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5327756 Apr 26, 2017
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@cortesi @mhils
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"""
This inline script allows conditional TLS Interception based
on a user-defined strategy.
Example:
> mitmdump -s tls_passthrough.py
1. curl --proxy http://localhost:8080 https://example.com --insecure
// works - we'll also see the contents in mitmproxy
2. curl --proxy http://localhost:8080 https://example.com --insecure
// still works - we'll also see the contents in mitmproxy
3. curl --proxy http://localhost:8080 https://example.com
// fails with a certificate error, which we will also see in mitmproxy
4. curl --proxy http://localhost:8080 https://example.com
// works again, but mitmproxy does not intercept and we do *not* see the contents
Authors: Maximilian Hils, Matthew Tuusberg
"""
import collections
import random
from enum import Enum
import mitmproxy
from mitmproxy import ctx
from mitmproxy.exceptions import TlsProtocolException
from mitmproxy.proxy.protocol import TlsLayer, RawTCPLayer
class InterceptionResult(Enum):
success = True
failure = False
skipped = None
class _TlsStrategy:
"""
Abstract base class for interception strategies.
"""
def __init__(self):
# A server_address -> interception results mapping
self.history = collections.defaultdict(lambda: collections.deque(maxlen=200))
def should_intercept(self, server_address):
"""
Returns:
True, if we should attempt to intercept the connection.
False, if we want to employ pass-through instead.
"""
raise NotImplementedError()
def record_success(self, server_address):
self.history[server_address].append(InterceptionResult.success)
def record_failure(self, server_address):
self.history[server_address].append(InterceptionResult.failure)
def record_skipped(self, server_address):
self.history[server_address].append(InterceptionResult.skipped)
class ConservativeStrategy(_TlsStrategy):
"""
Conservative Interception Strategy - only intercept if there haven't been any failed attempts
in the history.
"""
def should_intercept(self, server_address):
if InterceptionResult.failure in self.history[server_address]:
return False
return True
class ProbabilisticStrategy(_TlsStrategy):
"""
Fixed probability that we intercept a given connection.
"""
def __init__(self, p):
self.p = p
super(ProbabilisticStrategy, self).__init__()
def should_intercept(self, server_address):
return random.uniform(0, 1) < self.p
class TlsFeedback(TlsLayer):
"""
Monkey-patch _establish_tls_with_client to get feedback if TLS could be established
successfully on the client connection (which may fail due to cert pinning).
"""
def _establish_tls_with_client(self):
server_address = self.server_conn.address
try:
super(TlsFeedback, self)._establish_tls_with_client()
except TlsProtocolException as e:
tls_strategy.record_failure(server_address)
raise e
else:
tls_strategy.record_success(server_address)
# inline script hooks below.
tls_strategy = None
def load(l):
l.add_option(
"tlsstrat", int, 0, "TLS passthrough strategy (0-100)",
)
def configure(updated):
global tls_strategy
if ctx.options.tlsstrat > 0:
tls_strategy = ProbabilisticStrategy(float(ctx.options.tlsstrat) / 100.0)
else:
tls_strategy = ConservativeStrategy()
def next_layer(next_layer):
"""
This hook does the actual magic - if the next layer is planned to be a TLS layer,
we check if we want to enter pass-through mode instead.
"""
if isinstance(next_layer, TlsLayer) and next_layer._client_tls:
server_address = next_layer.server_conn.address
if tls_strategy.should_intercept(server_address):
# We try to intercept.
# Monkey-Patch the layer to get feedback from the TLSLayer if interception worked.
next_layer.__class__ = TlsFeedback
else:
# We don't intercept - reply with a pass-through layer and add a "skipped" entry.
mitmproxy.ctx.log("TLS passthrough for %s" % repr(next_layer.server_conn.address), "info")
next_layer_replacement = RawTCPLayer(next_layer.ctx, ignore=True)
next_layer.reply.send(next_layer_replacement)
tls_strategy.record_skipped(server_address)