diff --git a/hbd/server/http.py b/hbd/server/http.py
index e6d0525..542bfda 100644
--- a/hbd/server/http.py
+++ b/hbd/server/http.py
@@ -1089,6 +1089,9 @@ async def start(
         except Exception:
             return web.json_response({"error": "Invalid JSON"}, status=400)
 
+        if not isinstance(payload, dict):
+            return web.json_response({"error": "Invalid JSON"}, status=400)
+
         try:
             data = configio_mod.read_roundtrip(_config_path)
 
@@ -1112,7 +1115,17 @@ async def start(
                 configio_mod.apply_structured_section(data, "users", users_payload)
 
             if "oauth" in payload:
-                data["oauth"] = payload["oauth"]
+                existing_oauth = data.get("oauth") or {}
+                new_oauth = payload["oauth"]
+                for name, attrs in new_oauth.items():
+                    cs = attrs.get("client_secret", "")
+                    if not cs or cs == "•••":
+                        existing_cs = (existing_oauth.get(name) or {}).get("client_secret", "")
+                        if existing_cs:
+                            attrs["client_secret"] = existing_cs
+                        else:
+                            attrs.pop("client_secret", None)
+                data["oauth"] = new_oauth
 
             for section in ("notification_channels", "thresholds", "hosts", "dns"):
                 if section in payload:
@@ -1144,8 +1157,7 @@ async def start(
             return web.json_response({"error": "Invalid JSON"}, status=400)
 
         backup = body.get("backup", "")
-        expected_prefix = _config_path + ".bak."
-        if not backup or not backup.startswith(expected_prefix) or not os.path.exists(backup):
+        if not backup or backup not in configio_mod.list_backups(_config_path):
             return web.json_response({"error": "Invalid or missing backup"}, status=400)
 
         try:
diff --git a/tests/test_http_config_api.py b/tests/test_http_config_api.py
index 21bce09..2a0d82c 100644
--- a/tests/test_http_config_api.py
+++ b/tests/test_http_config_api.py
@@ -111,3 +111,63 @@ def test_rollback_restores_backup(tmp_path):
     configio.write_config(str(cfg), backup_data)
     restored = configio.read_roundtrip(str(cfg))
     assert restored["interval"] == 20
+
+
+def test_write_path_preserves_masked_password(tmp_path):
+    """The "•••" sentinel must preserve the existing hash, not write "•••" to disk."""
+    cfg = tmp_path / ".hb.yaml"
+    original_hash = "pbkdf2:sha256:original_hash"
+    cfg.write_text(
+        f"hbd_port: 50004\nusers:\n  alice:\n    full_name: Alice\n    admin: true\n    password: {original_hash}\n"
+    )
+    from hbd.server import configio
+    from hbd.server import users as users_mod
+    data = configio.read_roundtrip(str(cfg))
+    # Simulate what api_config_post does when client sends "•••" back
+    existing_users = data.get("users") or {}
+    users_payload = {"alice": {"full_name": "Alice", "admin": True, "password": "•••"}}
+    for username, attrs in users_payload.items():
+        pw = attrs.get("password", "")
+        if pw and pw != "•••" and not pw.startswith("pbkdf2:"):
+            attrs["password"] = users_mod.hash_password(pw)
+        elif not pw or pw == "•••":
+            existing_hash = (existing_users.get(username) or {}).get("password", "")
+            if existing_hash:
+                attrs["password"] = existing_hash
+            else:
+                attrs.pop("password", None)
+    configio.apply_structured_section(data, "users", users_payload)
+    configio.write_config(str(cfg), data)
+    data2 = configio.read_roundtrip(str(cfg))
+    assert data2["users"]["alice"]["password"] == original_hash, (
+        f"Expected original hash preserved, got: {data2['users']['alice']['password']!r}"
+    )
+
+
+def test_write_path_preserves_oauth_client_secret(tmp_path):
+    """The "•••" sentinel for oauth client_secret must preserve the existing secret."""
+    cfg = tmp_path / ".hb.yaml"
+    original_secret = "real_client_secret_value"
+    cfg.write_text(
+        f"hbd_port: 50004\noauth:\n  gitea:\n    type: gitea\n    url: https://git.example.com\n"
+        f"    client_id: cid123\n    client_secret: {original_secret}\n"
+    )
+    from hbd.server import configio
+    data = configio.read_roundtrip(str(cfg))
+    # Simulate what api_config_post does when client sends "•••" back for client_secret
+    existing_oauth = data.get("oauth") or {}
+    new_oauth = {"gitea": {"type": "gitea", "url": "https://git.example.com", "client_id": "cid123", "client_secret": "•••"}}
+    for name, attrs in new_oauth.items():
+        cs = attrs.get("client_secret", "")
+        if not cs or cs == "•••":
+            existing_cs = (existing_oauth.get(name) or {}).get("client_secret", "")
+            if existing_cs:
+                attrs["client_secret"] = existing_cs
+            else:
+                attrs.pop("client_secret", None)
+    data["oauth"] = new_oauth
+    configio.write_config(str(cfg), data)
+    data2 = configio.read_roundtrip(str(cfg))
+    assert data2["oauth"]["gitea"]["client_secret"] == original_secret, (
+        f"Expected original secret preserved, got: {data2['oauth']['gitea']['client_secret']!r}"
+    )