diff --git a/src/audio/mount_sound_manager.cpp b/src/audio/mount_sound_manager.cpp
index 0762ea26..92452161 100644
--- a/src/audio/mount_sound_manager.cpp
+++ b/src/audio/mount_sound_manager.cpp
@@ -495,7 +495,7 @@ MountType MountSoundManager::detectMountType(uint32_t creatureDisplayId) const {
 MountFamily MountSoundManager::detectMountFamilyFromPath(const std::string& modelPath) const {
     // Convert path to lowercase for matching
     std::string lower = modelPath;
-    for (char& c : lower) c = std::tolower(c);
+    for (char& c : lower) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
 
     // Check creature model path for family keywords
     if (lower.find("tallstrider") != std::string::npos ||
diff --git a/src/auth/auth_packets.cpp b/src/auth/auth_packets.cpp
index 258490ac..7ffef253 100644
--- a/src/auth/auth_packets.cpp
+++ b/src/auth/auth_packets.cpp
@@ -57,7 +57,8 @@ bool detectOutboundIPv4(std::array<uint8_t, 4>& outIp) {
 network::Packet LogonChallengePacket::build(const std::string& account, const ClientInfo& info) {
     // Convert account to uppercase
     std::string upperAccount = account;
-    std::transform(upperAccount.begin(), upperAccount.end(), upperAccount.begin(), ::toupper);
+    std::transform(upperAccount.begin(), upperAccount.end(), upperAccount.begin(),
+                   [](unsigned char c) { return static_cast<char>(std::toupper(c)); });
 
     // Calculate payload size (everything after cmd + error + size)
     // game(4) + version(3) + build(2) + platform(4) + os(4) + locale(4) +
diff --git a/src/auth/srp.cpp b/src/auth/srp.cpp
index 6d741920..154c4f55 100644
--- a/src/auth/srp.cpp
+++ b/src/auth/srp.cpp
@@ -117,8 +117,9 @@ std::vector<uint8_t> SRP::computeAuthHash(const std::string& username,
     // Convert to uppercase (WoW requirement)
     std::string upperUser = username;
     std::string upperPass = password;
-    std::transform(upperUser.begin(), upperUser.end(), upperUser.begin(), ::toupper);
-    std::transform(upperPass.begin(), upperPass.end(), upperPass.begin(), ::toupper);
+    auto toUpper = [](unsigned char c) { return static_cast<char>(std::toupper(c)); };
+    std::transform(upperUser.begin(), upperUser.end(), upperUser.begin(), toUpper);
+    std::transform(upperPass.begin(), upperPass.end(), upperPass.begin(), toUpper);
 
     // H(I:P)
     std::string combined = upperUser + ":" + upperPass;
diff --git a/src/game/world_packets.cpp b/src/game/world_packets.cpp
index a1012f42..7297298d 100644
--- a/src/game/world_packets.cpp
+++ b/src/game/world_packets.cpp
@@ -80,7 +80,7 @@ network::Packet AuthSessionPacket::build(uint32_t build,
     // Convert account name to uppercase
     std::string upperAccount = accountName;
     std::transform(upperAccount.begin(), upperAccount.end(),
-                   upperAccount.begin(), ::toupper);
+                   upperAccount.begin(), [](unsigned char c) { return static_cast<char>(std::toupper(c)); });
 
     LOG_INFO("Building CMSG_AUTH_SESSION for account: ", upperAccount);