diff --git a/gns3server/services/authentication.py b/gns3server/services/authentication.py
index 574c96c69..6103ee6f9 100644
--- a/gns3server/services/authentication.py
+++ b/gns3server/services/authentication.py
@@ -16,7 +16,9 @@
 
 from joserfc import jwt
 from joserfc.jwk import OctKey
-from joserfc.errors import JoseError
+from joserfc.errors import JoseError, BadSignatureError
+import base64
+import json
 import time
 from datetime import datetime, timedelta, timezone
 import bcrypt
@@ -34,6 +36,17 @@ log = logging.getLogger(__name__)
 DEFAULT_JWT_SECRET_KEY = "efd08eccec3bd0a1be2e086670e5efa90969c68d07e072d7354a76cea5e33d4e"
 
 
+def _extract_alg(token: str) -> str:
+    """Best-effort extraction of the unverified JWT header "alg" value — for logging only."""
+
+    try:
+        header_segment = token.split(".", 1)[0]
+        header = json.loads(base64.urlsafe_b64decode(header_segment + "=" * (-len(header_segment) % 4)))
+        return str(header.get("alg", "<missing>"))
+    except Exception:
+        return "<undecodable>"
+
+
 class AuthService:
 
     def hash_password(self, password: str) -> str:
@@ -75,32 +88,38 @@ class AuthService:
 
     def get_token_data(self, token: str, secret_key: str = None) -> TokenData:
 
-        credentials_exception = HTTPException(
-            status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Could not validate credentials",
-            headers={"WWW-Authenticate": "Bearer"},
-        )
+        def auth_error(detail: str) -> HTTPException:
+            return HTTPException(
+                status_code=status.HTTP_401_UNAUTHORIZED,
+                detail=detail,
+                headers={"WWW-Authenticate": "Bearer"},
+            )
+
+        if secret_key is None:
+            secret_key = Config.instance().settings.Controller.jwt_secret_key
+        if secret_key is None:
+            secret_key = DEFAULT_JWT_SECRET_KEY
+            log.error("A JWT secret key must be configured to secure the server, using an unsecured default key!")
+        algorithm = Config.instance().settings.Controller.jwt_algorithm
+        key = OctKey.import_key(secret_key)
         try:
-            if secret_key is None:
-                secret_key = Config.instance().settings.Controller.jwt_secret_key
-            if secret_key is None:
-                secret_key = DEFAULT_JWT_SECRET_KEY
-                log.error("A JWT secret key must be configured to secure the server, using an unsecured default key!")
-            algorithm = Config.instance().settings.Controller.jwt_algorithm
-            key = OctKey.import_key(secret_key)
             payload = jwt.decode(token, key, algorithms=[algorithm])
             username: str = payload.claims.get("sub")
             if username is None:
-                raise credentials_exception
+                raise auth_error("Invalid token: missing subject claim")
             # Validate the exp claim — joserfc does not validate time-based claims by default
             token_exp: int = payload.claims.get("exp", 0)
             if token_exp and time.time() > token_exp:
-                raise credentials_exception
+                raise auth_error("Token has expired")
             token_version: int = payload.claims.get("ver", 0)
             token_use: str = payload.claims.get("type", "access")
             token_data = TokenData(username=username, token_version=token_version, token_use=token_use)
-        except (JoseError, ValidationError, ValueError):
-            raise credentials_exception
+        except BadSignatureError as e:
+            log.error("JWT rejected: bad signature (header alg: '%s', error: %s)", _extract_alg(token), e)
+            raise auth_error("Invalid token signature")
+        except (JoseError, ValidationError, ValueError) as e:
+            log.error("JWT rejected: %s: %s (header alg: '%s')", type(e).__name__, e, _extract_alg(token))
+            raise auth_error(f"Invalid token ({type(e).__name__})")
         return token_data
 
     def get_username_from_token(self, token: str, secret_key: str = None) -> Optional[str]:
diff --git a/tests/api/routes/test_routes.py b/tests/api/routes/test_routes.py
index 5c8a59166..0854581f7 100644
--- a/tests/api/routes/test_routes.py
+++ b/tests/api/routes/test_routes.py
@@ -106,7 +106,7 @@ class TestRoutes:
                     async with aconnect_ws(path, client, params=params) as ws:
                         json_notification = await ws.receive_json()
                         assert json_notification['event'] == {
-                            'message': 'Could not authenticate while connecting to controller WebSocket: Could not validate credentials'
+                            'message': 'Could not authenticate while connecting to controller WebSocket: Invalid token (DecodeError)'
                         }