diff --git a/include/WinStd/COM.h b/include/WinStd/COM.h
index aa4fd601..271248b1 100644
--- a/include/WinStd/COM.h
+++ b/include/WinStd/COM.h
@@ -225,9 +225,10 @@ namespace winstd
         template<class _Traits, class _Ax>
         bstr(_In_ const std::basic_string<OLECHAR, _Traits, _Ax> &src)
         {
-            if (src.length() >= UINT_MAX)
-                throw std::invalid_argument("String too long");
-            m_h = SysAllocStringLen(src.c_str(), (UINT)src.length());
+            size_t len = src.length();
+            if (len > UINT_MAX)
+                throw std::invalid_argument("string too long");
+            m_h = SysAllocStringLen(src.c_str(), static_cast<UINT>(len));
             if (!m_h)
                 throw std::bad_alloc();
         }
diff --git a/include/WinStd/Common.h b/include/WinStd/Common.h
index 8532c085..a5c855fc 100644
--- a/include/WinStd/Common.h
+++ b/include/WinStd/Common.h
@@ -8,6 +8,7 @@
 
 #include <Windows.h>
 #include <assert.h>
+#include <intsafe.h>
 #include <stdarg.h>
 #include <tchar.h>
 #include <iostream>
@@ -203,6 +204,42 @@ typedef const BYTE *LPCBYTE;
 #pragma warning(disable: 4996)
 #pragma warning(disable: 4505) // Don't warn on unused code
 
+#ifdef _WIN64
+inline ULONGLONG ULongLongMult(ULONGLONG a, ULONGLONG b)
+{
+    ULONGLONG result;
+    if (SUCCEEDED(ULongLongMult(a, b, &result)))
+        return result;
+    throw std::invalid_argument("multiply overflow");
+}
+#else
+inline SIZE_T SIZETMult(SIZE_T a, SIZE_T b)
+{
+    SIZE_T result;
+    if (SUCCEEDED(SIZETMult(a, b, &result)))
+        return result;
+    throw std::invalid_argument("multiply overflow");
+}
+#endif
+
+#ifdef _WIN64
+inline ULONGLONG ULongLongAdd(ULONGLONG a, ULONGLONG b)
+{
+    ULONGLONG result;
+    if (SUCCEEDED(ULongLongAdd(a, b, &result)))
+        return result;
+    throw std::invalid_argument("add overflow");
+}
+#else
+inline SIZE_T SIZETAdd(SIZE_T a, SIZE_T b)
+{
+    SIZE_T result;
+    if (SUCCEEDED(SIZETAdd(a, b, &result)))
+        return result;
+    throw std::invalid_argument("add overflow");
+}
+#endif
+
 /// \addtogroup WinStdStrFormat
 /// @{
 
diff --git a/include/WinStd/Crypt.h b/include/WinStd/Crypt.h
index d92d41f3..effaa9bb 100644
--- a/include/WinStd/Crypt.h
+++ b/include/WinStd/Crypt.h
@@ -176,9 +176,14 @@ static _Success_(return != 0) BOOL CryptExportKey(_In_ HCRYPTKEY hKey, _In_ HCRY
 template<class _Ty, class _Ax>
 static _Success_(return != 0) BOOL CryptEncrypt(_In_ HCRYPTKEY hKey, _In_opt_ HCRYPTHASH hHash, _In_ BOOL Final, _In_ DWORD dwFlags, _Inout_ std::vector<_Ty, _Ax> &aData)
 {
+    SIZE_T
+        sDataLen = SIZETMult(aData.size(), sizeof(_Ty)),
+        sBufLen  = SIZETMult(aData.capacity(), sizeof(_Ty));
+    if (sDataLen > DWORD_MAX || sBufLen > DWORD_MAX)
+        throw std::invalid_argument("Data too big");
     DWORD
-        dwDataLen = (DWORD)(aData.size()     * sizeof(_Ty)),
-        dwBufLen  = (DWORD)(aData.capacity() * sizeof(_Ty)),
+        dwDataLen = static_cast<DWORD>(sDataLen),
+        dwBufLen  = static_cast<DWORD>(sBufLen),
         dwEncLen  = dwDataLen,
         dwResult;
 
