coap protocol tunneling
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coap (constrained application protocol) tunneling exploits header field manipulation and message id patterns in rfc 7252 implementations to create covert channels in iot environments.
technical description
coap is a specialized web transfer protocol designed for constrained devices and networks. operating over udp, coap provides a lightweight alternative to http for iot applications. the protocol’s header structure and option fields offer multiple vectors for covert data transmission.
covert channel opportunities:
- message id manipulation: 16-bit message identifier patterns
- token field exploitation: 0-8 byte client-generated tokens
- option field abuse: type-length-value encoded options
- uri-path encoding: covert data in resource paths
- block-wise transfer manipulation: sequence numbers and sizes
coap message format
header structure
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ver| t | tkl | code | message id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| token (if any, tkl bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
field descriptions
| field | size | description | covert potential |
|---|---|---|---|
| version | 2 bits | coap version (1) | none - fixed |
| type | 2 bits | confirmable/non-confirmable | low |
| token length | 4 bits | 0-8 bytes | medium |
| code | 8 bits | method/response code | low |
| message id | 16 bits | transaction identifier | high |
| token | 0-8 bytes | client-generated identifier | high |
| options | variable | request/response metadata | high |
| payload | variable | message content | high |
implementation: cceap educational framework
overview
cceap (https://github.com/cdpxe/cceap) provides educational coap covert channel implementations:
- academic research framework
- multiple encoding techniques demonstrated
- focus on educational understanding
- implementation examples in python
installation
# clone cceap repository
git clone https://github.com/cdpxe/cceap.git
cd cceap
# install dependencies
pip install -r requirements.txt
# install coap libraries
pip install aiocoap coapthon3
message id covert channel
# coap message id covert channel implementation
import asyncio
import struct
import time
from aiocoap import *
class coapmessageidchannel:
def __init__(self, server_host='localhost', server_port=5683):
self.server_host = server_host
self.server_port = server_port
self.context = none
async def initialize_client(self):
"""initialize coap client context"""
self.context = await context.create_client_context()
async def encode_in_message_id(self, data):
"""encode binary data using message id field"""
# convert data to 16-bit chunks
binary_data = ''.join(format(ord(c), '08b') for c in data)
# pad to multiple of 16 bits
while len(binary_data) % 16 != 0:
binary_data += '0'
message_ids = []
for i in range(0, len(binary_data), 16):
chunk = binary_data[i:i+16]
message_id = int(chunk, 2)
message_ids.append(message_id)
return message_ids
async def send_covert_message(self, covert_data, resource_path='/test'):
"""send covert data via message id manipulation"""
if not self.context:
await self.initialize_client()
message_ids = await self.encode_in_message_id(covert_data)
responses = []
print(f"encoding '{covert_data}' in {len(message_ids)} message ids")
for i, msg_id in enumerate(message_ids):
# create coap request with specific message id
request = message(code=get, uri=f'coap://{self.server_host}:{self.server_port}{resource_path}')
# manually set message id (requires low-level access)
# in practice, this would need modification of the coap library
print(f"sending message {i+1}/{len(message_ids)} with id {msg_id:04x}")
try:
response = await self.context.request(request).response
responses.append(response)
print(f"response: {response.code}")
except exception as e:
print(f"request failed: {e}")
# small delay to avoid flooding
await asyncio.sleep(0.1)
return responses
def decode_message_ids(self, message_ids):
"""decode data from sequence of message ids"""
binary_data = ''
for msg_id in message_ids:
# convert message id to 16-bit binary
binary_chunk = format(msg_id, '016b')
binary_data += binary_chunk
# convert binary to text
decoded_text = ''
for i in range(0, len(binary_data), 8):
byte_bits = binary_data[i:i+8]
if len(byte_bits) == 8 and byte_bits != '00000000':
decoded_text += chr(int(byte_bits, 2))
return decoded_text
# coap server for message id monitoring
class coapmessageidserver:
def __init__(self, port=5683):
self.port = port
self.received_message_ids = []
async def start_server(self):
"""start coap server to monitor message ids"""
root = resource.site()
root.add_resource(['test'], testresource(self))
