From efb4e25d7536583cbcebc60f99f08d8ada6304ac Mon Sep 17 00:00:00 2001
From: Avi Drissman <[email protected]>
Date: Thu, 24 Sep 2026 11:47:51 -0700
Subject: [PATCH] Guard against bad drags on the Mac

There are several scenarios in which an initiated drag can go bad.
Centralize the decision-making as to whether a drag can be initiated.

Bad drag #1:

An untrustworthy renderer could disrupt a drag in progress by starting a
second one. Starting with macOS 15, AppKit checks for multiple
concurrent drags and prohibits them, but there is no such check in
earlier versions.

Also set the key in preferences to ensure that AppKit prohibits
concurrent drags.

Bad drag #2:

An untrustworthy renderer could disrupt a menu (context or popup) in
progress by starting a drag. This is not accounted for by AppKit.

Fixed: 553163770
Fixed: 553163820
Fixed: 553160293
Change-Id: I66b15a2cf220548cda92c1d42230131a6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8452662
Commit-Queue: Bryan Oltman <[email protected]>
Reviewed-by: Bryan Oltman <[email protected]>
Commit-Queue: Avi Drissman <[email protected]>
Auto-Submit: Avi Drissman <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1704468}
---
diff --git a/components/remote_cocoa/app_shim/bridged_content_view.mm b/components/remote_cocoa/app_shim/bridged_content_view.mm
index d8f447aa3..58c8f6d 100644
--- a/components/remote_cocoa/app_shim/bridged_content_view.mm
+++ b/components/remote_cocoa/app_shim/bridged_content_view.mm
@@ -16,6 +16,7 @@
 #include "components/remote_cocoa/app_shim/native_widget_ns_window_host_helper.h"
 #include "components/remote_cocoa/common/native_widget_ns_window_host.mojom.h"
 #import "ui/base/cocoa/appkit_utils.h"
+#include "ui/base/cocoa/drag_permission.h"
 #include "ui/base/cocoa/find_pasteboard.h"
 #include "ui/base/cocoa/tracking_area.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
@@ -1704,11 +1705,18 @@
 }
 
 - (void)draggingSession:(NSDraggingSession*)session
+       willBeginAtPoint:(NSPoint)screenPoint {
+  ui::SetDragSessionInProgress(true);
+}
+
+- (void)draggingSession:(NSDraggingSession*)session
            endedAtPoint:(NSPoint)screenPoint
               operation:(NSDragOperation)operation {
+  ui::SetDragSessionInProgress(false);
   remote_cocoa::DragDropClient* client = [self dragDropClient];
-  if (client)
+  if (client) {
     client->EndDrag();
+  }
 }
 
 // NSAccessibility formal protocol implementation:
diff --git a/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm b/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
index 7581294..1c4d2363 100644
--- a/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
+++ b/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
@@ -48,6 +48,7 @@
 #include "ui/base/cocoa/animation_utils.h"
 #import "ui/base/cocoa/constrained_window/constrained_window_animation.h"
 #include "ui/base/cocoa/cursor_utils.h"
+#include "ui/base/cocoa/drag_permission.h"
 #include "ui/base/cocoa/remote_accessibility_api.h"
 #import "ui/base/cocoa/window_size_constants.h"
 #include "ui/base/emoji/emoji_panel_helper.h"
@@ -1406,6 +1407,10 @@
     DLOG(WARNING) << "BeginFileDrag failed: window or bridged_view is null";
     return;
   }
+  if (!ui::IsDragSessionInitiationAllowed()) {
+    DLOG(WARNING) << "BeginFileDrag failed: drag session not allowed";
+    return;
+  }
 
   NSURL* file_url = base::apple::FilePathToNSURL(file_drag_data->file_path);
   if (!file_url) {
diff --git a/content/app/mac_init.mm b/content/app/mac_init.mm
index dc4067c6..25452ebd 100644
--- a/content/app/mac_init.mm
+++ b/content/app/mac_init.mm
@@ -28,6 +28,19 @@
     // Chrome is unusable for a long period after returning from sleep.
     // https://crbug.com/41406192.
     @"NSAppSleepDisabled" : @YES,
+
+    // Prior to macOS 15, AppKit assumed that apps would not be so silly as to
+    // attempt to start a concurrent drag session while one was already in
+    // progress. Starting in macOS 15, AppKit guards against it, but it has a
+    // BoolAppConfig to allow it, presumably to allow for a case where this
+    // would be needed for compatibility. Given that starting a second drag
+    // while one is in progress could be used to mount an attack on the user,
+    // explicitly set the config. See -[NSCoreDragManager
+    // beginDraggingSessionWithItems:fromWindow:withClipRect:event:source:] for
+    // where the config is accessed, and see https://crbug.com/553160293,
+    // https://crbug.com/553163770, and https://crbug.com/553163820 for the
+    // rationale.
+    @"NSAllowMultipleDrags" : @NO,
   }];
 
