diff --git a/.gitignore b/.gitignore
index b73d7b9..bc7bcef 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ tokens.json
 config.json
 .active_tokens.json
 *.before-*
+.token_audit.log
diff --git a/common.py b/common.py
index 5f5561f..14bc735 100644
--- a/common.py
+++ b/common.py
@@ -129,6 +129,9 @@ async def oauth_authorize(request: Request):
     if client_id not in tokens:
         return HTMLResponse(f"<h1>Fehler</h1><p>Unbekannte Client ID: {client_id}</p>", status_code=400)
 
+    if not code_challenge:
+        return HTMLResponse("<h1>Fehler</h1><p>PKCE erforderlich (code_challenge fehlt).</p>", status_code=400)
+
     code = secrets.token_urlsafe(32)
     _auth_codes[code] = {
         "client_id": client_id,
@@ -171,19 +174,16 @@ async def oauth_token(request: Request):
         if code_data["client_id"] != client_id:
             return JSONResponse({"error": "invalid_grant", "error_description": "Client ID mismatch"}, status_code=400)
 
-        if code_data["code_challenge"]:
-            if not _verify_pkce(code_verifier, code_data["code_challenge"], code_data["code_challenge_method"]):
-                return JSONResponse({"error": "invalid_grant", "error_description": "PKCE verification failed"}, status_code=400)
+        # PKCE PFLICHT (kein Bypass mit leerem code_challenge)
+        if not code_data.get("code_challenge"):
+            return JSONResponse({"error": "invalid_grant", "error_description": "PKCE required"}, status_code=400)
+        if not _verify_pkce(code_verifier, code_data["code_challenge"], code_data["code_challenge_method"]):
+            return JSONResponse({"error": "invalid_grant", "error_description": "PKCE verification failed"}, status_code=400)
 
-        # Verify client_secret if provided, otherwise rely on PKCE alone
-        if client_secret:
-            user = _resolve_client(client_id, client_secret)
-            if not user:
-                return JSONResponse({"error": "invalid_client", "error_description": "Invalid client credentials"}, status_code=401)
-        else:
-            user = client_id
-            if user not in _load_tokens():
-                return JSONResponse({"error": "invalid_client"}, status_code=401)
+        # client_secret PFLICHT (confidential client) -- schliesst den Auth-Bypass
+        user = _resolve_client(client_id, client_secret)
+        if not user:
+            return JSONResponse({"error": "invalid_client", "error_description": "Invalid client credentials"}, status_code=401)
 
     elif grant_type == "client_credentials":
         user = _resolve_client(client_id, client_secret)