diff --git a/src/wp-includes/media.php b/src/wp-includes/media.php
index c40f7fb6984b9..fd215c800f9dc 100644
--- a/src/wp-includes/media.php
+++ b/src/wp-includes/media.php
@@ -6411,14 +6411,18 @@ function wp_get_image_editor_output_format( $filename, $mime_type ) {
  * @return bool Whether client-side media processing is enabled.
  */
 function wp_is_client_side_media_processing_enabled(): bool {
+	// This is due to SharedArrayBuffer requiring a secure context.
+	$host    = strtolower( (string) strtok( $_SERVER['HTTP_HOST'] ?? '', ':' ) );
+	$enabled = ( is_ssl() || 'localhost' === $host || str_ends_with( $host, '.localhost' ) );
+
 	/**
 	 * Filters whether client-side media processing is enabled.
 	 *
 	 * @since 7.0.0
 	 *
-	 * @param bool $enabled Whether client-side media processing is enabled. Default true.
+	 * @param bool $enabled Whether client-side media processing is enabled. Default true if the page is served in a secure context.
 	 */
-	return (bool) apply_filters( 'wp_client_side_media_processing_enabled', true );
+	return (bool) apply_filters( 'wp_client_side_media_processing_enabled', $enabled );
 }
 
 /**
@@ -6431,7 +6435,7 @@ function wp_set_client_side_media_processing_flag(): void {
 		return;
 	}
 
-	wp_add_inline_script( 'wp-block-editor', 'window.__clientSideMediaProcessing = true', 'before' );
+	wp_add_inline_script( 'wp-block-editor', 'window.__clientSideMediaProcessing = true;', 'before' );
 
 	$chromium_version = wp_get_chromium_major_version();
 
@@ -6477,6 +6481,10 @@ function wp_get_chromium_major_version(): ?int {
  * media processing in the editor. Uses Document-Isolation-Policy
  * on supported browsers (Chromium 137+).
  *
+ * Skips setup when a third-party page builder overrides the block
+ * editor via a custom `action` query parameter, as DIP would block
+ * same-origin iframe access that these editors rely on.
+ *
  * @since 7.0.0
  */
 function wp_set_up_cross_origin_isolation(): void {
@@ -6494,6 +6502,15 @@ function wp_set_up_cross_origin_isolation(): void {
 		return;
 	}
 
+	/*
+	 * Skip when a third-party page builder overrides the block editor.
+	 * DIP isolates the document into its own agent cluster,
+	 * which blocks same-origin iframe access that these editors rely on.
+	 */
+	if ( isset( $_GET['action'] ) && 'edit' !== $_GET['action'] ) {
+		return;
+	}
+
 	// Cross-origin isolation is not needed if users can't upload files anyway.
 	if ( ! current_user_can( 'upload_files' ) ) {
 		return;
diff --git a/tests/phpunit/tests/media/wpCrossOriginIsolation.php b/tests/phpunit/tests/media/wpCrossOriginIsolation.php
index 31f2e85975ee0..4fe5723bdc426 100644
--- a/tests/phpunit/tests/media/wpCrossOriginIsolation.php
+++ b/tests/phpunit/tests/media/wpCrossOriginIsolation.php
@@ -6,19 +6,36 @@
  * @group media
  * @covers ::wp_set_up_cross_origin_isolation
  * @covers ::wp_start_cross_origin_isolation_output_buffer
+ * @covers ::wp_is_client_side_media_processing_enabled
  */
 class Tests_Media_wpCrossOriginIsolation extends WP_UnitTestCase {
 
 	/**
 	 * Original HTTP_USER_AGENT value.
-	 *
-	 * @var string|null
 	 */
-	private $original_user_agent;
+	private ?string $original_user_agent;
+
+	/**
+	 * Original HTTP_HOST value.
+	 */
+	private ?string $original_http_host;
+
+	/**
+	 * Original HTTPS value.
+	 */
+	private ?string $original_https;
+
+	/**
+	 * Original $_GET['action'] value.
+	 */
+	private ?string $original_get_action;
 
 	public function set_up() {
 		parent::set_up();
-		$this->original_user_agent = isset( $_SERVER['HTTP_USER_AGENT'] ) ? $_SERVER['HTTP_USER_AGENT'] : null;
+		$this->original_user_agent = $_SERVER['HTTP_USER_AGENT'] ?? null;
+		$this->original_http_host  = $_SERVER['HTTP_HOST'] ?? null;
+		$this->original_https      = $_SERVER['HTTPS'] ?? null;
+		$this->original_get_action = $_GET['action'] ?? null;
 	}
 
 	public function tear_down() {
@@ -28,6 +45,24 @@ public function tear_down() {
 			$_SERVER['HTTP_USER_AGENT'] = $this->original_user_agent;
 		}
 
