A vulnerability exists in the HCI IEC 60870-5-104 that affects the RTU500 series product versions listed below. Incomplete or wrong received APDU frame layout may cause blocking on link layer. Error reason was an endless blocking when reading incoming frames on link layer with wrong length information of APDU or delayed reception of data octets. Only communication link of affected HCI IEC 60870-5-104 is blocked. If attack sequence stops the communication to the previously attacked link gets normal again.