Configurable limits on avatars (#11846)

Only allow files which file size and content types match configured
limits to be set as avatar.

Most of the inspiration from the non-test code comes from matrix-org/synapse-dinsic#19

changelog.d/11846.feature

@@ -0,0 +1 @@
+Allow configuring a maximum file size as well as a list of allowed content types for avatars.

docs/sample_config.yaml

@@ -471,6 +471,20 @@ limit_remote_rooms:
 #
 #allow_per_room_profiles: false
 
+# The largest allowed file size for a user avatar. Defaults to no restriction.
+#
+# Note that user avatar changes will not work if this is set without
+# using Synapse's media repository.
+#
+#max_avatar_size: 10M
+
+# The MIME types allowed for user avatars. Defaults to no restriction.
+#
+# Note that user avatar changes will not work if this is set without
+# using Synapse's media repository.
+#
+#allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
+
 # How long to keep redacted events in unredacted form in the database. After
 # this period redacted events get replaced with their redacted form in the DB.
 #

synapse/config/server.py

@@ -489,6 +489,19 @@ def read_config(self, config, **kwargs):
         # events with profile information that differ from the target's global profile.
         self.allow_per_room_profiles = config.get("allow_per_room_profiles", True)
 
+        # The maximum size an avatar can have, in bytes.
+        self.max_avatar_size = config.get("max_avatar_size")
+        if self.max_avatar_size is not None:
+            self.max_avatar_size = self.parse_size(self.max_avatar_size)
+
+        # The MIME types allowed for an avatar.
+        self.allowed_avatar_mimetypes = config.get("allowed_avatar_mimetypes")
+        if self.allowed_avatar_mimetypes and not isinstance(
+            self.allowed_avatar_mimetypes,
+            list,
+        ):
+            raise ConfigError("allowed_avatar_mimetypes must be a list")
+
         self.listeners = [parse_listener_def(x) for x in config.get("listeners", [])]
 
         # no_tls is not really supported any more, but let's grandfather it in
@@ -1168,6 +1181,20 @@ def generate_config_section(
         #
         #allow_per_room_profiles: false
 
+        # The largest allowed file size for a user avatar. Defaults to no restriction.
+        #
+        # Note that user avatar changes will not work if this is set without
+        # using Synapse's media repository.
+        #
+        #max_avatar_size: 10M
+
+        # The MIME types allowed for user avatars. Defaults to no restriction.
+        #
+        # Note that user avatar changes will not work if this is set without
+        # using Synapse's media repository.
+        #
+        #allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
+
         # How long to keep redacted events in unredacted form in the database. After
         # this period redacted events get replaced with their redacted form in the DB.
         #

synapse/handlers/profile.py

@@ -31,6 +31,8 @@
     create_requester,
     get_domain_from_id,
 )
+from synapse.util.caches.descriptors import cached
+from synapse.util.stringutils import parse_and_validate_mxc_uri
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -64,6 +66,11 @@ def __init__(self, hs: "HomeServer"):
         self.user_directory_handler = hs.get_user_directory_handler()
         self.request_ratelimiter = hs.get_request_ratelimiter()
 
+        self.max_avatar_size = hs.config.server.max_avatar_size
+        self.allowed_avatar_mimetypes = hs.config.server.allowed_avatar_mimetypes
+
+        self.server_name = hs.config.server.server_name
+
         if hs.config.worker.run_background_tasks:
             self.clock.looping_call(
                 self._update_remote_profile_cache, self.PROFILE_UPDATE_MS
@@ -286,6 +293,9 @@ async def set_avatar_url(
                 400, "Avatar URL is too long (max %i)" % (MAX_AVATAR_URL_LEN,)
             )
 
+        if not await self.check_avatar_size_and_mime_type(new_avatar_url):
+            raise SynapseError(403, "This avatar is not allowed", Codes.FORBIDDEN)
+
         avatar_url_to_set: Optional[str] = new_avatar_url
         if new_avatar_url == "":
             avatar_url_to_set = None
@@ -307,6 +317,63 @@ async def set_avatar_url(
 
         await self._update_join_states(requester, target_user)
 
