response.status shouldNotBe HttpStatusCode.Forbidden
+    }
+
     // -- extractHostname unit tests --
 
     @ParameterizedTest