fix: omit client_secret for public OAuth clients in token exchange

AbstractOAuthProvider.exchange_code always included client_secret in the
token request body. For a public client (PKCE-only,
token_endpoint_auth_method=none) client_secret is empty, so the request
sent client_secret="", which several IdPs reject since a public client
must not send client authentication. Include the field only when it is
set; the PKCE code_verifier, sent either way, is the public client's proof.

Confidential clients are unchanged. Adds test_token_exchange.py capturing
the token POST body for both the confidential and public modes.

Signed-off-by: Carlos Andrés Planchón Prestes <carlosandresplanchonprestes@gmail.com>

Author: carlosplanchon

crudauth/oauth/provider.py

@@ -163,15 +163,23 @@ async def exchange_code(
 
         Raises:
             httpx.HTTPStatusError: If the token endpoint returns an error status.
+
+        Note:
+            ``client_secret`` is included only when the provider actually has
+            one. A public client (PKCE-only, ``token_endpoint_auth_method=none``)
+            must not send client authentication - several IdPs reject an empty
+            ``client_secret`` outright - and its proof is the PKCE verifier,
+            which is sent either way.
         """
         httpx = _require_httpx()
         data = {
             "client_id": self.client_id,
-            "client_secret": self.client_secret,
             "code": code,
             "redirect_uri": self.redirect_uri,
             "grant_type": "authorization_code",
         }
+        if self.client_secret:
+            data["client_secret"] = self.client_secret
         if code_verifier:
             data["code_verifier"] = code_verifier
         req_headers = {"Accept": "application/json"}

tests/oauth/test_token_exchange.py

@@ -0,0 +1,66 @@
+"""Token-exchange client authentication: confidential vs public clients."""
+
+from __future__ import annotations
+
+import httpx
+
+from crudauth.oauth.providers.google import GoogleOAuthProvider
+
+
+def _fake_async_client(captured: dict):
+    """An httpx.AsyncClient stand-in that records the POST it receives."""
+
+    class FakeResponse:
+        def raise_for_status(self) -> None:
+            pass
+
+        def json(self) -> dict:
+            return {"access_token": "tok", "token_type": "Bearer"}
+
+    class FakeAsyncClient:
+        def __init__(self, *args, **kwargs):
+            pass
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, *exc):
+            return False
+
+        async def post(self, url, data=None, headers=None):
+            captured["url"] = url
+            captured["data"] = dict(data)
+            captured["headers"] = dict(headers or {})
+            return FakeResponse()
+
+    return FakeAsyncClient
+
+
+# --- confidential client (has a secret): client auth rides in the body --------
+async def test_exchange_code_sends_secret_for_confidential_client(monkeypatch) -> None:
+    captured: dict = {}
+    monkeypatch.setattr(httpx, "AsyncClient", _fake_async_client(captured))
+
+    prov = GoogleOAuthProvider("cid", "s3cret", "https://app/cb")
+    result = await prov.exchange_code("code-1", code_verifier="ver-1")
+
+    assert result["access_token"] == "tok"
+    assert captured["data"]["client_secret"] == "s3cret"
+    assert captured["data"]["code_verifier"] == "ver-1"
+    assert captured["data"]["grant_type"] == "authorization_code"
+
+
+# --- public client (no secret): the field must be absent, not empty -----------
+async def test_exchange_code_omits_secret_for_public_client(monkeypatch) -> None:
+    # A PKCE-only public client (token_endpoint_auth_method=none) must not send
+    # client authentication; several IdPs reject client_secret="" outright.
+    captured: dict = {}
+    monkeypatch.setattr(httpx, "AsyncClient", _fake_async_client(captured))
+
+    prov = GoogleOAuthProvider("cid", "", "https://app/cb")
+    await prov.exchange_code("code-1", code_verifier="ver-1")
+
+    assert "client_secret" not in captured["data"]
+    assert captured["data"]["client_id"] == "cid"
+    assert captured["data"]["code_verifier"] == "ver-1"
+    assert captured["data"]["grant_type"] == "authorization_code"