2 files changed, 6 insertions, 0 deletions
diff --git a/changelog.d/18139.misc b/changelog.d/18139.misc
new file mode 100644
index 0000000000..f753033b8e
--- /dev/null
+++ b/changelog.d/18139.misc
@@ -0,0 +1 @@
+Do not log at the exception-level when clients provide empty `since` token to `/sync` API.
diff --git a/synapse/types/__init__.py b/synapse/types/__init__.py
index 26783c5622..3d15c04faa 100644
--- a/synapse/types/__init__.py
+++ b/synapse/types/__init__.py
@@ -664,6 +664,11 @@ class RoomStreamToken(AbstractMultiWriterStreamToken):
 
     @classmethod
     async def parse(cls, store: "PurgeEventsStore", string: str) -> "RoomStreamToken":
+        # Check that it looks like a Synapse token first. We do this so that
+        # we don't log at the exception-level for obviously incorrect tokens.
+        if not string or string[0] not in ("s", "t", "m"):
+            raise SynapseError(400, f"Invalid room stream token {string:!r}")
+
         try:
             if string[0] == "s":
                 return cls(topological=None, stream=int(string[1:]))