diff --git a/homeassistant/components/auth/__init__.py b/homeassistant/components/auth/__init__.py
index a314a64adf0c..cc7590241863 100644
--- a/homeassistant/components/auth/__init__.py
+++ b/homeassistant/components/auth/__init__.py
@@ -291,6 +291,14 @@ class TokenView(HomeAssistantView):
         hass = request.app[KEY_HASS]
         data = cast(MultiDictProxy[str], await request.post())
 
+        # RFC 6749 3.2: parameters must not be included more than once.
+        if len(data) != len(set(data)) or any(
+            not isinstance(value, str) for value in data.values()
+        ):
+            return self.json(
+                {"error": "invalid_request"}, status_code=HTTPStatus.BAD_REQUEST
+            )
+
         grant_type = data.get("grant_type")
 
         # IndieAuth 6.3.5
diff --git a/homeassistant/components/auth/login_flow.py b/homeassistant/components/auth/login_flow.py
index b5134ce5205c..c73267b5f49e 100644
--- a/homeassistant/components/auth/login_flow.py
+++ b/homeassistant/components/auth/login_flow.py
@@ -365,6 +365,7 @@ class LoginFlowIndexView(LoginFlowBaseView):
                     r"^[A-Za-z0-9_-]{43}\Z"
                 ),
                 probatio.Optional("code_challenge_method"): str,
+                probatio.Optional("response_type"): str,
                 probatio.Optional(
                     "type", default="authorize"
                 ): str,  # not used, kept for backwards compatibility
@@ -380,6 +381,11 @@ class LoginFlowIndexView(LoginFlowBaseView):
         if not indieauth.verify_client_id(client_id):
             return self.json_message("Invalid client id", HTTPStatus.BAD_REQUEST)
 
+        if data.get("response_type", "code") != "code":
+            return self.json_message(
+                "Response type not supported", HTTPStatus.BAD_REQUEST
+            )
+
         code_challenge = data.get("code_challenge")
         code_challenge_method = data.get("code_challenge_method")
         if code_challenge_method is not None and not code_challenge:
diff --git a/tests/components/auth/test_init.py b/tests/components/auth/test_init.py
index 142d7a57827e..c2245c202453 100644
--- a/tests/components/auth/test_init.py
+++ b/tests/components/auth/test_init.py
@@ -6,8 +6,10 @@ import logging
 from typing import Any
 from unittest.mock import patch
 
+from aiohttp import FormData
 from aiohttp.test_utils import TestClient
 from freezegun.api import FrozenDateTimeFactory
+from multidict import MultiDict
 import pytest
 
 from homeassistant.auth import InvalidAuthError
@@ -800,6 +802,63 @@ async def _async_login_for_code(
     return step["result"]
 
 
+@pytest.mark.parametrize(
+    "parameter", ["client_id", "grant_type", "code", "redirect_uri", "code_verifier"]
+)
+async def test_pkce_token_request_rejects_duplicate_parameters(
+    hass: HomeAssistant,
+    aiohttp_client: ClientSessionGenerator,
+    parameter: str,
+) -> None:
+    """Test ambiguous token parameters are rejected without consuming the code."""
+    client = await async_setup_auth(hass, aiohttp_client, setup_api=True)
+    code = await _async_login_for_code(client, RFC7636_CHALLENGE)
+    token_data = MultiDict(
+        {
+            "client_id": CLIENT_ID,
+            "grant_type": "authorization_code",
+            "code": code,
+            "redirect_uri": CLIENT_REDIRECT_URI,
+            "code_verifier": RFC7636_VERIFIER,
+        }
+    )
+    token_data.add(parameter, "other")
+
+    resp = await client.post("/auth/token", data=token_data)
+
+    assert resp.status == HTTPStatus.BAD_REQUEST
+    assert (await resp.json())["error"] == "invalid_request"
+
+    resp = await client.post("/auth/token", data=dict(token_data))
+    assert resp.status == HTTPStatus.OK
+
+
+async def test_pkce_token_request_rejects_file_verifier(
+    hass: HomeAssistant, aiohttp_client: ClientSessionGenerator
+) -> None:
+    """Test a multipart file cannot be used as a code verifier."""
+    client = await async_setup_auth(hass, aiohttp_client, setup_api=True)
+    code = await _async_login_for_code(client, RFC7636_CHALLENGE)
+    token_data = {
+        "client_id": CLIENT_ID,
+        "grant_type": "authorization_code",
+        "code": code,
+        "redirect_uri": CLIENT_REDIRECT_URI,
+    }
+    form_data = FormData(token_data)
+    form_data.add_field("code_verifier", RFC7636_VERIFIER.encode(), filename="verifier")
+
+    resp = await client.post("/auth/token", data=form_data)
+
+    assert resp.status == HTTPStatus.BAD_REQUEST
+    assert (await resp.json())["error"] == "invalid_request"
+
+    resp = await client.post(
+        "/auth/token", data={**token_data, "code_verifier": RFC7636_VERIFIER}
+    )
+    assert resp.status == HTTPStatus.OK
+
+
 async def test_auth_code_pkce_success(
     hass: HomeAssistant, aiohttp_client: ClientSessionGenerator
 ) -> None:
diff --git a/tests/components/auth/test_login_flow.py b/tests/components/auth/test_login_flow.py
index 96cefdf7910f..609f95c67924 100644
--- a/tests/components/auth/test_login_flow.py
+++ b/tests/components/auth/test_login_flow.py
@@ -245,8 +245,14 @@ async def test_invalid_redirect_uri(
     assert data["message"] == "Invalid redirect URI"
 
 
+@pytest.mark.parametrize(
+    "authorization_data",
+    [{}, {"response_type": "code"}],
+)
 async def test_login_exist_user(
-    hass: HomeAssistant, aiohttp_client: ClientSessionGenerator
+    hass: HomeAssistant,
+    aiohttp_client: ClientSessionGenerator,
+    authorization_data: dict[str, str],
 ) -> None:
     """Test logging in with exist user."""
     client = await async_setup_auth(hass, aiohttp_client, setup_api=True)
@@ -261,6 +267,7 @@ async def test_login_exist_user(
             "client_id": CLIENT_ID,
             "handler": ["insecure_example", None],
             "redirect_uri": CLIENT_REDIRECT_URI,
+            **authorization_data,
         },
     )
     assert resp.status == HTTPStatus.OK
@@ -538,6 +545,12 @@ async def test_well_known_protected_resource_no_url(
             },
             "Message format incorrect",
         ),
+        (
+            {
+                "response_type": "token",
+            },
+            "Response type not supported",
+        ),
     ],
     ids=[
         "method_without_challenge",
@@ -545,6 +558,7 @@ async def test_well_known_protected_resource_no_url(
         "unsupported_plain_method",
         "challenge_too_short",
         "challenge_padded",
+        "unsupported_response_type",
     ],
 )
 async def test_login_flow_pkce_validation(