diff --git "a/sdk/lib/crt/mbstring/mbsncat.c" "b/sdk/lib/crt/mbstring/mbsncat.c"
index 21e63d0c0a1..ea3516a5a9e 100644
--- "a/sdk/lib/crt/mbstring/mbsncat.c"
+++ "b/sdk/lib/crt/mbstring/mbsncat.c"
@@ -12,6 +12,11 @@
 #include <mbstring.h>
 #include <string.h>
 
+static inline unsigned char* u_strncat( unsigned char* dst, const unsigned char* src, size_t len )
+{
+  return (unsigned char*)strncat( (char*)dst, (const char*)src, len);
+}
+
 size_t _mbclen2(const unsigned int s);
 unsigned char *_mbset (unsigned char *string, int c);
 
@@ -46,22 +51,33 @@ unsigned char *_mbsncat (unsigned char *dst, const unsigned char *src, size_t n)
  */
 unsigned char * _mbsnbcat(unsigned char *dst, const unsigned char *src, size_t n)
 {
-	unsigned char *d;
-	const unsigned char *s = src;
-	if (n != 0) {
-		d = dst + _mbslen(dst); // get the end of string
-		d += _mbclen2(*d); // move 1 or 2 up
+    MSVCRT_pthreadmbcinfo mbcinfo = get_mbcinfo();
 
-		do {
-			if ((*d++ = *s++) == 0)
-			{
-				while (--n != 0)
-					*d++ = 0;
-				break;
-			}
-			if ( !(n==1 && _ismbblead(*s)) )
-				n--;
-		} while (n > 0);
-	}
-	return dst;
+    if (!MSVCRT_CHECK_PMT(dst && src))
+        return NULL;
+
+    if (mbcinfo->ismbcodepage)
+    {
+        unsigned char *res = dst;
+        while (*dst)
+        {
+            if (_ismbblead(*dst++))
+            {
+                if (*dst)
+                {
+                    dst++;
+                }
+                else
+                {
+                    /* as per msdn overwrite the lead byte in front of '\0' */
+                    dst--;
+                    break;
+                }
+            }
+        }
+        while (*src && n--) *dst++ = *src++;
+        *dst = '\0';
+        return res;
+    }
+    return u_strncat(dst, src, n); /* ASCII CP */
 }