fix(client): validate LSYN payload length before parsing remote layouts

fixes: #10107

A malicious server can send a language synchronisation (LSYN) message
with a single-byte payload. size()-2 then underflows to SIZE_MAX and
substr(2, 2) throws std::out_of_range, terminating the client.

Reject odd-length payloads before mutating state, mirroring the
existing DSOP even-length check; an empty payload is still treated as
a normal clear.

src/lib/deskflow/KeyboardLayoutManager.cpp

@@ -35,13 +35,16 @@ KeyboardLayoutManager::KeyboardLayoutManager(const std::vector<std::string> &loc
 
 void KeyboardLayoutManager::setRemoteLayouts(const std::string_view &remoteLayouts)
 {
+  if (!remoteLayouts.empty() && remoteLayouts.size() % 2 != 0) {
+    LOG_ERR("remote layouts are the incorrect size, can not process them");
+    return;
+  }
+
   m_remoteLayouts.clear();
-  if (!remoteLayouts.empty()) {
+  for (size_t i = 0; i + 2 <= remoteLayouts.size(); i += 2) {
-    for (size_t i = 0; i <= remoteLayouts.size() - 2; i += 2) {
     auto rLangs = remoteLayouts.substr(i, 2);
     m_remoteLayouts.emplace_back(rLangs);
   }
-  }
   LOG_INFO("remote layouts: %s", vectorToString(m_remoteLayouts, ", ").c_str());
 }
 

src/unittests/deskflow/KeyboardLayoutManagerTests.cpp

@@ -26,6 +26,22 @@ void KeyboardLayoutManagerTests::remoteLayouts()
   QVERIFY(manager.getRemoteLayouts().empty());
 }
 
+void KeyboardLayoutManagerTests::remoteLayouts_tooShort_returnsEmpty()
+{
+  deskflow::KeyboardLayoutManager manager({"ru", "en", "uk"});
+
+  manager.setRemoteLayouts("a");
+  QVERIFY(manager.getRemoteLayouts().empty());
+}
+
+void KeyboardLayoutManagerTests::remoteLayouts_oddLength_returnsEmpty()
+{
+  deskflow::KeyboardLayoutManager manager({"ru", "en", "uk"});
+
+  manager.setRemoteLayouts("rue");
+  QVERIFY(manager.getRemoteLayouts().empty());
+}
+
 void KeyboardLayoutManagerTests::localLayout()
 {
   std::vector<std::string> localLayouts = {"ru", "en", "uk"};

src/unittests/deskflow/KeyboardLayoutManagerTests.h

@@ -15,6 +15,8 @@ private Q_SLOTS:
   void initTestCase();
   // Test are run in order top to bottom
   void remoteLayouts();
+  void remoteLayouts_tooShort_returnsEmpty();
+  void remoteLayouts_oddLength_returnsEmpty();
   void localLayout();
   void missedLayout();
   void serializeLocalLayouts();