[sans-io] tlsconfig: generate proper certificates

mitmproxy/addons/tlsconfig.py

@@ -1,19 +1,24 @@
 import os
-from typing import Optional, Tuple, cast
+from typing import List, Optional, Tuple, TypedDict, cast
 
 from OpenSSL import SSL, crypto
-
 from mitmproxy import certs, ctx, exceptions
 from mitmproxy.net import tls as net_tls
 from mitmproxy.options import CONF_BASENAME
-from mitmproxy.proxy.protocol.tls import CIPHER_ID_NAME_MAP, DEFAULT_CLIENT_CIPHERS
+from mitmproxy.proxy.protocol.tls import DEFAULT_CLIENT_CIPHERS
 from mitmproxy.proxy2 import context
 from mitmproxy.proxy2.layers import tls
 
 
-def alpn_select_callback(conn: SSL.Connection, options):
-    server_alpn = conn.get_app_data()["server_alpn"]
-    http2 = conn.get_app_data()["http2"]
+class AppData(TypedDict):
+    server_alpn: Optional[bytes]
+    http2: bool
+
+
+def alpn_select_callback(conn: SSL.Connection, options: List[bytes]):
+    app_data: AppData = conn.get_app_data()
+    server_alpn = app_data["server_alpn"]
+    http2 = app_data["http2"]
     if server_alpn and server_alpn in options:
         return server_alpn
     http_alpns = tls.HTTP_ALPNS if http2 else tls.HTTP1_ALPNS
@@ -25,16 +30,57 @@ def alpn_select_callback(conn: SSL.Connection, options):
 
 
 class TlsConfig:
+    """
+    This addon supplies the proxy core with the desired OpenSSL connection objects to negotiate TLS.
+    """
     certstore: certs.CertStore = None
 
+    # TODO: We should support configuring TLS 1.3 cipher suites (https://github.com/mitmproxy/mitmproxy/issues/4260)
     # TODO: We should re-use SSL.Context options here, if only for TLS session resumption.
     #       This may require patches to pyOpenSSL, as some functionality is only exposed on contexts.
+    # TODO: This addon should manage the following options itself, which are current defined in mitmproxy/options.py:
+    #  - upstream_cert
+    #  - add_upstream_certs_to_client_chain
+    #  - ssl_version_client
+    #  - ssl_version_server
+    #  - ciphers_client
+    #  - ciphers_server
+    #  - key_size
+    #  - certs
 
     def get_cert(self, context: context.Context) -> Tuple[certs.Cert, SSL.PKey, str]:
-        # FIXME
-        return self.certstore.get_cert(
-            context.client.sni or b"localhost", [context.client.sni or b"localhost"]
-        )
+        """
+        This function determines the Common Name (CN), Subject Alternative Names (SANs) and Organization Name
+        our certificate should have and then fetches a matching cert from the certstore.
+        """
+        altnames: List[bytes] = []
+        organization: Optional[str] = None
+
+        # Use upstream certificate if available.
+        if context.server.certificate_list:
+            upstream_cert = context.server.certificate_list[0]
+            if upstream_cert.cn:
+                altnames.append(upstream_cert.cn)
+            altnames.extend(upstream_cert.altnames)
+            if upstream_cert.organization:
+                organization = upstream_cert.organization
+
+        # Add SNI. If not available, try the server address as well.
+        if context.client.sni:
+            altnames.append(context.client.sni)
+        elif context.server.address:
+            altnames.append(context.server.address[0])
+
+        # As a last resort, add *something* so that we have a certificate to serve.
+        if not altnames:
+            altnames.append(b"mitmproxy")
+
+        # only keep first occurrence of each hostname
+        altnames = list(dict.fromkeys(altnames))
+
+        # RFC 2818: If a subjectAltName extension of type dNSName is present, that MUST be used as the identity.
+        # In other words, the Common Name is irrelevant then.
+        return self.certstore.get_cert(altnames[0], altnames, organization)
 