+    @cached()
+    async def check_avatar_size_and_mime_type(self, mxc: str) -> bool:
+        """Check that the size and content type of the avatar at the given MXC URI are
+        within the configured limits.
+
+        Args:
+            mxc: The MXC URI at which the avatar can be found.
+
+        Returns:
+             A boolean indicating whether the file can be allowed to be set as an avatar.
+        """
+        if not self.max_avatar_size and not self.allowed_avatar_mimetypes:
+            return True
+
+        server_name, _, media_id = parse_and_validate_mxc_uri(mxc)
+
+        if server_name == self.server_name:
+            media_info = await self.store.get_local_media(media_id)
+        else:
+            media_info = await self.store.get_cached_remote_media(server_name, media_id)
+
+        if media_info is None:
+            # Both configuration options need to access the file's metadata, and
+            # retrieving remote avatars just for this becomes a bit of a faff, especially
+            # if e.g. the file is too big. It's also generally safe to assume most files
+            # used as avatar are uploaded locally, or if the upload didn't happen as part
+            # of a PUT request on /avatar_url that the file was at least previewed by the
+            # user locally (and therefore downloaded to the remote media cache).
+            logger.warning("Forbidding avatar change to %s: avatar not on server", mxc)
+            return False
+
+        if self.max_avatar_size:
+            # Ensure avatar does not exceed max allowed avatar size
+            if media_info["media_length"] > self.max_avatar_size:
+                logger.warning(
+                    "Forbidding avatar change to %s: %d bytes is above the allowed size "
+                    "limit",
+                    mxc,
+                    media_info["media_length"],
+                )
+                return False
+
+        if self.allowed_avatar_mimetypes:
+            # Ensure the avatar's file type is allowed
+            if (
+                self.allowed_avatar_mimetypes
+                and media_info["media_type"] not in self.allowed_avatar_mimetypes
+            ):
+                logger.warning(
+                    "Forbidding avatar change to %s: mimetype %s not allowed",
+                    mxc,
+                    media_info["media_type"],
+                )
+                return False
+
+        return True
+
     async def on_profile_query(self, args: JsonDict) -> JsonDict:
         """Handles federation profile query requests."""
 

synapse/handlers/room_member.py

@@ -590,6 +590,12 @@ async def update_membership_locked(
                 errcode=Codes.BAD_JSON,
             )
 
+        if "avatar_url" in content:
+            if not await self.profile_handler.check_avatar_size_and_mime_type(
+                content["avatar_url"],
+            ):
+                raise SynapseError(403, "This avatar is not allowed", Codes.FORBIDDEN)
+
         # The event content should *not* include the authorising user as
         # it won't be properly signed. Strip it out since it might come
         # back from a client updating a display name / avatar.

tests/handlers/test_profile.py

@@ -11,12 +11,13 @@
 # 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, Dict
 from unittest.mock import Mock
 
 import synapse.types
 from synapse.api.errors import AuthError, SynapseError
 from synapse.rest import admin
+from synapse.server import HomeServer
 from synapse.types import UserID
 
 from tests import unittest
@@ -46,7 +47,7 @@ def register_query_handler(query_type, handler):
         )
         return hs
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor, clock, hs: HomeServer):
         self.store = hs.get_datastore()
 
         self.frank = UserID.from_string("@1234abcd:test")
@@ -248,3 +249,92 @@ def test_set_my_avatar_if_disabled(self):
             ),
             SynapseError,
         )
+
+    def test_avatar_constraints_no_config(self):
+        """Tests that the method to check an avatar against configured constraints skips
+        all of its check if no constraint is configured.
+        """
+        # The first check that's done by this method is whether the file exists; if we
+        # don't get an error on a non-existing file then it means all of the checks were
+        # successfully skipped.
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/unknown_file")
+        )
+        self.assertTrue(res)
+
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_constraints_missing(self):
+        """Tests that an avatar isn't allowed if the file at the given MXC URI couldn't
+        be found.
+        """
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/unknown_file")
+        )
+        self.assertFalse(res)
+
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_constraints_file_size(self):
+        """Tests that a file that's above the allowed file size is forbidden but one
+        that's below it is allowed.
+        """
+        self._setup_local_files(
+            {
+                "small": {"size": 40},
+                "big": {"size": 60},
+            }
+        )
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/small")
+        )
+        self.assertTrue(res)
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/big")
+        )
+        self.assertFalse(res)
+
+    @unittest.override_config({"allowed_avatar_mimetypes": ["image/png"]})
+    def test_avatar_constraint_mime_type(self):
+        """Tests that a file with an unauthorised MIME type is forbidden but one with
+        an authorised content type is allowed.
+        """
+        self._setup_local_files(
+            {
+                "good": {"mimetype": "image/png"},
+                "bad": {"mimetype": "application/octet-stream"},
+            }
+        )
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/good")
+        )
+        self.assertTrue(res)
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/bad")
+        )
+        self.assertFalse(res)
+
+    def _setup_local_files(self, names_and_props: Dict[str, Dict[str, Any]]):
+        """Stores metadata about files in the database.
+
+        Args:
+            names_and_props: A dictionary with one entry per file, with the key being the
+                file's name, and the value being a dictionary of properties. Supported
+                properties are "mimetype" (for the file's type) and "size" (for the
+                file's size).
+        """
+        store = self.hs.get_datastore()
+
+        for name, props in names_and_props.items():
+            self.get_success(
+                store.store_local_media(
+                    media_id=name,
+                    media_type=props.get("mimetype", "image/png"),
+                    time_now_ms=self.clock.time_msec(),
+                    upload_name=None,
+                    media_length=props.get("size", 50),
+                    user_id=UserID.from_string("@rin:test"),
+                )
+            )