await context.create_server_context(root, bind=('localhost', self.port))
print(f"coap message id monitoring server started on port {self.port}")
# keep server running
await asyncio.get_event_loop().create_future()
def log_message_id(self, message_id):
"""log received message id"""
self.received_message_ids.append(message_id)
print(f"logged message id: {message_id:04x}")
# attempt decoding after every few messages
if len(self.received_message_ids) % 4 == 0:
self.attempt_decode()
def attempt_decode(self):
"""attempt to decode collected message ids"""
if len(self.received_message_ids) < 2:
return
channel = coapmessageidchannel()
decoded = channel.decode_message_ids(self.received_message_ids)
if decoded.strip():
print(f"decoded message: '{decoded}'")
class testresource(resource.resource):
def __init__(self, server):
super().__init__()
self.server = server
async def render_get(self, request):
"""handle get requests and log message ids"""
# extract message id from request
# note: this requires access to underlying coap message
# in practice, would need coap library modification
message_id = getattr(request, 'mid', 0)
self.server.log_message_id(message_id)
return message(payload=b'coap response', code=content)
# usage
async def run_covert_channel():
# start server
server = coapmessageidserver()
server_task = asyncio.create_task(server.start_server())
# small delay for server startup
await asyncio.sleep(1)
# send covert message
client = coapmessageidchannel()
await client.send_covert_message('secret')
# let server process
await asyncio.sleep(2)
# asyncio.run(run_covert_channel())
token field covert channel
# coap token field covert channel
import os
import base64
from aiocoap import *
class coaptokenchannel:
def __init__(self):
self.context = none
async def initialize_context(self):
"""initialize coap context"""
self.context = await context.create_client_context()
def encode_in_token(self, data, max_token_length=8):
"""encode data in coap token fields"""
# convert data to bytes
data_bytes = data.encode() if isinstance(data, str) else data
# split into token-sized chunks
token_chunks = []
for i in range(0, len(data_bytes), max_token_length):
chunk = data_bytes[i:i+max_token_length]
token_chunks.append(chunk)
return token_chunks
async def send_token_covert_data(self, data, server_uri):
"""send covert data using token field"""
if not self.context:
await self.initialize_context()
token_chunks = self.encode_in_token(data)
print(f"sending {len(token_chunks)} coap requests with token data")
responses = []
for i, token_data in enumerate(token_chunks):
# create request with custom token
request = message(
code=get,
uri=server_uri,
token=token_data # embed covert data in token
)
print(f"token {i+1}: {token_data.hex()}")
try:
response = await self.context.request(request).response
responses.append(response)
except exception as e:
print(f"request {i+1} failed: {e}")
await asyncio.sleep(0.2)
return responses
def decode_from_tokens(self, token_list):
"""decode data from token sequence"""
decoded_data = b''
for token in token_list:
decoded_data += token
try:
return decoded_data.decode('utf-8')
except unicodedecodeerror:
return decoded_data.hex()
# coap option field covert channel
class coapoptionchannel:
def __init__(self):
self.context = none
# coap option numbers for covert use
self.covert_options = {
'custom_1': 65000, # experimental range
'custom_2': 65001,
'custom_3': 65002
}
async def initialize_context(self):
"""initialize coap context"""
self.context = await context.create_client_context()
def create_covert_options(self, data):
"""create coap options with covert data"""
data_bytes = data.encode() if isinstance(data, str) else data
options = {}
# split data across multiple options
chunk_size = 20 # reasonable option value size
chunks = [data_bytes[i:i+chunk_size] for i in range(0, len(data_bytes), chunk_size)]
option_keys = list(self.covert_options.keys())
for i, chunk in enumerate(chunks):
if i < len(option_keys):
option_num = self.covert_options[option_keys[i]]
options[option_num] = chunk
return options
async def send_option_covert_data(self, data, server_uri):
"""send covert data using coap options"""
if not self.context:
await self.initialize_context()
covert_options = self.create_covert_options(data)
# create request with covert options
request = message(code=get, uri=server_uri)
# add covert options (requires low-level coap manipulation)
for option_num, option_value in covert_options.items():
# note: this requires coap library that supports custom options
print(f"adding option {option_num}: {option_value.hex()}")
try:
response = await self.context.request(request).response
return response
except exception as e:
print(f"option covert request failed: {e}")
return none
# uri-path covert channel
class coapuripathchannel:
