diff --git a/sys/winnt/winnt.c b/sys/winnt/winnt.c
index 967fa6e4e..e7a2040a6 100644
--- a/sys/winnt/winnt.c
+++ b/sys/winnt/winnt.c
@@ -241,11 +241,11 @@ VA_DECL(const char *, s)
         end_screen();
     if (WINDOWPORT("tty")) {
         buf[0] = '\n';
-        (void) vsprintf(&buf[1], s, VA_ARGS);
+        (void) vsnprintf(&buf[1], sizeof buf - (1 + sizeof "\n"), s, VA_ARGS);
         Strcat(buf, "\n");
         msmsg(buf);
     } else {
-        (void) vsprintf(buf, s, VA_ARGS);
+        (void) vsnprintf(buf, sizeof buf - sizeof "\n", s, VA_ARGS);
         Strcat(buf, "\n");
         raw_printf(buf);
     }