diff --git a/ChangeLog b/ChangeLog
index e777c33..a925eac 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2017-01-02  Szczepan Zalega <szczepan@nitrokey.com>
+
+	* tool/upgrade_by_passwd.py: Add file extention check.
+
 2017-02-01  NIIBE Yutaka  <gniibe@fsij.org>
 
 	* tool/upgrade_by_passwd.py (main): More verbose messages
diff --git a/tool/upgrade_by_passwd.py b/tool/upgrade_by_passwd.py
index 18d9b46..f76f9e2 100755
--- a/tool/upgrade_by_passwd.py
+++ b/tool/upgrade_by_passwd.py
@@ -124,6 +124,9 @@ if __name__ == '__main__':
         passwd = getpass("Admin password: ")
     filename_regnual = sys.argv[1]
     filename_upgrade = sys.argv[2]
+    if not filename_regnual.endswith('bin') or not filename_upgrade.endswith('bin'):
+        print("Both input files must be in binary format (*.bin)!")
+        exit(1)
     f = open(filename_regnual,"rb")
     data_regnual = f.read()
     f.close()