def __init__(self):
self.context = none
async def initialize_context(self):
"""initialize coap context"""
self.context = await context.create_client_context()
def encode_in_uri_path(self, data):
"""encode data in uri path segments"""
# base64 encode data for uri safety
encoded_data = base64.urlsafe_b64encode(data.encode()).decode()
# split into path segments
segment_size = 10
segments = [encoded_data[i:i+segment_size]
for i in range(0, len(encoded_data), segment_size)]
# create path with legitimate-looking prefixes
path_segments = []
prefixes = ['api', 'v1', 'device', 'sensor', 'data', 'status']
for i, segment in enumerate(segments):
prefix = prefixes[i % len(prefixes)]
path_segments.append(f"{prefix}_{segment}")
return '/' + '/'.join(path_segments)
async def send_uri_covert_data(self, data, base_server_uri):
"""send covert data encoded in uri path"""
if not self.context:
await self.initialize_context()
covert_path = self.encode_in_uri_path(data)
full_uri = base_server_uri.rstrip('/') + covert_path
print(f"covert uri: {full_uri}")
request = message(code=get, uri=full_uri)
try:
response = await self.context.request(request).response
return response
except exception as e:
print(f"uri covert request failed: {e}")
return none
def decode_from_uri_path(self, uri_path):
"""decode data from uri path"""
# extract encoded segments
segments = uri_path.strip('/').split('/')
encoded_parts = []
for segment in segments:
if '_' in segment:
# extract data part after prefix
data_part = segment.split('_', 1)[1]
encoded_parts.append(data_part)
# reconstruct encoded data
full_encoded = ''.join(encoded_parts)
try:
# decode base64
decoded_bytes = base64.urlsafe_b64decode(full_encoded + '==') # padding
return decoded_bytes.decode('utf-8')
except exception as e:
print(f"uri decoding failed: {e}")
return none
# usage example
async def demonstrate_coap_covert_channels():
"""demonstrate various coap covert channel techniques"""
server_uri = 'coap://localhost:5683'
print("=== coap token channel ===")
token_channel = coaptokenchannel()
await token_channel.send_token_covert_data('hidden', server_uri + '/test')
print("\n=== coap uri path channel ===")
uri_channel = coapuripathchannel()
await uri_channel.send_uri_covert_data('secret', server_uri)
print("\n=== coap option channel ===")
option_channel = coapoptionchannel()
await option_channel.send_option_covert_data('covert', server_uri + '/test')
# asyncio.run(demonstrate_coap_covert_channels())
block-wise transfer covert channel
implementation
# coap block-wise transfer covert channel
import struct
import math
from aiocoap import *
class coapblockwisechannel:
def __init__(self):
self.context = none
self.block_size_exponents = [4, 5, 6, 7, 8, 9, 10] # 16 to 1024 bytes
async def initialize_context(self):
"""initialize coap context"""
self.context = await context.create_client_context()
def encode_in_block_parameters(self, data):
"""encode data using block size and sequence manipulation"""
# convert data to binary
binary_data = ''.join(format(ord(c), '08b') for c in data)
# encode in block size exponent (3 bits per block)
block_parameters = []
for i in range(0, len(binary_data), 3):
chunk = binary_data[i:i+3].ljust(3, '0') # pad if needed
block_size_idx = int(chunk, 2)
if block_size_idx < len(self.block_size_exponents):
block_size_exp = self.block_size_exponents[block_size_idx]
else:
block_size_exp = self.block_size_exponents[0] # fallback
block_parameters.append({
'size_exponent': block_size_exp,
'block_number': i // 3, # sequence number
'more_blocks': i + 3 < len(binary_data)
})
return block_parameters
async def send_blockwise_covert_data(self, data, server_uri):
"""send covert data using block-wise transfers"""
if not self.context:
await self.initialize_context()
block_params = self.encode_in_block_parameters(data)
print(f"sending {len(block_params)} block requests with covert data")
responses = []
for i, params in enumerate(block_params):
# create block option value
# format: num=block_number, m=more_flag, szx=size_exponent
block_value = (params['block_number'] << 4) | \
(1 if params['more_blocks'] else 0) << 3 | \
(params['size_exponent'] - 4) # szx encoding
print(f"block {i}: num={params['block_number']}, " +
f"szx={params['size_exponent']}, more={params['more_blocks']}")
# create coap request with block1 option
request = message(
code=post,
uri=server_uri,
payload=b'x' * (2 ** params['size_exponent']) # dummy payload
)
# add block1 option (would need coap library support)
# request.opt.block1 = (params['block_number'], params['more_blocks'], params['size_exponent'])
try:
response = await self.context.request(request).response
responses.append(response)
except exception as e:
print(f"block request {i} failed: {e}")
