diff --git a/dlls/msvcr100/msvcr100.spec b/dlls/msvcr100/msvcr100.spec
index 36452977926..0f57555e454 100644
--- a/dlls/msvcr100/msvcr100.spec
+++ b/dlls/msvcr100/msvcr100.spec
@@ -1126,7 +1126,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/msvcr110/msvcr110.spec b/dlls/msvcr110/msvcr110.spec
index b057d5b85ad..072cef0f0c9 100644
--- a/dlls/msvcr110/msvcr110.spec
+++ b/dlls/msvcr110/msvcr110.spec
@@ -1483,7 +1483,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/msvcr120/msvcr120.spec b/dlls/msvcr120/msvcr120.spec
index 5f24c3f7971..55a0a4b1d2d 100644
--- a/dlls/msvcr120/msvcr120.spec
+++ b/dlls/msvcr120/msvcr120.spec
@@ -1494,7 +1494,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/msvcr80/msvcr80.spec b/dlls/msvcr80/msvcr80.spec
index 42e215d7f79..38d157f8a9a 100644
--- a/dlls/msvcr80/msvcr80.spec
+++ b/dlls/msvcr80/msvcr80.spec
@@ -798,7 +798,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/msvcr90/msvcr90.spec b/dlls/msvcr90/msvcr90.spec
index be847415309..58cb3ef2e92 100644
--- a/dlls/msvcr90/msvcr90.spec
+++ b/dlls/msvcr90/msvcr90.spec
@@ -776,7 +776,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/msvcrt/mbcs.c b/dlls/msvcrt/mbcs.c
index 01d290b2647..245dc843ba2 100644
--- a/dlls/msvcrt/mbcs.c
+++ b/dlls/msvcrt/mbcs.c
@@ -2336,20 +2336,22 @@ unsigned char* CDECL _mbsnbcat(unsigned char *dst, const unsigned char *src, siz
     return _mbsnbcat_l(dst, src, len, NULL);
 }
 
-int CDECL _mbsnbcat_s(unsigned char *dst, size_t size, const unsigned char *src, size_t len)
+/*********************************************************************
+ *		_mbsnbcat_s_l(MSVCRT.@)
+ */
+int CDECL _mbsnbcat_s_l(unsigned char *dst, size_t size, const unsigned char *src, size_t len, _locale_t locale)
 {
     unsigned char *ptr = dst;
     size_t i;
+    pthreadmbcinfo mbcinfo;
 
-    if (!dst && !size && !src && !len)
+    if (!dst && !size && !len)
         return 0;
 
-    if (!dst || !size || !src)
+    if (!MSVCRT_CHECK_PMT(dst && size && src))
     {
         if (dst && size)
             *dst = '\0';
-
-        *_errno() = EINVAL;
         return EINVAL;
     }
 
@@ -2364,9 +2366,14 @@ int CDECL _mbsnbcat_s(unsigned char *dst, size_t size, const unsigned char *src,
         return EINVAL;
     }
 
+    if (locale)
+        mbcinfo = locale->mbcinfo;
+    else
+        mbcinfo = get_mbcinfo();
+
     /* If necessary, check that the character preceding the null terminator is
      * a lead byte and move the pointer back by one for later overwrite. */
-    if (ptr != dst && get_mbcinfo()->ismbcodepage && _ismbblead(*(ptr - 1)))
+    if (ptr != dst && mbcinfo->ismbcodepage && _ismbblead_l(*(ptr - 1), locale))
         size++, ptr--;
 
     for (i = 0; *src && i < len; i++)
@@ -2386,6 +2393,14 @@ int CDECL _mbsnbcat_s(unsigned char *dst, size_t size, const unsigned char *src,
     return 0;
 }
 
+/*********************************************************************
+ *		_mbsnbcat_s(MSVCRT.@)
+ */
+int CDECL _mbsnbcat_s(unsigned char *dst, size_t size, const unsigned char *src, size_t len)
+{
+    return _mbsnbcat_s_l(dst, size, src, len, NULL);
+}
+
 /*********************************************************************
  *		_mbsncat(MSVCRT.@)
  */
diff --git a/dlls/msvcrt/msvcrt.spec b/dlls/msvcrt/msvcrt.spec
index a997a4e61d5..738279a2cd3 100644
--- a/dlls/msvcrt/msvcrt.spec
+++ b/dlls/msvcrt/msvcrt.spec
@@ -747,7 +747,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-# stub _mbsnbcat_s_l(str long ptr long ptr)
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
diff --git a/dlls/ucrtbase/ucrtbase.spec b/dlls/ucrtbase/ucrtbase.spec
index f0b078e09e7..46cea0359b4 100644
--- a/dlls/ucrtbase/ucrtbase.spec
+++ b/dlls/ucrtbase/ucrtbase.spec
@@ -642,7 +642,7 @@
 @ cdecl _mbsnbcat(str str long)
 @ cdecl _mbsnbcat_l(str str long ptr)
 @ cdecl _mbsnbcat_s(str long ptr long)
-@ stub _mbsnbcat_s_l
+@ cdecl _mbsnbcat_s_l(str long ptr long ptr)
 @ cdecl _mbsnbcmp(str str long)
 @ cdecl _mbsnbcmp_l(str str long ptr)
 @ cdecl _mbsnbcnt(ptr long)
@@ -1211,7 +1211,7 @@
 @ cdecl _o__mbsnbcat(str str long) _mbsnbcat
 @ cdecl _o__mbsnbcat_l(str str long ptr) _mbsnbcat_l
 @ cdecl _o__mbsnbcat_s(str long ptr long) _mbsnbcat_s
-@ stub _o__mbsnbcat_s_l
+@ cdecl _o__mbsnbcat_s_l(str long ptr long ptr) _mbsnbcat_s_l
 @ cdecl _o__mbsnbcmp(str str long) _mbsnbcmp
 @ cdecl _o_mbsnbcmp_l(str str long ptr) _mbsnbcmp_l
 @ cdecl _o__mbsnbcnt(ptr long) _mbsnbcnt