@@ -226,7 +231,10 @@ static _Success_(return != 0) BOOL CryptEncrypt(_In_ HCRYPTKEY hKey, _In_opt_ HC
 template<class _Ty, class _Ax>
 static _Success_(return != 0) BOOL CryptDecrypt(_In_ HCRYPTKEY hKey, _In_opt_ HCRYPTHASH hHash, _In_ BOOL Final, _In_ DWORD dwFlags, _Inout_ std::vector<_Ty, _Ax> &aData)
 {
-    DWORD dwDataLen = (DWORD)(aData.size() * sizeof(_Ty));
+    SIZE_T sDataLen = SIZETMult(aData.size(), sizeof(_Ty));
+    if (sDataLen > DWORD_MAX)
+        throw std::invalid_argument("Data too big");
+    DWORD dwDataLen = static_cast<DWORD>(sDataLen);
 
     if (CryptDecrypt(hKey, hHash, Final, dwFlags, reinterpret_cast<BYTE*>(aData.data()), &dwDataLen)) {
         // Decryption succeeded.
diff --git a/include/WinStd/WLAN.h b/include/WinStd/WLAN.h
index e9ae90da..10a5fac0 100644
--- a/include/WinStd/WLAN.h
+++ b/include/WinStd/WLAN.h
@@ -33,23 +33,25 @@ extern DWORD (WINAPI *pfnWlanReasonCodeToString)(__in DWORD dwReasonCode, __in D
 template<class _Traits, class _Ax>
 static DWORD WlanReasonCodeToString(_In_ DWORD dwReasonCode, _Inout_ std::basic_string<wchar_t, _Traits, _Ax> &sValue, __reserved PVOID pReserved)
 {
-    DWORD dwSize = 0;
+    SIZE_T sSize = 0;
 
     if (!::pfnWlanReasonCodeToString)
         return ERROR_CALL_NOT_IMPLEMENTED;
 
     for (;;) {
         // Increment size and allocate buffer.
-        dwSize += 1024;
-        std::unique_ptr<wchar_t[]> szBuffer(new wchar_t[dwSize]);
+        sSize = SIZETAdd(sSize, 1024);
+        if (sSize > DWORD_MAX)
+            throw std::runtime_exception("Data too big");
+        std::unique_ptr<wchar_t[]> szBuffer(new wchar_t[sSize]);
 
         // Try!
-        DWORD dwResult = ::pfnWlanReasonCodeToString(dwReasonCode, dwSize, szBuffer.get(), pReserved);
+        DWORD dwResult = ::pfnWlanReasonCodeToString(dwReasonCode, static_cast<DWORD>(sSize), szBuffer.get(), pReserved);
         if (dwResult == ERROR_SUCCESS) {
-            DWORD dwLength = (DWORD)wcsnlen(szBuffer.get(), dwSize);
-            if (dwLength < dwSize - 1) {
+            SIZE_T sLength = wcsnlen(szBuffer.get(), sSize);
+            if (sLength < sSize - 1) {
                 // Buffer was long enough.
-                sValue.assign(szBuffer.get(), dwLength);
+                sValue.assign(szBuffer.get(), sLength);
                 return ERROR_SUCCESS;
             }
         } else {
diff --git a/include/WinStd/Win.h b/include/WinStd/Win.h
index 677ae684..a106c17d 100644
--- a/include/WinStd/Win.h
+++ b/include/WinStd/Win.h
@@ -177,21 +177,23 @@ static _Success_(return != 0) BOOL GetFileVersionInfoW(_In_z_ LPCWSTR lptstrFile
 
 /// @copydoc ExpandEnvironmentStringsW()
 template<class _Traits, class _Ax>
-static _Success_(return != 0) DWORD ExpandEnvironmentStringsA(_In_z_ LPCSTR lpSrc, _Out_ std::basic_string<char, _Traits, _Ax> &sValue) noexcept
+static _Success_(return != 0) DWORD ExpandEnvironmentStringsA(_In_z_ LPCSTR lpSrc, _Out_ std::basic_string<char, _Traits, _Ax> &sValue)
 {
     assert(0); // TODO: Test this code.
 