await asyncio.sleep(0.1)
return responses
def decode_block_parameters(self, block_params):
"""decode data from block parameters"""
binary_data = ''
# sort by block number to ensure correct order
sorted_params = sorted(block_params, key=lambda x: x['block_number'])
for params in sorted_params:
# find size exponent index
if params['size_exponent'] in self.block_size_exponents:
size_idx = self.block_size_exponents.index(params['size_exponent'])
else:
size_idx = 0
# convert to 3-bit binary
binary_chunk = format(size_idx, '03b')
binary_data += binary_chunk
# convert binary to text
decoded_text = ''
for i in range(0, len(binary_data), 8):
byte_bits = binary_data[i:i+8]
if len(byte_bits) == 8:
char_code = int(byte_bits, 2)
if 32 <= char_code <= 126: # printable ascii
decoded_text += chr(char_code)
return decoded_text
# coap observe covert channel
class coapobservechannel:
def __init__(self):
self.context = none
self.observe_values = []
async def initialize_context(self):
"""initialize coap context"""
self.context = await context.create_client_context()
async def encode_in_observe_sequence(self, data, server_uri):
"""encode data in observe notification sequence numbers"""
if not self.context:
await self.initialize_context()
# start observe relationship
request = message(code=get, uri=server_uri, observe=0)
try:
# this would require custom coap server that manipulates observe values
response = await self.context.request(request).response
print(f"started observe relationship: {response.code}")
# in practice, the server would send notifications with
# sequence numbers that encode the covert data
except exception as e:
print(f"observe setup failed: {e}")
def decode_observe_sequence(self, observe_values):
"""decode data from observe notification sequence"""
# extract data from observe option values
binary_data = ''
for observe_val in observe_values:
# use observe value as 8-bit data (0-255)
if 0 <= observe_val <= 255:
binary_chunk = format(observe_val, '08b')
binary_data += binary_chunk
# convert to text
decoded_text = ''
for i in range(0, len(binary_data), 8):
byte_bits = binary_data[i:i+8]
if len(byte_bits) == 8:
decoded_text += chr(int(byte_bits, 2))
return decoded_text
# complete coap covert channel suite
class coapcovertchannelsuite:
def __init__(self, server_uri='coap://localhost:5683'):
self.server_uri = server_uri
self.channels = {
'token': coaptokenchannel(),
'uri_path': coapuripathchannel(),
'option': coapoptionchannel(),
'blockwise': coapblockwisechannel(),
'observe': coapobservechannel()
}
async def send_multi_channel_data(self, data, channel_types=['token', 'uri_path']):
"""send data across multiple covert channels simultaneously"""
results = {}
for channel_type in channel_types:
if channel_type in self.channels:
channel = self.channels[channel_type]
try:
if channel_type == 'token':
result = await channel.send_token_covert_data(data, self.server_uri + '/test')
elif channel_type == 'uri_path':
result = await channel.send_uri_covert_data(data, self.server_uri)
elif channel_type == 'option':
result = await channel.send_option_covert_data(data, self.server_uri + '/test')
elif channel_type == 'blockwise':
result = await channel.send_blockwise_covert_data(data, self.server_uri + '/upload')
elif channel_type == 'observe':
result = await channel.encode_in_observe_sequence(data, self.server_uri + '/events')
results[channel_type] = result
except exception as e:
print(f"{channel_type} channel failed: {e}")
results[channel_type] = none
return results
async def analyze_channel_performance(self, data, iterations=5):
"""analyze performance characteristics of coap covert channels"""
performance = {}
for channel_name, channel in self.channels.items():
start_time = time.time()
success_count = 0
for i in range(iterations):
try:
if channel_name == 'token':
await channel.send_token_covert_data(f"{data}_{i}", self.server_uri + '/test')
# ... other channel types
success_count += 1
except exception as e:
print(f"{channel_name} iteration {i} failed: {e}")
end_time = time.time()
performance[channel_name] = {
'success_rate': success_count / iterations,
'avg_time_per_message': (end_time - start_time) / iterations,
'total_time': end_time - start_time
}
return performance
# usage
async def run_comprehensive_coap_test():
"""run comprehensive coap covert channel test"""
suite = coapcovertchannelsuite()
# test individual channels
test_data = "covert message"
results = await suite.send_multi_channel_data(test_data, ['token', 'uri_path'])
print("coap covert channel results:")
for channel, result in results.items():
status = "success" if result else "failed"
print(f" {channel}: {status}")
# performance analysis
performance = await suite.analyze_channel_performance("test", iterations=3)
