Implement login blocking based on SAML attributes (#8052)

Hopefully this mostly speaks for itself. I also did a bit of cleaning up of the
error handling.

Fixes #8047

changelog.d/8052.feature

@@ -0,0 +1 @@
+Allow login to be blocked based on the values of SAML attributes.

docs/sample_config.yaml

@@ -1577,6 +1577,17 @@ saml2_config:
   #
   #grandfathered_mxid_source_attribute: upn
 
+  # It is possible to configure Synapse to only allow logins if SAML attributes
+  # match particular values. The requirements can be listed under
+  # `attribute_requirements` as shown below. All of the listed attributes must
+  # match for the login to be permitted.
+  #
+  #attribute_requirements:
+  #  - attribute: userGroup
+  #    value: "staff"
+  #  - attribute: department
+  #    value: "sales"
+
   # Directory in which Synapse will try to find the template files below.
   # If not set, default templates from within the Synapse package will be used.
   #

synapse/config/_util.py

@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import Any, List
+
+import jsonschema
+
+from synapse.config._base import ConfigError
+from synapse.types import JsonDict
+
+
+def validate_config(json_schema: JsonDict, config: Any, config_path: List[str]) -> None:
+    """Validates a config setting against a JsonSchema definition
+
+    This can be used to validate a section of the config file against a schema
+    definition. If the validation fails, a ConfigError is raised with a textual
+    description of the problem.
+
+    Args:
+        json_schema: the schema to validate against
+        config: the configuration value to be validated
+        config_path: the path within the config file. This will be used as a basis
+           for the error message.
+    """
+    try:
+        jsonschema.validate(config, json_schema)
+    except jsonschema.ValidationError as e:
+        # copy `config_path` before modifying it.
+        path = list(config_path)
+        for p in list(e.path):
+            if isinstance(p, int):
+                path.append("<item %i>" % p)
+            else:
+                path.append(str(p))
+
+        raise ConfigError(
+            "Unable to parse configuration: %s at %s" % (e.message, ".".join(path))
+        )

synapse/config/saml2_config.py

@@ -15,7 +15,9 @@
 # limitations under the License.
 
 import logging
+from typing import Any, List
 
+import attr
 import jinja2
 import pkg_resources
 
@@ -23,6 +25,7 @@ from synapse.python_dependencies import DependencyException, check_requirements
 from synapse.util.module_loader import load_module, load_python_module
 
 from ._base import Config, ConfigError
+from ._util import validate_config
 
 logger = logging.getLogger(__name__)
 
@@ -80,6 +83,11 @@ class SAML2Config(Config):
 
         self.saml2_enabled = True
 
+        attribute_requirements = saml2_config.get("attribute_requirements") or []
+        self.attribute_requirements = _parse_attribute_requirements_def(
+            attribute_requirements
+        )
+
         self.saml2_grandfathered_mxid_source_attribute = saml2_config.get(
             "grandfathered_mxid_source_attribute", "uid"
         )
@@ -341,6 +349,17 @@ class SAML2Config(Config):
           #
           #grandfathered_mxid_source_attribute: upn
 
+          # It is possible to configure Synapse to only allow logins if SAML attributes
+          # match particular values. The requirements can be listed under
+          # `attribute_requirements` as shown below. All of the listed attributes must
+          # match for the login to be permitted.
+          #
+          #attribute_requirements:
+          #  - attribute: userGroup
+          #    value: "staff"
+          #  - attribute: department
+          #    value: "sales"
+
           # Directory in which Synapse will try to find the template files below.
           # If not set, default templates from within the Synapse package will be used.
           #
@@ -368,3 +387,34 @@ class SAML2Config(Config):
         """ % {
             "config_dir_path": config_dir_path
         }
+
+
+@attr.s(frozen=True)
+class SamlAttributeRequirement:
+    """Object describing a single requirement for SAML attributes."""
+
+    attribute = attr.ib(type=str)
+    value = attr.ib(type=str)
+
+    JSON_SCHEMA = {
+        "type": "object",
+        "properties": {"attribute": {"type": "string"}, "value": {"type": "string"}},
+        "required": ["attribute", "value"],
+    }
+
+
+ATTRIBUTE_REQUIREMENTS_SCHEMA = {
+    "type": "array",
+    "items": SamlAttributeRequirement.JSON_SCHEMA,
+}
+
+
+def _parse_attribute_requirements_def(
+    attribute_requirements: Any,
+) -> List[SamlAttributeRequirement]:
+    validate_config(
+        ATTRIBUTE_REQUIREMENTS_SCHEMA,
+        attribute_requirements,
+        config_path=["saml2_config", "attribute_requirements"],
+    )
+    return [SamlAttributeRequirement(**x) for x in attribute_requirements]

synapse/handlers/saml_handler.py

@@ -14,15 +14,16 @@
 # limitations under the License.
 import logging
 import re
-from typing import Callable, Dict, Optional, Set, Tuple
+from typing import TYPE_CHECKING, Callable, Dict, Optional, Set, Tuple
 
 import attr
 import saml2
 import saml2.response
 from saml2.client import Saml2Client
 
