diff --git a/ui/base/clipboard/clipboard_format_type_win.cc b/ui/base/clipboard/clipboard_format_type_win.cc
--- a/ui/base/clipboard/clipboard_format_type_win.cc
+++ b/ui/base/clipboard/clipboard_format_type_win.cc
@@ -15,6 +15,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/synchronization/lock.h"
 #include "ui/base/clipboard/clipboard_constants.h"

 namespace ui {
@@ -224,6 +225,17 @@ const ClipboardFormatType& ClipboardFormatType::FileDescriptorType() {
   return *format;
 }

+namespace {
+// FileContentTypeMap() is touched from base::ThreadPool workers
+// (ExtractVirtualFiles in clipboard_util_win.cc) and from the UI thread
+// (HasVirtualFilenames / SetFileContents). std::map mutation is not
+// thread-safe; guard it.
+base::Lock& FileContentTypeMapLock() {
+  static base::NoDestructor<base::Lock> lock;
+  return *lock;
+}
+}  // namespace
+
 // static
 std::map<LONG, ClipboardFormatType>& ClipboardFormatType::FileContentTypeMap() {
   static base::NoDestructor<std::map<LONG, ClipboardFormatType>>
@@ -234,6 +246,7 @@ std::map<LONG, ClipboardFormatType>& ClipboardFormatType::FileContentTypeMap() {
 // static
 const ClipboardFormatType& ClipboardFormatType::FileContentAtIndexType(
     LONG index) {
+  base::AutoLock auto_lock(FileContentTypeMapLock());
   auto& index_to_type_map = FileContentTypeMap();

   auto insert_or_assign_result = index_to_type_map.insert(