print("\nperformance analysis:")
for channel, stats in performance.items():
print(f" {channel}:")
print(f" success rate: {stats['success_rate']:.1%}")
print(f" avg time: {stats['avg_time_per_message']:.2f}s")
# asyncio.run(run_comprehensive_coap_test())
traffic analysis and detection
coap packet analysis
# coap covert channel detection
import struct
import statistics
from collections import defaultdict, counter
class coapcovertdetector:
def __init__(self):
self.client_stats = defaultdict(lambda: {
'message_ids': [],
'tokens': [],
'uri_paths': [],
'options': [],
'request_count': 0,
'first_seen': none,
'last_seen': none
})
# detection thresholds
self.thresholds = {
'high_entropy': 6.0,
'unusual_message_id_pattern': 0.8, # correlation threshold
'max_token_entropy': 5.0,
'suspicious_uri_length': 50,
'unusual_option_codes': [65000, 65001, 65002] # experimental range
}
def analyze_coap_packet(self, timestamp, src_ip, dst_ip, coap_data):
"""analyze coap packet for covert channel indicators"""
if len(coap_data) < 4:
return # invalid coap packet
# parse coap header
header = struct.unpack('!bbh', coap_data[:4])
version_type_tkl = header[0]
code = header[1]
message_id = header[2]
version = (version_type_tkl >> 6) & 0x03
msg_type = (version_type_tkl >> 4) & 0x03
token_length = version_type_tkl & 0x0f
if version != 1: # only coap version 1
return
# extract token
token = b''
if token_length > 0 and len(coap_data) > 4:
token = coap_data[4:4+token_length]
# update client statistics
client_key = (src_ip, dst_ip)
stats = self.client_stats[client_key]
stats['message_ids'].append(message_id)
stats['tokens'].append(token)
stats['request_count'] += 1
if stats['first_seen'] is none:
stats['first_seen'] = timestamp
stats['last_seen'] = timestamp
# parse options and uri path (simplified)
options_start = 4 + token_length
if len(coap_data) > options_start:
self.parse_coap_options(coap_data[options_start:], stats)
# check for anomalies
self.check_coap_anomalies(client_key)
def parse_coap_options(self, options_data, stats):
"""parse coap options for analysis"""
i = 0
option_number = 0
while i < len(options_data):
if options_data[i] == 0xff: # payload marker
break
if i >= len(options_data):
break
# parse option delta and length
option_byte = options_data[i]
option_delta = (option_byte >> 4) & 0x0f
option_length = option_byte & 0x0f
i += 1
# handle extended delta/length (simplified)
if option_delta == 13:
if i < len(options_data):
option_delta = 13 + options_data[i]
i += 1
elif option_delta == 14:
if i + 1 < len(options_data):
option_delta = 269 + struct.unpack('!h', options_data[i:i+2])[0]
i += 2
if option_length == 13:
if i < len(options_data):
option_length = 13 + options_data[i]
i += 1
elif option_length == 14:
if i + 1 < len(options_data):
option_length = 269 + struct.unpack('!h', options_data[i:i+2])[0]
i += 2
option_number += option_delta
# extract option value
if option_length > 0 and i + option_length <= len(options_data):
option_value = options_data[i:i + option_length]
i += option_length
stats['options'].append((option_number, option_value))
# check for uri-path (option 11)
if option_number == 11:
try:
uri_segment = option_value.decode('utf-8')
stats['uri_paths'].append(uri_segment)
except:
pass
else:
break
def check_coap_anomalies(self, client_key):
"""check for coap covert channel anomalies"""
stats = self.client_stats[client_key]
if stats['request_count'] < 5: # need minimum samples
return
anomalies = []
# message id analysis
if len(stats['message_ids']) >= 10:
msg_id_anomalies = self.analyze_message_id_patterns(stats['message_ids'])
anomalies.extend(msg_id_anomalies)
# token analysis
if len(stats['tokens']) >= 5:
token_anomalies = self.analyze_token_patterns(stats['tokens'])
anomalies.extend(token_anomalies)
# uri path analysis
if stats['uri_paths']:
uri_anomalies = self.analyze_uri_patterns(stats['uri_paths'])
anomalies.extend(uri_anomalies)
# option analysis
if stats['options']:
option_anomalies = self.analyze_option_patterns(stats['options'])
anomalies.extend(option_anomalies)
if anomalies:
src_ip, dst_ip = client_key
print(f"coap anomalies detected: {src_ip} -> {dst_ip}")
for anomaly in anomalies:
print(f" {anomaly}")
def analyze_message_id_patterns(self, message_ids):
"""analyze message id patterns for covert channels"""
anomalies = []
# check for sequential patterns
if len(message_ids) >= 5:
differences = [message_ids[i] - message_ids[i-1] for i in range(1, len(message_ids))]
# consistent differences suggest encoding
if len(set(differences)) <= 2 and 0 not in set(differences):
anomalies.append(f"sequential message id pattern detected")
# entropy analysis
if len(message_ids) >= 10:
entropy = self.calculate_entropy(message_ids)
if entropy > 14: # high entropy for 16-bit values
