Fix error in is_mine_id when encountering a malformed ID

Fixes #13040.

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13746.bugfix

@@ -0,0 +1 @@
+Fix a long standing bug where Synapse would fail to handle malformed user IDs or room aliases gracefully in certain cases.

synapse/server.py

@@ -341,7 +341,17 @@ def is_mine(self, domain_specific_string: DomainSpecificString) -> bool:
         return domain_specific_string.domain == self.hostname
 
     def is_mine_id(self, string: str) -> bool:
-        return string.split(":", 1)[1] == self.hostname
+        """Determines whether a user ID or room alias originates from this homeserver.
+
+        Returns:
+            `True` if the hostname part of the user ID or room alias matches this
+            homeserver.
+            `False` otherwise, or if the user ID or room alias is malformed.
+        """
+        localpart_hostname = string.split(":", 1)
+        if len(localpart_hostname) < 2:
+            return False
+        return localpart_hostname[1] == self.hostname
 
     @cache_in_self
     def get_clock(self) -> Clock:

tests/test_types.py

@@ -13,11 +13,35 @@
 # limitations under the License.
 
 from synapse.api.errors import SynapseError
-from synapse.types import RoomAlias, UserID, map_username_to_mxid_localpart
+from synapse.types import (
+    RoomAlias,
+    UserID,
+    get_domain_from_id,
+    get_localpart_from_id,
+    map_username_to_mxid_localpart,
+)
 
 from tests import unittest
 
 
+class IsMineIDTests(unittest.HomeserverTestCase):
+    def test_is_mine_id(self) -> None:
+        self.assertTrue(self.hs.is_mine_id("@user:test"))
+        self.assertTrue(self.hs.is_mine_id("#room:test"))
+        self.assertTrue(self.hs.is_mine_id("invalid:test"))
+
+        self.assertFalse(self.hs.is_mine_id("@user:test\0"))
+        self.assertFalse(self.hs.is_mine_id("@user"))
+
+    def test_two_colons(self) -> None:
+        """Test handling of IDs containing more than one colon."""
+        # The domain starts after the first colon.
+        # These functions must interpret things consistently.
+        self.assertFalse(self.hs.is_mine_id("@user:test:test"))
+        self.assertEqual("user", get_localpart_from_id("@user:test:test"))
+        self.assertEqual("test:test", get_domain_from_id("@user:test:test"))
+
+
 class UserIDTestCase(unittest.HomeserverTestCase):
     def test_parse(self):
         user = UserID.from_string("@1234abcd:test")