diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04d95a9f..3f3d9d73 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 * Migrated `setup.py`, `pytest.ini`, `ruff.toml` and `.coveragerc` to `pyproject.toml` to allow Dependabot to discover outdated dependencies #350
 * Tightened `requests` upper bound to `<2.33.0` matching the ceiling imposed by `autosubmit` v4.1.17 dependency (`requests<=2.32.5`)
 * `/v3/login` answers 401 instead of 500 when the CAS ticket verification response is not valid XML #383
+* `/v4/auth/cas/v2/login` returns 401 instead of 500 when the CAS server's ticket validation response is not a valid CAS document #383
 
 ### Release v4.2.0 - Release date: 2026-09-28
 
diff --git a/autosubmit_api/routers/v4/auth.py b/autosubmit_api/routers/v4/auth.py
index 4631724e..e2fa2271 100644
--- a/autosubmit_api/routers/v4/auth.py
+++ b/autosubmit_api/routers/v4/auth.py
@@ -1,4 +1,5 @@
 from http import HTTPStatus
+import xml.etree.ElementTree as ET
 from typing import Optional
 from fastapi import APIRouter, Depends, Request
 from fastapi.responses import JSONResponse, RedirectResponse
@@ -8,6 +9,7 @@
 from autosubmit_api.auth import ProtectionLevels, auth_token_dependency, oidc
 from autosubmit_api.auth.utils import validate_client
 from autosubmit_api.auth.utils import generate_jwt_token
+from autosubmit_api.logger import logger
 from autosubmit_api.models.responses import AuthResponse, LoginResponse
 
 
@@ -66,7 +68,11 @@ async def cas_v2_login(
 
     # There is a ticket, the request come from CAS as callback.
     # need call `verify_ticket()` to validate ticket and get user profile.
-    user, attributes, pgtiou = cas_client.verify_ticket(ticket)
+    try:
+        user, attributes, pgtiou = cas_client.verify_ticket(ticket)
+    except (ET.ParseError, IndexError, AttributeError, ValueError) as exc:
+        logger.warning("Unexpected CAS ticket validation response: %s", exc)
+        user = None
 
     if not user:
         return JSONResponse(
diff --git a/tests/test_endpoints_v4.py b/tests/test_endpoints_v4.py
index 7af69fa9..d2adc3f2 100644
--- a/tests/test_endpoints_v4.py
+++ b/tests/test_endpoints_v4.py
@@ -44,6 +44,74 @@ def test_invalid_client(
         response = fixture_fastapi_client.get(self.endpoint, params={"service": "asd"})
         assert response.status_code == HTTPStatus.UNAUTHORIZED
 
+    @pytest.mark.parametrize(
+        "cas_response, expected_status, expected_user",
+        [
+            pytest.param(
+                b"<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>"
+                b"<cas:authenticationSuccess><cas:user>jdoe</cas:user>"
+                b"</cas:authenticationSuccess></cas:serviceResponse>",
+                HTTPStatus.OK,
+                "jdoe",
+                id="success",
+            ),
+            pytest.param(
+                b"<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>"
+                b"<cas:authenticationFailure code='INVALID_TICKET'>"
+                b"Ticket not recognized</cas:authenticationFailure>"
+                b"</cas:serviceResponse>",
+                HTTPStatus.UNAUTHORIZED,
+                None,
+                id="failure",
+            ),
+            pytest.param(
+                b"<html>Service unavailable",
+                HTTPStatus.UNAUTHORIZED,
+                None,
+                id="not_xml",
+            ),
+            pytest.param(
+                b"<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>"
+                b"</cas:serviceResponse>",
+                HTTPStatus.UNAUTHORIZED,
+                None,
+                id="empty_service_response",
+            ),
+            pytest.param(
+                b"<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>"
+                b"<cas:authenticationSuccess></cas:authenticationSuccess>"
+                b"</cas:serviceResponse>",
+                HTTPStatus.UNAUTHORIZED,
+                None,
+                id="success_without_user",
+            ),
+            pytest.param(
+                b"<serviceResponse><authenticationSuccess><user>jdoe</user>"
+                b"</authenticationSuccess></serviceResponse>",
+                HTTPStatus.UNAUTHORIZED,
+                None,
+                id="no_namespace",
+            ),
+        ],
+    )
+    def test_ticket_validation(
+        self,
+        fixture_fastapi_client: TestClient,
+        monkeypatch: pytest.MonkeyPatch,
+        cas_response: bytes,
+        expected_status: HTTPStatus,
+        expected_user: str | None,
+    ):
+        monkeypatch.setattr(
+            "cas.CASClientV2.get_verification_response",
+            lambda self, ticket: cas_response,
+        )
+        response = fixture_fastapi_client.get(
+            self.endpoint, params={"ticket": "ST-1-test"}
+        )
+        assert response.status_code == expected_status
+        assert response.json()["user"] == expected_user
+
 
 class TestOIDCLogin:
     endpoint = "/v4/auth/oidc/login"