   // Disable NSAutoFillHeuristicController on macOS 26.0 and 26.1. On those OS
diff --git a/content/app_shim_remote_cocoa/web_contents_view_cocoa.mm b/content/app_shim_remote_cocoa/web_contents_view_cocoa.mm
index 94da754..60bb2a2 100644
--- a/content/app_shim_remote_cocoa/web_contents_view_cocoa.mm
+++ b/content/app_shim_remote_cocoa/web_contents_view_cocoa.mm
@@ -20,6 +20,7 @@
 #include "ui/base/clipboard/clipboard_constants.h"
 #include "ui/base/clipboard/clipboard_util_mac.h"
 #include "ui/base/clipboard/custom_data_helper.h"
+#include "ui/base/cocoa/drag_permission.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/events/event_utils.h"
 #include "ui/events/platform_event.h"
@@ -241,8 +242,9 @@
                         image:(NSImage*)image
                        offset:(NSPoint)offset
                  isPrivileged:(BOOL)isPrivileged {
-  if (!_host)
+  if (!_host || !ui::IsDragSessionInitiationAllowed()) {
     return;
+  }
 
   NSPoint mouseLocation = [self.window mouseLocationOutsideOfEventStream];
   NSEvent* dragEvent = [NSEvent mouseEventWithType:NSEventTypeLeftMouseDragged
@@ -325,10 +327,16 @@
   return _dragOperation;
 }
 
+- (void)draggingSession:(NSDraggingSession*)session
+       willBeginAtPoint:(NSPoint)screenPoint {
+  ui::SetDragSessionInProgress(true);
+}
+
 // Called when a drag initiated in our view ends.
 - (void)draggingSession:(NSDraggingSession*)session
            endedAtPoint:(NSPoint)screenPoint
               operation:(NSDragOperation)operation {
+  ui::SetDragSessionInProgress(false);
   if (!_host) {
     return;
   }
diff --git a/ui/base/cocoa/drag_permission.h b/ui/base/cocoa/drag_permission.h
new file mode 100644
index 0000000..841e1d21
--- /dev/null
+++ b/ui/base/cocoa/drag_permission.h
@@ -0,0 +1,37 @@
+// Copyright 2026 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_BASE_COCOA_DRAG_PERMISSION_H_
+#define UI_BASE_COCOA_DRAG_PERMISSION_H_
+
+#include "base/component_export.h"
+
+// There are several times when it would be bad to allow a drag to be initiated.
+// These are often not accounted for by AppKit, which assumes a reasonable and
+// non-adversarial usage of its APIs. This file handles coordination of defense
+// against those attacks.
+//
+// This class is meant to prevent the initiation of a drag session while:
+//   - a drag session is already in progress
+//   - a menu tracking session is already in progress
+//
+// macOS 15+ does prevent the first, but it does not prevent the second. Given
+// that these are security concerns, take this into our own hands.
+//
+// See https://crbug.com/553160293, https://crbug.com/553163770, and
+// https://crbug.com/553163820 for the concerns with concurrent drag sessions.
+
+namespace ui {
+
+// Returns whether a drag session may be initiated.
+COMPONENT_EXPORT(UI_BASE)
+bool IsDragSessionInitiationAllowed();
+
+// Sets the state of whether a drag session is in progress.
+COMPONENT_EXPORT(UI_BASE)
+void SetDragSessionInProgress(bool in_progress);
+
+}  // namespace ui
+
+#endif  // UI_BASE_COCOA_DRAG_PERMISSION_H_
diff --git a/ui/base/cocoa/drag_permission.mm b/ui/base/cocoa/drag_permission.mm
new file mode 100644
index 0000000..7008f90
--- /dev/null
+++ b/ui/base/cocoa/drag_permission.mm
@@ -0,0 +1,33 @@
+// Copyright 2026 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/base/cocoa/drag_permission.h"
+
+#import <AppKit/AppKit.h>
+
+#include "base/check_op.h"
+
+namespace ui {
+
+namespace {
+
+// There is SPI to inquire as to the existence of a current drag session
+// ([_NSDragManager isDragging]). However, usage of SPI is avoided if it is easy
+// to do so, and it is easy in this case.
+bool gDragSessionIsInProgress;
+
+}  // namespace
+
+bool IsDragSessionInitiationAllowed() {
+  CHECK_EQ(NSRunLoop.currentRunLoop, NSRunLoop.mainRunLoop);
+  return !gDragSessionIsInProgress &&
+         NSRunLoop.currentRunLoop.currentMode != NSEventTrackingRunLoopMode;
+}
+
+void SetDragSessionInProgress(bool in_progress) {
+  CHECK_NE(gDragSessionIsInProgress, in_progress);
+  gDragSessionIsInProgress = in_progress;
+}
+
+}  // namespace ui
diff --git a/ui/views/cocoa/drag_drop_client_mac.mm b/ui/views/cocoa/drag_drop_client_mac.mm
index 2a583d1..22934a2 100644
--- a/ui/views/cocoa/drag_drop_client_mac.mm
+++ b/ui/views/cocoa/drag_drop_client_mac.mm
@@ -10,6 +10,7 @@
 #import "components/remote_cocoa/app_shim/bridged_content_view.h"
 #import "components/remote_cocoa/app_shim/native_widget_ns_window_bridge.h"
 #include "components/remote_cocoa/app_shim/window_move_loop.h"
+#include "ui/base/cocoa/drag_permission.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/base/dragdrop/mojom/drag_drop_types.mojom.h"
 #import "ui/base/dragdrop/os_exchange_data_provider_mac.h"
@@ -38,6 +39,10 @@
     return;
   }
 
