diff --git a/dlls/kernel32/tests/actctx.c b/dlls/kernel32/tests/actctx.c
index 56211c46040..20e5e42b620 100644
--- a/dlls/kernel32/tests/actctx.c
+++ b/dlls/kernel32/tests/actctx.c
@@ -2853,7 +2853,6 @@ static void test_CreateActCtx(void)
         handle = CreateActCtxW(&ctxW);
         if (!test[i].error)
         {
-            todo_wine
             ok(handle != INVALID_HANDLE_VALUE, "CreateActCtx error %lu\n", GetLastError());
             ReleaseActCtx(handle);
         }
@@ -2868,7 +2867,6 @@ static void test_CreateActCtx(void)
             ctxW.lpSource = sourceW; /* source without hModule must point to valid PE */
         SetLastError(0xdeadbeef);
         handle = CreateActCtxW(&ctxW);
-        todo_wine_if(i != 4)
         ok(handle != INVALID_HANDLE_VALUE, "CreateActCtx error %lu\n", GetLastError());
         ReleaseActCtx(handle);
 
diff --git a/dlls/ntdll/actctx.c b/dlls/ntdll/actctx.c
index 9379d20b6c1..fbffe691559 100644
--- a/dlls/ntdll/actctx.c
+++ b/dlls/ntdll/actctx.c
@@ -5261,9 +5261,17 @@ NTSTATUS WINAPI RtlCreateActivationContext( HANDLE *handle, const void *ptr )
 
     TRACE("%p %08lx\n", pActCtx, pActCtx ? pActCtx->dwFlags : 0);
 
-    if (!pActCtx || pActCtx->cbSize < sizeof(*pActCtx) ||
-        (pActCtx->dwFlags & ~ACTCTX_FLAGS_ALL))
+#define CHECK_LIMIT( field ) (pActCtx->cbSize >= RTL_SIZEOF_THROUGH_FIELD( ACTCTXW, field ))
+    if (!pActCtx || (pActCtx->dwFlags & ~ACTCTX_FLAGS_ALL) ||
+        !CHECK_LIMIT( lpSource ) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_PROCESSOR_ARCHITECTURE_VALID) && !CHECK_LIMIT( wProcessorArchitecture )) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_LANGID_VALID) && !CHECK_LIMIT( wLangId )) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_ASSEMBLY_DIRECTORY_VALID) && !CHECK_LIMIT( lpAssemblyDirectory )) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_RESOURCE_NAME_VALID) && !CHECK_LIMIT( lpResourceName )) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_APPLICATION_NAME_VALID) && !CHECK_LIMIT( lpApplicationName )) ||
+        ((pActCtx->dwFlags & ACTCTX_FLAG_HMODULE_VALID) && !CHECK_LIMIT( hModule )))
         return STATUS_INVALID_PARAMETER;
+#undef CHECK_LIMIT
 
     if ((pActCtx->dwFlags & ACTCTX_FLAG_RESOURCE_NAME_VALID) && !pActCtx->lpResourceName)
         return STATUS_INVALID_PARAMETER;