diff --git a/src/caldav_sync.py b/src/caldav_sync.py
index a02112e..10ca51c 100644
--- a/src/caldav_sync.py
+++ b/src/caldav_sync.py
@@ -105,6 +105,25 @@ def _to_utc_naive(dt):
     return datetime(dt.year, dt.month, dt.day), True
 
 
+def _find_existing_event(db, pending, uid_val, calendar_id):
+    """Find the event to update for THIS calendar.
+
+    CalendarEvent.uid is the global primary key, so an unscoped lookup by uid
+    returns whatever row holds that VEVENT uid — including another owner's.
+    The old code then reassigned that row's calendar_id, moving (stealing)
+    another user's event into the syncing calendar whenever the two share a
+    uid (shared/subscribed/public calendars, or two accounts on one server).
+    Scope the lookup to the calendar being synced; a genuine cross-user uid
+    collision then fails the PK insert inside the per-calendar try/except
+    instead of hijacking the row. (import_ics was already fixed this way.)
+    """
+    from core.database import CalendarEvent
+    return pending.get(uid_val) or db.query(CalendarEvent).filter(
+        CalendarEvent.uid == uid_val,
+        CalendarEvent.calendar_id == calendar_id,
+    ).first()
+
+
 def _sync_blocking(owner: str, url: str, username: str, password: str) -> dict:
     """The actual sync — synchronous, intended to run in a threadpool.
     Returns counts: {calendars, events, deleted, errors}."""
@@ -235,9 +254,7 @@ def _sync_blocking(owner: str, url: str, username: str, password: str) -> dict:
                             else ""
                         )
 
-                        existing = pending.get(uid_val) or db.query(CalendarEvent).filter(
-                            CalendarEvent.uid == uid_val,
-                        ).first()
+                        existing = _find_existing_event(db, pending, uid_val, local_cal.id)
                         if existing:
                             existing.calendar_id = local_cal.id
                             existing.summary = summary
diff --git a/tests/test_caldav_sync_uid_scope.py b/tests/test_caldav_sync_uid_scope.py
new file mode 100644
index 0000000..dee737f
--- /dev/null
+++ b/tests/test_caldav_sync_uid_scope.py
@@ -0,0 +1,76 @@
+"""CalDAV sync must not hijack another user's event via a shared VEVENT uid.
+
+CalendarEvent.uid is the global primary key. _sync_blocking looked up the
+existing event by uid with NO calendar scope, so when user B synced a uid
+that user A's calendar already held, the query returned A's row and the sync
+reassigned its calendar_id to B's calendar — stealing A's event. The lookup
+must be scoped to the calendar being synced.
+"""
+import tempfile
+import uuid
+from datetime import datetime
+
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import NullPool
+
+import core.database as cdb
+from core.database import CalendarEvent, CalendarCal
+from src.caldav_sync import _find_existing_event
+
+_TMPDB = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+_ENGINE = create_engine(f"sqlite:///{_TMPDB.name}", connect_args={"check_same_thread": False}, poolclass=NullPool)
+cdb.Base.metadata.create_all(_ENGINE)
+_TS = sessionmaker(bind=_ENGINE, autoflush=False, autocommit=False)
+
+
+def _setup():
+    db = _TS()
+    try:
+        db.query(CalendarEvent).delete(); db.query(CalendarCal).delete()
+        db.add(CalendarCal(id="calA", owner="alice", name="A"))
+        db.add(CalendarCal(id="calB", owner="bob", name="B"))
+        # dtstart/dtend are NOT NULL in the schema, so seed valid values.
+        db.add(CalendarEvent(
+            uid="shared@svc", calendar_id="calA", summary="Alice event",
+            dtstart=datetime(2026, 6, 4, 9, 0), dtend=datetime(2026, 6, 4, 10, 0),
+        ))
+        db.commit()
+    finally:
+        db.close()
+
+
+def test_lookup_for_other_calendar_does_not_find_a_users_event():
+    _setup()
+    db = _TS()
+    try:
+        # Bob's calendar syncing the same uid must NOT resolve Alice's row.
+        assert _find_existing_event(db, {}, "shared@svc", "calB") is None
+        # Same calendar still resolves its own event (normal update path).
+        own = _find_existing_event(db, {}, "shared@svc", "calA")
+        assert own is not None and own.calendar_id == "calA"
+    finally:
+        db.close()
+
+
+def test_alice_event_is_not_moved():
+    _setup()
+    db = _TS()
+    try:
+        # Simulate the (fixed) sync deciding there is no existing row for calB.
+        assert _find_existing_event(db, {}, "shared@svc", "calB") is None
+        ev = db.query(CalendarEvent).filter(CalendarEvent.uid == "shared@svc").first()
+        assert ev.calendar_id == "calA"  # unchanged — not hijacked
+    finally:
+        db.close()
+
+
+def test_pending_takes_precedence():
+    _setup()
+    db = _TS()
+    try:
+        sentinel = object()
+        assert _find_existing_event(db, {"shared@svc": sentinel}, "shared@svc", "calB") is sentinel
+    finally:
+        db.close()