diff --git a/lldb/source/Host/windows/FileSystem.cpp b/lldb/source/Host/windows/FileSystem.cpp
--- a/lldb/source/Host/windows/FileSystem.cpp
+++ b/lldb/source/Host/windows/FileSystem.cpp
@@ -101,6 +101,8 @@
   std::wstring wpath;
   if (!llvm::ConvertUTF8toWide(path, wpath))
     return -1;
+  // All other bits are rejected by _wsopen_s
+  mode = mode & (_S_IREAD | _S_IWRITE);
   int result;
   ::_wsopen_s(&result, wpath.c_str(), flags, _SH_DENYNO, mode);
   return result;