diff --git a/libwvdrmengine/cdm/core/include/device_files.h b/libwvdrmengine/cdm/core/include/device_files.h
index d23a5d45..8b3926ef 100644
--- a/libwvdrmengine/cdm/core/include/device_files.h
+++ b/libwvdrmengine/cdm/core/include/device_files.h
@@ -5,6 +5,7 @@
 #ifndef WVCDM_CORE_DEVICE_FILES_H_
 #define WVCDM_CORE_DEVICE_FILES_H_
 
+#include <mutex>
 #include <set>
 #include <string>
 #include <vector>
@@ -369,6 +370,7 @@ class DeviceFiles {
 #endif
 
   static std::set<std::string> reserved_license_ids_;
+  static std::mutex reserved_license_ids_mutex_;
 
   wvutil::FileSystem* file_system_;
   CdmSecurityLevel security_level_;
diff --git a/libwvdrmengine/cdm/core/src/device_files.cpp b/libwvdrmengine/cdm/core/src/device_files.cpp
index 2602171b..e18fac0a 100644
--- a/libwvdrmengine/cdm/core/src/device_files.cpp
+++ b/libwvdrmengine/cdm/core/src/device_files.cpp
@@ -87,6 +87,8 @@ using video_widevine_client::sdk::
   }
 
 namespace wvcdm {
+using UniqueLock = std::unique_lock<std::mutex>;
+
 namespace {
 const char kEmptyFileName[] = "";
 const char kFalse[] = "false";
@@ -364,6 +366,7 @@ const char* DeviceFiles::ResponseTypeToString(ResponseType type) {
 
 // static
 std::set<std::string> DeviceFiles::reserved_license_ids_;
+std::mutex DeviceFiles::reserved_license_ids_mutex_;
 
 DeviceFiles::DeviceFiles(wvutil::FileSystem* file_system)
     : file_system_(file_system),
@@ -847,6 +850,7 @@ bool DeviceFiles::StoreLicense(const CdmLicenseData& license_data,
   std::string serialized_file;
   file.SerializeToString(&serialized_file);
 
+  UniqueLock lock(reserved_license_ids_mutex_);
   reserved_license_ids_.erase(license_data.key_set_id);
   *result = StoreFileWithHash(license_data.key_set_id + kLicenseFileNameExt,
                               serialized_file);
@@ -984,18 +988,21 @@ bool DeviceFiles::DeleteAllFiles() {
 
 bool DeviceFiles::LicenseExists(const std::string& key_set_id) {
   RETURN_FALSE_IF_UNINITIALIZED();
+  UniqueLock lock(reserved_license_ids_mutex_);
   return reserved_license_ids_.count(key_set_id) ||
          FileExists(key_set_id + kLicenseFileNameExt);
 }
 
 bool DeviceFiles::ReserveLicenseId(const std::string& key_set_id) {
   RETURN_FALSE_IF_UNINITIALIZED();
+  UniqueLock lock(reserved_license_ids_mutex_);
   reserved_license_ids_.insert(key_set_id);
   return true;
 }
 
 bool DeviceFiles::UnreserveLicenseId(const std::string& key_set_id) {
   RETURN_FALSE_IF_UNINITIALIZED();
+  UniqueLock lock(reserved_license_ids_mutex_);
   reserved_license_ids_.erase(key_set_id);
   return true;
 }