anomalies.append(f"high message id entropy: {entropy:.2f}")
# check for repeated patterns
id_counts = counter(message_ids)
most_common = id_counts.most_common(1)[0]
if most_common[1] > len(message_ids) * 0.3: # >30% repetition
anomalies.append(f"repeated message id: {most_common[0]:04x}")
return anomalies
def analyze_token_patterns(self, tokens):
"""analyze token patterns for covert channels"""
anomalies = []
# check token lengths
token_lengths = [len(token) for token in tokens]
if statistics.stdev(token_lengths) if len(token_lengths) > 1 else 0 > 2:
anomalies.append("highly variable token lengths")
# check token entropy
for i, token in enumerate(tokens[-5:]): # check recent tokens
if len(token) > 0:
entropy = self.calculate_entropy(token)
if entropy > self.thresholds['max_token_entropy']:
anomalies.append(f"high entropy token: {token.hex()}")
# check for base64-like patterns
for token in tokens[-5:]:
if len(token) > 4:
try:
decoded = token.decode('ascii')
if self.looks_like_base64(decoded):
anomalies.append(f"base64-like token: {decoded}")
except:
pass
return anomalies
def analyze_uri_patterns(self, uri_paths):
"""analyze uri path patterns for covert channels"""
anomalies = []
# check for unusually long uri segments
for uri_segment in uri_paths:
if len(uri_segment) > self.thresholds['suspicious_uri_length']:
anomalies.append(f"long uri segment: {uri_segment[:20]}...")
# check for base64 encoding in uri
if self.looks_like_base64(uri_segment):
anomalies.append(f"base64-like uri segment: {uri_segment}")
# check for high entropy
entropy = self.calculate_entropy(uri_segment.encode())
if entropy > self.thresholds['high_entropy']:
anomalies.append(f"high entropy uri segment: {uri_segment}")
return anomalies
def analyze_option_patterns(self, options):
"""analyze coap options for covert channels"""
anomalies = []
# check for unusual option numbers
option_numbers = [opt[0] for opt in options]
for option_num in option_numbers:
if option_num in self.thresholds['unusual_option_codes']:
anomalies.append(f"experimental option code: {option_num}")
# very high option numbers are suspicious
if option_num > 60000:
anomalies.append(f"high option number: {option_num}")
# check option value entropy
for option_num, option_value in options[-10:]: # recent options
if len(option_value) > 4:
entropy = self.calculate_entropy(option_value)
if entropy > self.thresholds['high_entropy']:
anomalies.append(f"high entropy option {option_num}: {option_value.hex()}")
return anomalies
def calculate_entropy(self, data):
"""calculate shannon entropy"""
from collections import counter
import math
if not data:
return 0
if isinstance(data[0], int): # list of integers
counts = counter(data)
else: # bytes
counts = counter(data)
total = len(data)
probs = [count/total for count in counts.values()]
return -sum(p * math.log2(p) for p in probs if p > 0)
def looks_like_base64(self, text):
"""heuristic to detect base64 encoding"""
import re
if len(text) < 4:
return false
# base64 character pattern
base64_pattern = re.compile(r'^[a-za-z0-9+/_-]+=*$')
if not base64_pattern.match(text):
return false
# length should be multiple of 4
if len(text) % 4 not in [0, 2, 3]: # account for padding
return false
# high entropy suggests encoding
entropy = self.calculate_entropy(text.encode())
return entropy > 4.0
# usage
detector = coapcovertdetector()
# simulate coap packet analysis
coap_packet = b'\x44\x01\x12\x34token123/api/data' # example coap packet
detector.analyze_coap_packet(time.time(), '192.168.1.100', '192.168.1.1', coap_packet)
countermeasures
coap server hardening
# coap server with covert channel prevention
from aiocoap import *
import hashlib
import time
class securecoapserver:
def __init__(self, port=5683):
self.port = port
# security policies
self.security_policy = {
'max_token_length': 4, # reduce from 8
'allowed_message_types': [0, 1], # con, non
'max_uri_path_length': 20,
'max_payload_size': 64,
'allowed_option_codes': [3, 7, 8, 11, 12, 14, 15, 17, 20, 35, 39, 60], # standard options only
'rate_limit_per_ip': 10, # requests per minute
'max_block_size': 64 # limit block transfers
}
# rate limiting
self.client_requests = defaultdict(list)
async def start_server(self):
"""start secure coap server"""
root = resource.site()
root.add_resource(['secure'], secureresource(self))
root.add_resource(['test'], secureresource(self))
await context.create_server_context(root, bind=('localhost', self.port))
print(f"secure coap server started on port {self.port}")
await asyncio.get_event_loop().create_future()
def validate_request(self, request, client_ip):
"""validate coap request against security policy"""
violations = []
# rate limiting
current_time = time.time()
client_history = self.client_requests[client_ip]
