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Fix DoS vulnerability: validate quantity field in read request handlers #359

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Fix DoS vulnerability: validate quantity field in read request handlers #359

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812ab7d
Validate quantity and address range in read request handlers to preve…
jamesgol Jul 26, 2026
1640a80
Add tests for read request quantity and address validation
jamesgol Jul 26, 2026
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54 changes: 54 additions & 0 deletions src/modbus-server-response-handler.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,15 @@ export default class ModbusServerResponseHandler<FR extends ModbusAbstractRespon
return
}

/* Modbus spec: FC01 quantity must be 1-2000 (0x0001 to 0x07D0) */
if (request.body.count < 1 || request.body.count > 0x7D0) {
debug('illegal data value, quantity out of range: %d', request.body.count)
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x03)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

this._server.emit('preReadCoils', request, cb)

const responseBody = ReadCoilsResponseBody.fromRequest(request.body, this._server.coils)
Expand All @@ -133,6 +142,15 @@ export default class ModbusServerResponseHandler<FR extends ModbusAbstractRespon
return
}

/* Modbus spec: FC02 quantity must be 1-2000 (0x0001 to 0x07D0) */
if (request.body.count < 1 || request.body.count > 0x7D0) {
debug('illegal data value, quantity out of range: %d', request.body.count)
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x03)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

this._server.emit('preReadDiscreteInputs', request, cb)

const responseBody = ReadDiscreteInputsResponseBody.fromRequest(request.body, this._server.discrete)
Expand Down Expand Up @@ -192,6 +210,24 @@ export default class ModbusServerResponseHandler<FR extends ModbusAbstractRespon
return
}

/* Modbus spec: FC03 quantity must be 1-125 (0x0001 to 0x007D) */
if (request.body.count < 1 || request.body.count > 0x7D) {
debug('illegal data value, quantity out of range: %d', request.body.count)
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x03)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

/* Validate start address + count does not exceed holding register buffer */
if ((request.body.start + request.body.count) * 2 > this._server.holding.length) {
debug('illegal data address')
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x02)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

this._server.emit('preReadHoldingRegisters', request, cb)

const responseBody = ReadHoldingRegistersResponseBody.fromRequest(request.body, this._server.holding)
Expand All @@ -216,6 +252,24 @@ export default class ModbusServerResponseHandler<FR extends ModbusAbstractRespon
return
}

/* Modbus spec: FC04 quantity must be 1-125 (0x0001 to 0x007D) */
if (request.body.count < 1 || request.body.count > 0x7D) {
debug('illegal data value, quantity out of range: %d', request.body.count)
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x03)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

/* Validate start address + count does not exceed input register buffer */
if ((request.body.start + request.body.count) * 2 > this._server.input.length) {
debug('illegal data address')
const exceptionBody = new ExceptionResponseBody(request.body.fc, 0x02)
const exceptionResponse = this._fromRequest(request, exceptionBody)
cb(exceptionResponse.createPayload())
return exceptionResponse
}

this._server.emit('preReadInputRegisters', request, cb)

const responseBody = ReadInputRegistersResponseBody.fromRequest(request.body, this._server.input)
Expand Down
250 changes: 249 additions & 1 deletion test/tcp-server.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,9 @@ describe('TCP Server Tests.', function () {

server = new Modbus.server.TCP(socket, {
holding: Buffer.alloc(12, 0x00),
coils: Buffer.from([0x55, 0x55, 0x55])
input: Buffer.alloc(12, 0x00),
coils: Buffer.from([0x55, 0x55, 0x55]),
discrete: Buffer.from([0xAA, 0xAA, 0xAA])
})
})

Expand Down Expand Up @@ -320,4 +322,250 @@ describe('TCP Server Tests.', function () {
socket.emit('data', request)
})
})

describe('Read Request Quantity Validation Tests.', function () {
it('should return exception 0x03 for FC01 read coils with quantity of 0', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x01, // function code
0x00, 0x00, // starting address
0x00, 0x00 // quantity 0 (below minimum of 1)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x81, // exception for FC01
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x03 for FC02 read discrete inputs with quantity of 0', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x02, // function code
0x00, 0x00, // starting address
0x00, 0x00 // quantity 0 (below minimum of 1)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x82, // exception for FC02
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x03 for FC03 read holding registers with quantity > 125', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x03, // function code
0x00, 0x00, // starting address
0x00, 0x7E // quantity 126 (exceeds 125 limit)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x83, // exception for FC03
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x03 for FC04 read input registers with quantity > 125', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x04, // function code
0x00, 0x00, // starting address
0x00, 0x7E // quantity 126 (exceeds 125 limit)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x84, // exception for FC04
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x03 for FC03 with quantity of 0', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x03, // function code
0x00, 0x00, // starting address
0x00, 0x00 // quantity 0 (below minimum of 1)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x83, // exception for FC03
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x02 for FC03 when start + count exceeds holding buffer', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x03, // function code
0x00, 0x04, // starting address 4
0x00, 0x04 // quantity 4 (address 4 + 4 = 8 registers = 16 bytes > 12 byte buffer)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x83, // exception for FC03
0x02 // ILLEGAL DATA ADDRESS
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should return exception 0x02 for FC04 when start + count exceeds input buffer', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x04, // function code
0x00, 0x04, // starting address 4
0x00, 0x04 // quantity 4 (address 4 + 4 = 8 registers = 16 bytes > 12 byte buffer)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x84, // exception for FC04
0x02 // ILLEGAL DATA ADDRESS
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should not crash when FC03 quantity would overflow byte count (DoS regression)', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x03, // function code
0x05, 0x39, // starting address 1337
0x02, 0x69 // quantity 617 (byte count 1234 overflows UInt8)
])
const expectedResponse = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x03, // byte count
0x02, // unit id
0x83, // exception for FC03
0x03 // ILLEGAL DATA VALUE
])

socket.write = (response) => {
assert.deepEqual(expectedResponse, response)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})

it('should accept FC03 read holding registers at max valid quantity for buffer', function (done) {
const request = Buffer.from([
0x00, 0x01, // transaction id
0x00, 0x00, // protocol
0x00, 0x06, // byte count
0x02, // unit id
0x03, // function code
0x00, 0x00, // starting address 0
0x00, 0x06 // quantity 6 (exactly fills 12-byte holding buffer)
])

socket.write = (response) => {
assert.equal(response.readUInt8(7), 0x03)
done()
}

socket.emit('connection', socket)
socket.emit('data', request)
})
})
})