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

3 files changed, 100 insertions, 0 deletions

diff --git a/synapse/config/server.py b/synapse/config/server.py
index f200d0c1f1..a460cf25b4 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -489,6 +489,19 @@ class ServerConfig(Config):
         # 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 @@ class ServerConfig(Config):
         #
         #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.
         #
diff --git a/synapse/handlers/profile.py b/synapse/handlers/profile.py
index 6b5a6ded8b..36e3ad2ba9 100644
--- a/synapse/handlers/profile.py
+++ b/synapse/handlers/profile.py
@@ -31,6 +31,8 @@ from synapse.types import (
     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 @@ class ProfileHandler:
         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 @@ class ProfileHandler:
                 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 @@ class ProfileHandler:
 
         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."""
 
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 6aa910dd10..3dd5e1b6e4 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -590,6 +590,12 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                 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.