# remove old requests (older than 1 minute)
client_history[:] = [req_time for req_time in client_history
if current_time - req_time < 60]
if len(client_history) >= self.security_policy['rate_limit_per_ip']:
violations.append("rate limit exceeded")
client_history.append(current_time)
# token validation
if hasattr(request, 'token') and len(request.token) > self.security_policy['max_token_length']:
violations.append(f"token too long: {len(request.token)} bytes")
# uri path validation
if hasattr(request, 'opt') and request.opt.uri_path:
full_path = '/'.join(request.opt.uri_path)
if len(full_path) > self.security_policy['max_uri_path_length']:
violations.append(f"uri path too long: {len(full_path)}")
# check for suspicious patterns
if self.contains_suspicious_patterns(full_path):
violations.append("suspicious uri path pattern")
# payload size validation
if hasattr(request, 'payload') and len(request.payload) > self.security_policy['max_payload_size']:
violations.append(f"payload too large: {len(request.payload)} bytes")
# option validation (if accessible)
# note: this would require low-level coap library access
return violations
def contains_suspicious_patterns(self, text):
"""check for suspicious patterns in text"""
# high entropy check
entropy = self.calculate_entropy(text.encode())
if entropy > 4.5:
return true
# base64 pattern check
import re
base64_pattern = re.compile(r'[a-za-z0-9+/]{20,}={0,2}')
if base64_pattern.search(text):
return true
# excessive length for single segment
segments = text.split('/')
if any(len(segment) > 15 for segment in segments):
return true
return false
def calculate_entropy(self, data):
"""calculate shannon entropy"""
from collections import counter
import math
if not data:
return 0
counts = counter(data)
probs = [count/len(data) for count in counts.values()]
return -sum(p * math.log2(p) for p in probs if p > 0)
def normalize_response(self, response):
"""normalize response to prevent information leakage"""
# use consistent message ids (hash-based)
if hasattr(response, 'mid'):
# generate deterministic message id
hash_input = f"{time.time()//60}".encode() # change every minute
response.mid = int(hashlib.md5(hash_input).hexdigest()[:4], 16)
# limit response payload
if hasattr(response, 'payload') and len(response.payload) > 64:
response.payload = response.payload[:64]
return response
class secureresource(resource.resource):
def __init__(self, server):
super().__init__()
self.server = server
async def render_get(self, request):
"""handle get requests with security validation"""
# extract client ip (simplified)
client_ip = '127.0.0.1' # would extract from request in practice
# validate request
violations = self.server.validate_request(request, client_ip)
if violations:
print(f"security violations from {client_ip}: {violations}")
return message(code=bad_request, payload=b'request blocked')
# process legitimate request
response = message(code=content, payload=b'secure coap response')
# normalize response
return self.server.normalize_response(response)
async def render_post(self, request):
"""handle post requests with security validation"""
client_ip = '127.0.0.1'
violations = self.server.validate_request(request, client_ip)
if violations:
print(f"security violations from {client_ip}: {violations}")
return message(code=bad_request, payload=b'request blocked')
return message(code=created, payload=b'resource created')
# coap traffic normalization proxy
class coapnormalizationproxy:
def __init__(self, upstream_server, bind_port=5684):
self.upstream_server = upstream_server
self.bind_port = bind_port
self.context = none
async def start_proxy(self):
"""start coap normalization proxy"""
self.context = await context.create_client_context()
root = resource.site()
root.add_resource(['.well-known', 'core'], proxyresource(self))
root.add_resource(['proxy'], proxyresource(self))
await context.create_server_context(root, bind=('localhost', self.bind_port))
print(f"coap normalization proxy on port {self.bind_port}")
await asyncio.get_event_loop().create_future()
async def normalize_and_forward(self, request):
"""normalize request and forward to upstream"""
# create normalized request
normalized_request = self.normalize_coap_request(request)
# forward to upstream server
try:
response = await self.context.request(normalized_request).response
# normalize response
normalized_response = self.normalize_coap_response(response)
return normalized_response
except exception as e:
print(f"upstream request failed: {e}")
return message(code=bad_gateway, payload=b'upstream error')
def normalize_coap_request(self, request):
"""normalize coap request to remove covert channels"""
# create clean request
normalized = message(
code=request.code,
uri=f"coap://{self.upstream_server}/normalized"