+  if (!ui::IsDragSessionInitiationAllowed()) {
+    return;
+  }
+
   exchange_data_ = std::move(data);
   source_operation_ = operation;
   is_drag_source_ = true;
@@ -97,9 +102,17 @@
     }
   }
 
-  [bridge_->ns_view() beginDraggingSessionWithItems:drag_items
-                                              event:event
-                                             source:bridge_->ns_view()];
+  NSDraggingSession* session =
+      [bridge_->ns_view() beginDraggingSessionWithItems:drag_items
+                                                  event:event
+                                                 source:bridge_->ns_view()];
+  if (!session) {
+    // The return value from -beginDraggingSessionWithItems:event:source: is
+    // marked as nonnull, but in the edge case where there already is an ongoing
+    // dragging session, nil is returned. (The issue with the SDK is filed as
+    // FB24907244.) In that case, bail.
+    return;
+  }
 
   // Since Drag and drop is asynchronous on the Mac, spin a nested run loop for
   // consistency with other platforms.
@@ -107,12 +120,12 @@
   quit_closure_ = run_loop.QuitClosure();
   run_loop.Run();
 
-  // As of MacOS 26, dragging with a trackpad results in a leftover mouse-up
+  // As of macOS 26, dragging with a trackpad results in a leftover mouse-up
   // event remaining in the queue when the drag session is terminated. This
   // ensures any leftover message is pumped to avoid handling the "release"
   // action as a click.
   [NSApp nextEventMatchingMask:NSEventMaskLeftMouseUp
-                     untilDate:[NSDate distantPast]
+                     untilDate:NSDate.distantPast
                         inMode:NSEventTrackingRunLoopMode
                        dequeue:YES];
 }
diff --git a/ui/views/cocoa/drag_drop_client_mac_unittest.mm b/ui/views/cocoa/drag_drop_client_mac_unittest.mm
index 15de877..5f4f25d 100644
--- a/ui/views/cocoa/drag_drop_client_mac_unittest.mm
+++ b/ui/views/cocoa/drag_drop_client_mac_unittest.mm
@@ -40,7 +40,12 @@
                                                 source:(id<NSDraggingSource>)
                                                            source {
   ++g_begin_dragging_session_call_count;
-  return nil;
+
+  // Return a fake dragging session to pretend that the dragging initiation was
+  // successful. The only thing done to the return object is a nil check, so a
+  // raw NSObject will do, and if that ever changes, a blank object will explode
+  // immediately, alerting to the issue.
+  return (NSDraggingSession*)[[NSObject alloc] init];
 }
 @end
 
diff --git a/ui/base/BUILD.gn b/ui/base/BUILD.gn
index 39c46572..78a13bb 100644
--- a/ui/base/BUILD.gn
+++ b/ui/base/BUILD.gn
@@ -319,6 +319,8 @@
       "cocoa/default_command_dispatcher_delegate.mm",
       "cocoa/defaults_utils.h",
       "cocoa/defaults_utils.mm",
+      "cocoa/drag_permission.h",
+      "cocoa/drag_permission.mm",
       "cocoa/find_pasteboard.h",
       "cocoa/find_pasteboard.mm",
       "cocoa/menu_utils.h",

diff --git a/ui/views/cocoa/drag_drop_client_mac_unittest.mm b/ui/views/cocoa/drag_drop_client_mac_unittest.mm
index 15de877..5f4f25d 100644
--- a/ui/views/cocoa/drag_drop_client_mac_unittest.mm
+++ b/ui/views/cocoa/drag_drop_client_mac_unittest.mm
@@ -40,7 +40,12 @@
                                                 source:(id<NSDraggingSource>)
                                                            source {
   ++g_begin_dragging_session_call_count;
-  return nil;
+
+  // Return a fake dragging session to pretend that the dragging initiation was
+  // successful. The only thing done to the return object is a nil check, so a
+  // raw NSObject will do, and if that ever changes, a blank object will explode
+  // immediately, alerting to the issue.
+  return (NSDraggingSession*)[[NSObject alloc] init];
 }
 @end