-from synapse.api.errors import SynapseError
+from synapse.api.errors import AuthError, SynapseError
 from synapse.config import ConfigError
+from synapse.config.saml2_config import SamlAttributeRequirement
 from synapse.http.servlet import parse_string
 from synapse.http.site import SynapseRequest
 from synapse.module_api import ModuleApi
@@ -34,6 +35,9 @@ from synapse.types import (
 from synapse.util.async_helpers import Linearizer
 from synapse.util.iterutils import chunk_seq
 
+if TYPE_CHECKING:
+    import synapse.server
+
 logger = logging.getLogger(__name__)
 
 
@@ -49,7 +53,7 @@ class Saml2SessionData:
 
 
 class SamlHandler:
-    def __init__(self, hs):
+    def __init__(self, hs: "synapse.server.HomeServer"):
         self._saml_client = Saml2Client(hs.config.saml2_sp_config)
         self._auth = hs.get_auth()
         self._auth_handler = hs.get_auth_handler()
@@ -62,6 +66,7 @@ class SamlHandler:
         self._grandfathered_mxid_source_attribute = (
             hs.config.saml2_grandfathered_mxid_source_attribute
         )
+        self._saml2_attribute_requirements = hs.config.saml2.attribute_requirements
 
         # plugin to do custom mapping from saml response to mxid
         self._user_mapping_provider = hs.config.saml2_user_mapping_provider_class(
@@ -73,7 +78,7 @@ class SamlHandler:
         self._auth_provider_id = "saml"
 
         # a map from saml session id to Saml2SessionData object
-        self._outstanding_requests_dict = {}
+        self._outstanding_requests_dict = {}  # type: Dict[str, Saml2SessionData]
 
         # a lock on the mappings
         self._mapping_lock = Linearizer(name="saml_mapping", clock=self._clock)
@@ -165,11 +170,18 @@ class SamlHandler:
                 saml2.BINDING_HTTP_POST,
                 outstanding=self._outstanding_requests_dict,
             )
+        except saml2.response.UnsolicitedResponse as e:
+            # the pysaml2 library helpfully logs an ERROR here, but neglects to log
+            # the session ID. I don't really want to put the full text of the exception
+            # in the (user-visible) exception message, so let's log the exception here
+            # so we can track down the session IDs later.
+            logger.warning(str(e))
+            raise SynapseError(400, "Unexpected SAML2 login.")
         except Exception as e:
-            raise SynapseError(400, "Unable to parse SAML2 response: %s" % (e,))
+            raise SynapseError(400, "Unable to parse SAML2 response: %s." % (e,))
 
         if saml2_auth.not_signed:
-            raise SynapseError(400, "SAML2 response was not signed")
+            raise SynapseError(400, "SAML2 response was not signed.")
 
         logger.debug("SAML2 response: %s", saml2_auth.origxml)
         for assertion in saml2_auth.assertions:
@@ -188,6 +200,9 @@ class SamlHandler:
             saml2_auth.in_response_to, None
         )
 
+        for requirement in self._saml2_attribute_requirements:
+            _check_attribute_requirement(saml2_auth.ava, requirement)
+
         remote_user_id = self._user_mapping_provider.get_remote_user_id(
             saml2_auth, client_redirect_url
         )
@@ -294,6 +309,21 @@ class SamlHandler:
             del self._outstanding_requests_dict[reqid]
 
 
+def _check_attribute_requirement(ava: dict, req: SamlAttributeRequirement):
+    values = ava.get(req.attribute, [])
+    for v in values:
+        if v == req.value:
+            return
+
+    logger.info(
+        "SAML2 attribute %s did not match required value '%s' (was '%s')",
+        req.attribute,
+        req.value,
+        values,
+    )
+    raise AuthError(403, "You are not authorized to log in here.")
+
+
 DOT_REPLACE_PATTERN = re.compile(
     ("[^%s]" % (re.escape("".join(mxid_localpart_allowed_characters)),))
 )

synapse/res/templates/saml_error.html

@@ -2,10 +2,17 @@
 <html lang="en">
 <head>
     <meta charset="UTF-8">
-    <title>SSO error</title>
+    <title>SSO login error</title>
 </head>
 <body>
-    <p>Oops! Something went wrong during authentication<span id="errormsg"></span>.</p>
+{# a 403 means we have actively rejected their login #}
+{% if code == 403 %}
+    <p>You are not allowed to log in here.</p>
+{% else %}
+    <p>
+        There was an error during authentication:
+    </p>
+    <div id="errormsg" style="margin:20px 80px">{{ msg }}</div>
     <p>
         If you are seeing this page after clicking a link sent to you via email, make
         sure you only click the confirmation link once, and that you open the
@@ -37,9 +44,9 @@
         // to print one.
         let errorDesc = new URLSearchParams(searchStr).get("error_description")
         if (errorDesc) {
-
-            document.getElementById("errormsg").innerText = ` ("${errorDesc}")`;
+            document.getElementById("errormsg").innerText = errorDesc;
         }
     </script>
+{% endif %}
 </body>
 </html>