diff --git a/libvncserver/main.c b/libvncserver/main.c
index 1c9a4e7..b6bd930 100644
--- a/libvncserver/main.c
+++ b/libvncserver/main.c
@@ -242,7 +242,7 @@ rfbDefaultLog(const char *format, ...)
 
     time(&log_clock);
     strftime(buf, 255, "%d/%m/%Y %X ", localtime(&log_clock));
-    fprintf(stderr,buf);
+    fprintf(stderr, "%s", buf);
 
     vfprintf(stderr, format, args);
     fflush(stderr);