diff --git a/chrome/updater/app/app_server_win.cc b/chrome/updater/app/app_server_win.cc
--- a/chrome/updater/app/app_server_win.cc
+++ b/chrome/updater/app/app_server_win.cc
@@ -261,13 +261,18 @@ void AppServerWin::Stop() {
   VLOG(2) << __func__ << ": COM server is shutting down.";
   if (IsSystemInstall(updater_scope())) {
     // Call `on_service_stopping_` to allow for incoming COM activation requests
     // received while the service is shutting down to be handled by a new
     // service process.
-    // It is possible for `Stop` to be called multiple times, so check for a
-    // valid `on_service_stopping_` callback before calling `Run`.
-    if (on_service_stopping_) {
-      std::move(on_service_stopping_).Run();
+    // `Stop` runs on a COM RPC thread and may be invoked concurrently from
+    // multiple RPC threads (and the SCM control-dispatcher thread); move the
+    // callback out under a lock so only one caller runs it.
+    base::OnceClosure on_service_stopping;
+    {
+      base::AutoLock lock(on_service_stopping_lock_);
+      on_service_stopping = std::move(on_service_stopping_);
+    }
+    if (on_service_stopping) {
+      std::move(on_service_stopping).Run();
     }
   }
   UnregisterClassObjects();
@@ -283,9 +288,13 @@ void AppServerWin::Stop() {
 }

 HRESULT AppServerWin::RunCOMServer(base::OnceClosure on_service_stopping) {
-  on_service_stopping_ = std::move(on_service_stopping);
+  {
+    base::AutoLock lock(on_service_stopping_lock_);
+    on_service_stopping_ = std::move(on_service_stopping);
+  }
   absl::Cleanup reset_on_service_stopping = [&] {
+    base::AutoLock lock(on_service_stopping_lock_);
     on_service_stopping_.Reset();
   };
   return Run();
 }
diff --git a/chrome/updater/app/app_server_win.h b/chrome/updater/app/app_server_win.h
--- a/chrome/updater/app/app_server_win.h
+++ b/chrome/updater/app/app_server_win.h
@@ -11,7 +11,9 @@

 #include "base/functional/callback.h"
 #include "base/memory/scoped_refptr.h"
+#include "base/synchronization/lock.h"
 #include "base/task/sequenced_task_runner.h"
+#include "base/thread_annotations.h"
 #include "chrome/updater/app/app_server.h"
 #include "chrome/updater/update_service.h"
 #include "chrome/updater/update_service_internal.h"
@@ -110,7 +112,9 @@ class AppServerWin : public AppServer {
   std::unique_ptr<UpdateServiceInternalStub> active_duty_internal_stub_;
   std::unique_ptr<UpdateServiceStub> active_duty_stub_;

-  base::OnceClosure on_service_stopping_;
+  // Guards `on_service_stopping_`, which is read/moved on COM RPC threads.
+  base::Lock on_service_stopping_lock_;
+  base::OnceClosure on_service_stopping_ GUARDED_BY(on_service_stopping_lock_);
 };

 // Returns the singleton AppServerWin instance.