+		if ( null === $this->original_http_host ) {
+			unset( $_SERVER['HTTP_HOST'] );
+		} else {
+			$_SERVER['HTTP_HOST'] = $this->original_http_host;
+		}
+
+		if ( null === $this->original_https ) {
+			unset( $_SERVER['HTTPS'] );
+		} else {
+			$_SERVER['HTTPS'] = $this->original_https;
+		}
+
+		if ( null === $this->original_get_action ) {
+			unset( $_GET['action'] );
+		} else {
+			$_GET['action'] = $this->original_get_action;
+		}
+
 		// Clean up any output buffers started during tests.
 		while ( ob_get_level() > 1 ) {
 			ob_end_clean();
@@ -124,6 +159,32 @@ public function test_does_not_start_output_buffer_for_safari() {
 		$this->assertSame( $level_before, $level_after, 'Output buffer should not be started for Safari.' );
 	}
 
+	/**
+	 * @ticket 64803
+	 */
+	public function test_client_side_processing_disabled_on_non_secure_origin() {
+		$_SERVER['HTTP_HOST'] = 'example.com';
+		$_SERVER['HTTPS']     = '';
+
+		$this->assertFalse(
+			wp_is_client_side_media_processing_enabled(),
+			'Client-side media processing should be disabled on non-secure, non-localhost origins.'
+		);
+	}
+
+	/**
+	 * @ticket 64803
+	 */
+	public function test_client_side_processing_enabled_on_localhost() {
+		$_SERVER['HTTP_HOST'] = 'localhost';
+		$_SERVER['HTTPS']     = '';
+
+		$this->assertTrue(
+			wp_is_client_side_media_processing_enabled(),
+			'Client-side media processing should be enabled on localhost.'
+		);
+	}
+
 	/**
 	 * This test must run in a separate process because the output buffer
 	 * callback sends HTTP headers via header(), which would fail in the
diff --git a/tests/phpunit/tests/rest-api/rest-attachments-controller.php b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
index 93cd4211c93ba..7fa4545a38d9c 100644
--- a/tests/phpunit/tests/rest-api/rest-attachments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
@@ -3162,6 +3162,12 @@ static function ( $data ) use ( &$captured_data ) {
 	 * @requires function imagejpeg
 	 */
 	public function test_sideload_scaled_image() {
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+		// Reinitialize REST server so the sideload route is registered.
+		global $wp_rest_server;
+		$wp_rest_server = new Spy_REST_Server();
+		do_action( 'rest_api_init', $wp_rest_server );
+
 		wp_set_current_user( self::$author_id );
 
 		// First, create an attachment.
@@ -3215,6 +3221,12 @@ public function test_sideload_scaled_image() {
 	 * @requires function imagejpeg
 	 */
 	public function test_sideload_scaled_image_requires_auth() {
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+		// Reinitialize REST server so the sideload route is registered.
+		global $wp_rest_server;
+		$wp_rest_server = new Spy_REST_Server();
+		do_action( 'rest_api_init', $wp_rest_server );
+
 		wp_set_current_user( self::$author_id );
 
 		// Create an attachment.
@@ -3244,6 +3256,12 @@ public function test_sideload_scaled_image_requires_auth() {
 	 * @ticket 64737
 	 */
 	public function test_sideload_route_includes_scaled_enum() {
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+		// Reinitialize REST server so the sideload route is registered.
+		global $wp_rest_server;
+		$wp_rest_server = new Spy_REST_Server();
+		do_action( 'rest_api_init', $wp_rest_server );
+
 		$server = rest_get_server();
 		$routes = $server->get_routes();
 
@@ -3266,6 +3284,12 @@ public function test_sideload_route_includes_scaled_enum() {
 	 * @requires function imagejpeg
 	 */
 	public function test_sideload_scaled_unique_filename() {
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+		// Reinitialize REST server so the sideload route is registered.
+		global $wp_rest_server;
+		$wp_rest_server = new Spy_REST_Server();
+		do_action( 'rest_api_init', $wp_rest_server );
+
 		wp_set_current_user( self::$author_id );
 
 		// Create an attachment.
@@ -3300,6 +3324,12 @@ public function test_sideload_scaled_unique_filename() {
 	 * @requires function imagejpeg
 	 */
 	public function test_sideload_scaled_unique_filename_conflict() {
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+		// Reinitialize REST server so the sideload route is registered.
+		global $wp_rest_server;
+		$wp_rest_server = new Spy_REST_Server();
+		do_action( 'rest_api_init', $wp_rest_server );
+
 		wp_set_current_user( self::$author_id );
 
 		// Create the first attachment.
diff --git a/tests/phpunit/tests/rest-api/rest-schema-setup.php b/tests/phpunit/tests/rest-api/rest-schema-setup.php
index 0e0e00b934359..01dad836981bc 100644
--- a/tests/phpunit/tests/rest-api/rest-schema-setup.php
+++ b/tests/phpunit/tests/rest-api/rest-schema-setup.php
@@ -16,6 +16,9 @@ class WP_Test_REST_Schema_Initialization extends WP_Test_REST_TestCase {
 	public function set_up() {
 		parent::set_up();
 
+		// Ensure client-side media processing is enabled so the sideload route is registered.
+		add_filter( 'wp_client_side_media_processing_enabled', '__return_true' );
+
 		/** @var WP_REST_Server $wp_rest_server */
 		global $wp_rest_server;
 		$wp_rest_server = new Spy_REST_Server();