Index: tsm/tsm-vte.c
--- tsm/tsm-vte.c.orig
+++ tsm/tsm-vte.c
@@ -439,9 +439,9 @@ static void vte_write_fnkey(struct tsm_vte *vte, bool 
 	if (fnmod > 1) {
 		if (index <= 0)
 			index = 1;
-		len = sprintf(buf, "\e[%d;%d%c", index, fnmod, c);
+		len = snprintf(buf, sizeof(buf), "\e[%d;%d%c", index, fnmod, c);
 	} else {
-		len = sprintf(buf, "\e%c%.0d%c", ss3 ? 'O' : '[', index, c);
+		len = snprintf(buf, sizeof(buf), "\e%c%.0d%c", ss3 ? 'O' : '[', index, c);
 	}
 	vte_write(vte, buf, len);
 }
