diff --git a/gateway/authz_mixin.py b/gateway/authz_mixin.py
index 0232288eed..2c97016e57 100644
--- a/gateway/authz_mixin.py
+++ b/gateway/authz_mixin.py
@@ -159,8 +159,6 @@ def _normalize_nostr_allow_entries(entries: set) -> set:
 def _principal_matches_allowlist(source, user_id: str, allowed_ids: set) -> bool:
     """Whether *user_id* (under any platform-specific alias) is in *allowed_ids*."""
     check_ids = {user_id}
-    if "@" in user_id:
-        check_ids.add(user_id.split("@")[0])
 
     # WhatsApp (Baileys + Cloud): phone<->LID / JID aliases match the same principal.
     if source.platform in {Platform.WHATSAPP, Platform.WHATSAPP_CLOUD}:
diff --git a/tests/gateway/test_allowlist_qualified_ids.py b/tests/gateway/test_allowlist_qualified_ids.py
new file mode 100644
index 0000000000..3798051b4b
--- /dev/null
+++ b/tests/gateway/test_allowlist_qualified_ids.py
@@ -0,0 +1,131 @@
+"""Allowlist matching compares qualified user_ids, not bare '@' localparts.
+
+The generic ``user_id.split("@")[0]`` alias in ``_principal_matches_allowlist``
+was added for WhatsApp JIDs (``<phone>@s.whatsapp.net``) before WhatsApp got a
+dedicated alias expansion. Left unconditional, it makes a bare allowlist entry
+``alice`` admit ``alice@<any domain>`` on every platform whose user_id is
+'@'-shaped (email, Google Chat, iMessage handles) — domains the sender controls.
+"""
+
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock
+
+from gateway.authz_mixin import _principal_matches_allowlist
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.session import SessionSource
+
+
+def _source(platform: Platform, user_id: str) -> SessionSource:
+    return SessionSource(
+        platform=platform,
+        user_id=user_id,
+        chat_id=user_id,
+        user_name="tester",
+        chat_type="dm",
+    )
+
+
+def test_bare_localpart_entry_does_not_admit_foreign_domain():
+    """alice@evil.example must not match an allowlist holding bare ``alice``."""
+    source = _source(Platform.EMAIL, "alice@evil.example")
+    assert _principal_matches_allowlist(source, "alice@evil.example", {"alice"}) is False
+
+
+def test_qualified_entry_still_matches():
+    source = _source(Platform.EMAIL, "alice@corp.example")
+    assert _principal_matches_allowlist(
+        source, "alice@corp.example", {"alice@corp.example"}
+    ) is True
+
+
+def test_whatsapp_bare_phone_entry_still_matches_jid():
+    """The original intent survives via the scoped WhatsApp expansion, not the
+    generic split: a bare phone allowlist entry matches the sender's JID."""
+    source = _source(Platform.WHATSAPP, "15550000001@s.whatsapp.net")
+    assert _principal_matches_allowlist(
+        source, "15550000001@s.whatsapp.net", {"15550000001"}
+    ) is True
+
+
+def test_whatsapp_device_suffix_jid_matches_bare_phone():
+    source = _source(Platform.WHATSAPP, "15550000001:47@s.whatsapp.net")
+    assert _principal_matches_allowlist(
+        source, "15550000001:47@s.whatsapp.net", {"15550000001"}
+    ) is True
+
+
+# ------------------------------------------------------------- full authz path
+
+def _make_runner(platform: Platform, config: GatewayConfig):
+    from gateway.run import GatewayRunner
+
+    runner = object.__new__(GatewayRunner)
+    runner.config = config
+    adapter = SimpleNamespace(send=AsyncMock())
+    runner.adapters = {platform: adapter}
+    runner.pairing_store = MagicMock()
+    runner.pairing_store.is_approved.return_value = False
+    runner.pairing_store._is_rate_limited.return_value = False
+    runner._running_agents = {}
+    runner._running_agents_ts = {}
+    runner._update_prompts = {}
+    runner.hooks = SimpleNamespace(dispatch=AsyncMock(return_value=None))
+    runner._sessions = {}
+    return runner, adapter
+
+
+def test_email_allowlist_bare_localpart_rejects_foreign_domain(monkeypatch):
+    """End to end: EMAIL_ALLOWED_USERS=alice must not authorize alice@evil.example."""
+    for key in ("EMAIL_ALLOWED_USERS", "EMAIL_ALLOW_ALL_USERS", "GATEWAY_ALLOWED_USERS"):
+        monkeypatch.delenv(key, raising=False)
+    monkeypatch.setenv("EMAIL_ALLOWED_USERS", "alice")
+
+    runner, _adapter = _make_runner(
+        Platform.EMAIL,
+        GatewayConfig(platforms={Platform.EMAIL: PlatformConfig(enabled=True)}),
+    )
+
+    assert runner._is_user_authorized(_source(Platform.EMAIL, "alice@evil.example")) is False
+    assert runner._is_user_authorized(_source(Platform.EMAIL, "mallory@other.example")) is False
+
+
+def test_global_allowlist_bare_localpart_rejects_foreign_domain(monkeypatch):
+    """The reachable widening: BlueBubbles registers no platform allowlist env,
+    so GATEWAY_ALLOWED_USERS is the only gate and the adapter does no sender
+    check of its own — an iMessage email handle is matched as-is. A bare
+    ``alice`` entry must not admit alice@evil.example."""
+    for key in ("GATEWAY_ALLOWED_USERS", "GATEWAY_ALLOW_ALL_USERS"):
+        monkeypatch.delenv(key, raising=False)
+    monkeypatch.setenv("GATEWAY_ALLOWED_USERS", "alice")
+
+    runner, _adapter = _make_runner(
+        Platform.BLUEBUBBLES,
+        GatewayConfig(platforms={Platform.BLUEBUBBLES: PlatformConfig(enabled=True)}),
+    )
+
+    assert runner._is_user_authorized(
+        _source(Platform.BLUEBUBBLES, "alice@evil.example")
+    ) is False
+    assert runner._is_user_authorized(
+        _source(Platform.BLUEBUBBLES, "alice@alice.example")
+    ) is False
+
+
+def test_global_allowlist_qualified_entry_still_authorizes(monkeypatch):
+    """Positive control on the same path: a fully-qualified entry still admits
+    its exact sender, so the gate is not simply rejecting everything."""
+    for key in ("GATEWAY_ALLOWED_USERS", "GATEWAY_ALLOW_ALL_USERS"):
+        monkeypatch.delenv(key, raising=False)
+    monkeypatch.setenv("GATEWAY_ALLOWED_USERS", "alice@corp.example")
+
+    runner, _adapter = _make_runner(
+        Platform.BLUEBUBBLES,
+        GatewayConfig(platforms={Platform.BLUEBUBBLES: PlatformConfig(enabled=True)}),
+    )
+
+    assert runner._is_user_authorized(
+        _source(Platform.BLUEBUBBLES, "alice@corp.example")
+    ) is True
+    assert runner._is_user_authorized(
+        _source(Platform.BLUEBUBBLES, "alice@evil.example")
+    ) is False