-    for (DWORD dwSizeOut = (DWORD)strlen(lpSrc) + 0x100;;) {
-        DWORD dwSizeIn = dwSizeOut;
+    for (SIZE_T sSizeOut = SIZETAdd(strlen(lpSrc), 0x100);;) {
+        if (sSizeOut > DWORD_MAX)
+            throw std::invalid_argument("String too big");
+        DWORD dwSizeIn = static_cast<DWORD>(sSizeOut);
         std::unique_ptr<char[]> szBuffer(new char[(size_t)dwSizeIn + 2]); // Note: ANSI version requires one extra char.
-        dwSizeOut = ::ExpandEnvironmentStringsA(lpSrc, szBuffer.get(), dwSizeIn);
-        if (dwSizeOut == 0) {
+        sSizeOut = ::ExpandEnvironmentStringsA(lpSrc, szBuffer.get(), dwSizeIn);
+        if (sSizeOut == 0) {
             // Error or zero-length input.
             break;
-        } else if (dwSizeOut <= dwSizeIn) {
+        } else if (sSizeOut <= dwSizeIn) {
             // The buffer was sufficient.
-            sValue.assign(szBuffer.get(), dwSizeOut - 1);
-            return dwSizeOut;
+            sValue.assign(szBuffer.get(), sSizeOut - 1);
+            return static_cast<DWORD>(sSizeOut);
         }
     }
 
@@ -205,19 +207,21 @@ static _Success_(return != 0) DWORD ExpandEnvironmentStringsA(_In_z_ LPCSTR lpSr
 /// \sa [ExpandEnvironmentStrings function](https://msdn.microsoft.com/en-us/library/windows/desktop/ms724265.aspx)
 ///
 template<class _Traits, class _Ax>
-static _Success_(return != 0) DWORD ExpandEnvironmentStringsW(_In_z_ LPCWSTR lpSrc, _Out_ std::basic_string<wchar_t, _Traits, _Ax> &sValue) noexcept
+static _Success_(return != 0) DWORD ExpandEnvironmentStringsW(_In_z_ LPCWSTR lpSrc, _Out_ std::basic_string<wchar_t, _Traits, _Ax> &sValue)
 {
-    for (DWORD dwSizeOut = (DWORD)wcslen(lpSrc) + 0x100;;) {
-        DWORD dwSizeIn = dwSizeOut;
+    for (SIZE_T sSizeOut = SIZETAdd(wcslen(lpSrc), 0x100);;) {
+        if (sSizeOut > DWORD_MAX)
+            throw std::invalid_argument("String too big");
+        DWORD dwSizeIn = static_cast<DWORD>(sSizeOut);
         std::unique_ptr<wchar_t[]> szBuffer(new wchar_t[(size_t)dwSizeIn + 1]);
-        dwSizeOut = ::ExpandEnvironmentStringsW(lpSrc, szBuffer.get(), dwSizeIn);
-        if (dwSizeOut == 0) {
+        sSizeOut = ::ExpandEnvironmentStringsW(lpSrc, szBuffer.get(), dwSizeIn);
+        if (sSizeOut == 0) {
             // Error or zero-length input.
             break;
-        } else if (dwSizeOut <= dwSizeIn) {
+        } else if (sSizeOut <= dwSizeIn) {
             // The buffer was sufficient.
-            sValue.assign(szBuffer.get(), dwSizeOut - 1);
-            return dwSizeOut;
+            sValue.assign(szBuffer.get(), sSizeOut - 1);
+            return static_cast<DWORD>(sSizeOut);
         }
     }