1 files changed, 33 insertions, 0 deletions
diff --git a/synapse/types.py b/synapse/types.py
index 1aa426fcbb..1eeda0b72f 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -12,6 +12,7 @@
 # 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.
+import string
 
 from synapse.api.errors import SynapseError
 
@@ -156,6 +157,38 @@ class GroupID(DomainSpecificString):
     """Structure representing a group ID."""
     SIGIL = "+"
 
+    @classmethod
+    def from_string(cls, s):
+        group_id = super(GroupID, cls).from_string(s)
+        if not group_id.localpart:
+            raise SynapseError(
+                400,
+                "Group ID cannot be empty",
+            )
+
+        if contains_invalid_mxid_characters(group_id.localpart):
+            raise SynapseError(
+                400,
+                "Group ID can only contain characters a-z, 0-9, or '=_-./'",
+            )
+
+        return group_id
+
+
+mxid_localpart_allowed_characters = set("_-./=" + string.ascii_lowercase + string.digits)
+
+
+def contains_invalid_mxid_characters(localpart):
+    """Check for characters not allowed in an mxid or groupid localpart
+
+    Args:
+        localpart (basestring): the localpart to be checked
+
+    Returns:
+        bool: True if there are any naughty characters
+    """
+    return any(c not in mxid_localpart_allowed_characters for c in localpart)
+
 
 class StreamToken(
     namedtuple("Token", (