     def tls_clienthello(self, tls_clienthello: tls.ClientHelloData):
         context = tls_clienthello.context
@@ -61,9 +107,9 @@ class TlsConfig:
         tls_method, tls_options = net_tls.VERSION_CHOICES[ctx.options.ssl_version_client]
         cert, key, chain_file = self.get_cert(tls_start.context)
         if ctx.options.add_upstream_certs_to_client_chain:
-            raise NotImplementedError()
+            extra_chain_certs = tls_start.context.server.certificate_list
         else:
-            extra_chain_certs = None
+            extra_chain_certs = ()
         ssl_ctx = net_tls.create_server_context(
             cert=cert,
             key=key,
@@ -76,10 +122,10 @@ class TlsConfig:
             extra_chain_certs=extra_chain_certs,
         )
         tls_start.ssl_conn = SSL.Connection(ssl_ctx)
-        tls_start.ssl_conn.set_app_data({
-            "server_alpn": tls_start.context.server.alpn,
-            "http2": ctx.options.http2,
-        })
+        tls_start.ssl_conn.set_app_data(AppData(
+            server_alpn=tls_start.context.server.alpn,
+            http2=ctx.options.http2,
+        ))
         tls_start.ssl_conn.set_accept_state()
 
     def create_proxy_server_ssl_conn(self, tls_start: tls.TlsStartData) -> None:
@@ -104,10 +150,10 @@ class TlsConfig:
             if ctx.options.ciphers_server:
                 server.cipher_list = ctx.options.ciphers_server.split(":")
             elif client.cipher_list:
-                server.cipher_list = [
-                    x for x in client.cipher_list
-                    if x in CIPHER_ID_NAME_MAP
-                ]
+                # We used to filter for known ciphers here, but that doesn't seem to make sense.
+                # According to OpenSSL docs, the control string str should be universally
+                # usable and not depend on details of the library configuration (ciphers compiled in).
+                server.cipher_list = list(client.cipher_list)
 
         args = net_tls.client_arguments_from_options(ctx.options)
 

mitmproxy/proxy2/context.py

@@ -25,7 +25,22 @@ class Connection:
 
     tls: bool = False
     tls_established: bool = False
-    certificate_chain: Optional[Sequence[certs.Cert]] = None
+    certificate_list: Optional[Sequence[certs.Cert]] = None
+    """
+    The TLS certificate list as sent by the peer. 
+    The first certificate is the end-entity certificate.
+    
+    [RFC 8446] Prior to TLS 1.3, "certificate_list" ordering required each
+    certificate to certify the one immediately preceding it; however,
+    some implementations allowed some flexibility.  Servers sometimes
+    send both a current and deprecated intermediate for transitional
+    purposes, and others are simply configured incorrectly, but these
+    cases can nonetheless be validated properly.  For maximum
+    compatibility, all implementations SHOULD be prepared to handle
+    potentially extraneous certificates and arbitrary orderings from any
+    TLS version, with the exception of the end-entity certificate which
+    MUST be first.
+    """
     alpn: Optional[bytes] = None
     alpn_offers: Sequence[bytes] = ()
     cipher_list: Sequence[bytes] = ()

mitmproxy/proxy2/layers/tls.py

@@ -190,7 +190,7 @@ class _TLSLayer(tunnel.TunnelLayer):
             self.conn.tls_established = True
             self.conn.sni = self.tls.get_servername()
             self.conn.alpn = self.tls.get_alpn_proto_negotiated()
-            self.conn.certificate_chain = [certs.Cert(x) for x in all_certs]
+            self.conn.certificate_list = [certs.Cert(x) for x in all_certs]
             self.conn.cipher_list = self.tls.get_cipher_list()
             self.conn.tls_version = self.tls.get_protocol_version_name()
             self.conn.timestamp_tls_setup = time.time()