)
# sanitize payload
if hasattr(request, 'payload') and request.payload:
# limit payload size
clean_payload = request.payload[:64]
# remove potential covert markers
clean_payload = clean_payload.replace(b'\xaa\xbb', b'')
clean_payload = clean_payload.replace(b'\xff\xfe', b'')
normalized.payload = clean_payload
# use standardized token
normalized.token = b'norm'
return normalized
def normalize_coap_response(self, response):
"""normalize coap response"""
# create clean response
normalized = message(
code=response.code,
payload=response.payload[:64] if response.payload else b''
)
# use predictable message id
normalized.mid = int(time.time()) & 0xffff
return normalized
class proxyresource(resource.resource):
def __init__(self, proxy):
super().__init__()
self.proxy = proxy
async def render_get(self, request):
"""proxy get requests"""
return await self.proxy.normalize_and_forward(request)
async def render_post(self, request):
"""proxy post requests"""
return await self.proxy.normalize_and_forward(request)
# usage
async def run_secure_coap_infrastructure():
"""run secure coap server and proxy"""
# start secure server
server = securecoapserver(5683)
server_task = asyncio.create_task(server.start_server())
# start normalization proxy
proxy = coapnormalizationproxy('localhost:5683', 5684)
proxy_task = asyncio.create_task(proxy.start_proxy())
# run both
await asyncio.gather(server_task, proxy_task)
# asyncio.run(run_secure_coap_infrastructure())
advantages and limitations
advantages
- iot environment integration: blends with legitimate iot traffic
- udp-based protocol: connectionless communication reduces detection
- multiple encoding vectors: tokens, options, uris, block transfers
- constrained device compatibility: works with resource-limited devices
- standard protocol compliance: maintains coap protocol correctness
limitations
- limited deployment: coap not as widely deployed as http
- udp unreliability: packet loss affects covert communication
- small header fields: limited capacity per packet
- protocol inspection: coap parsers can detect anomalies
- iot security focus: increasing attention to iot protocol security
performance characteristics
capacity analysis
| technique | capacity per packet | reliability | stealth level |
|---|---|---|---|
| token field | 8 bytes | high | high |
| message id | 2 bytes | high | medium |
| uri path | 50+ bytes | high | low |
| options | 20+ bytes | medium | high |
| payload | 1024+ bytes | high | low |
coap vs other protocols
# coap covert channel performance comparison
def compare_covert_protocols():
"""compare coap with other iot protocols for covert channels"""
protocols = {
'coap': {
'transport': 'udp',
'header_size': 4,
'max_payload': 1024,
'covert_fields': ['token', 'message_id', 'options', 'uri'],
'covert_capacity_per_packet': 40, # conservative estimate
'detection_difficulty': 'medium'
},
'mqtt': {
'transport': 'tcp',
'header_size': 2,
'max_payload': 256000000, # 256mb theoretical
'covert_fields': ['client_id', 'topic', 'payload'],
'covert_capacity_per_packet': 100,
'detection_difficulty': 'low'
},
'http': {
'transport': 'tcp',
'header_size': 'variable',
'max_payload': 'unlimited',
'covert_fields': ['headers', 'uri', 'payload', 'cookies'],
'covert_capacity_per_packet': 1000,
'detection_difficulty': 'low'
},
'dns': {
'transport': 'udp',
'header_size': 12,
'max_payload': 512, # standard
'covert_fields': ['queries', 'responses', 'subdomains'],
'covert_capacity_per_packet': 200,
'detection_difficulty': 'medium'
}
}
print("iot covert channel protocol comparison:")
for protocol, specs in protocols.items():
print(f"\n{protocol.upper()}:")
print(f" transport: {specs['transport']}")
print(f" covert capacity: ~{specs['covert_capacity_per_packet']} bytes/packet")
print(f" detection difficulty: {specs['detection_difficulty']}")
print(f" covert fields: {', '.join(specs['covert_fields'])}")
# compare_covert_protocols()
real-world applications
iot device compromise
- command and control for compromised iot devices
- data exfiltration from smart home systems
- coordination between distributed sensors
industrial espionage
- monitoring industrial iot deployments
- extracting sensor data from smart factories
- maintaining access in operational technology networks
research and testing
- iot security assessment frameworks
- covert channel capacity studies
- protocol implementation testing
references
- rfc 7252: the constrained application protocol (coap)
- rfc 7641: observing resources in the constrained application protocol
- rfc 7959: block-wise transfers in the constrained application protocol
- “security considerations for coap” - ietf core working group
- cceap educational framework - network covert channels research