diff --git a/backend/chainlit/auth/cookie.py b/backend/chainlit/auth/cookie.py
index 1cb4cc5ace..7ff72f3abc 100644
--- a/backend/chainlit/auth/cookie.py
+++ b/backend/chainlit/auth/cookie.py
@@ -1,3 +1,4 @@
+import hmac
 import os
 from typing import Literal, Optional, cast
 
@@ -24,7 +25,7 @@
 )
 _cookie_secure = _cookie_samesite == "none"
 if _cookie_root_path := os.environ.get("CHAINLIT_ROOT_PATH", None):
-    _cookie_path = os.environ.get(_cookie_root_path, "/")
+    _cookie_path = _cookie_root_path
 else:
     _cookie_path = os.environ.get("CHAINLIT_AUTH_COOKIE_PATH", "/")
 _state_cookie_lifetime = int(
@@ -186,11 +187,15 @@ def set_oauth_state_cookie(response: Response, token: str):
 
 
 def validate_oauth_state_cookie(request: Request, state: str):
-    """Check the state from the oauth provider against the browser cookie."""
+    """Check the state from the oauth provider against the browser cookie.
+
+    Uses ``hmac.compare_digest`` for constant-time comparison to prevent
+    timing-based side-channel leakage of the expected state value.
+    """
 
     oauth_state = request.cookies.get(_state_cookie_name)
 
-    if oauth_state != state:
+    if oauth_state is None or not hmac.compare_digest(oauth_state, state):
         raise Exception("oauth state does not correspond")
 
 
diff --git a/backend/tests/auth/test_cookie.py b/backend/tests/auth/test_cookie.py
index 5f5c3848a5..a9482305a7 100644
--- a/backend/tests/auth/test_cookie.py
+++ b/backend/tests/auth/test_cookie.py
@@ -133,6 +133,68 @@ def test_overwrite_shorter_token_unchunked(client):
     assert len(chunk_cookies) == 0, f"Found {len(chunk_cookies)} residual cookies"
 
 
+def test_cookie_path_uses_chainlit_root_path_value(monkeypatch):
+    """CHAINLIT_ROOT_PATH must be used as the cookie path directly.
+
+    Before the fix, the code read::
+
+        _cookie_path = os.environ.get(_cookie_root_path, "/")
+
+    where ``_cookie_root_path`` was the *value* of ``CHAINLIT_ROOT_PATH``,
+    not a key.  This caused ``_cookie_path`` to always be ``"/"`` (the
+    default) whenever ``CHAINLIT_ROOT_PATH`` was set, because the value
+    (e.g. ``"/app"``) is not a valid environment variable name.  Cookie
+    deletion then used the wrong path, leaving stale cookies on the client.
+    """
+    monkeypatch.setenv("CHAINLIT_ROOT_PATH", "/myapp")
+    monkeypatch.delenv("CHAINLIT_AUTH_COOKIE_PATH", raising=False)
+    importlib.reload(cookie_module)
+    assert cookie_module._cookie_path == "/myapp", (
+        f"Expected _cookie_path to be '/myapp' but got '{cookie_module._cookie_path}'. "
+        "CHAINLIT_ROOT_PATH value should be used directly as the cookie path."
+    )
+
+
+def test_validate_oauth_state_cookie_rejects_missing_state(monkeypatch):
+    """validate_oauth_state_cookie must raise when the state cookie is absent."""
+    from chainlit.auth.cookie import validate_oauth_state_cookie
+    from starlette.requests import Request as StarletteRequest
+
+    scope = {
+        "type": "http",
+        "headers": [],
+        "query_string": b"",
+        "method": "GET",
+        "path": "/",
+    }
+    request = StarletteRequest(scope)
+    # Cookies dict is empty — oauth_state cookie is absent
+    with pytest.raises(Exception, match="oauth state does not correspond"):
+        validate_oauth_state_cookie(request, "expected_state")
+
+
+def test_validate_oauth_state_cookie_accepts_correct_state():
+    """validate_oauth_state_cookie must not raise when states match."""
+    from chainlit.auth.cookie import validate_oauth_state_cookie
+    from starlette.requests import Request as StarletteRequest
+
+    state_value = "secret_state_token"
+
+    # Build a minimal request with the oauth_state cookie set
+    scope = {
+        "type": "http",
+        "headers": [
+            (b"cookie", f"oauth_state={state_value}".encode()),
+        ],
+        "query_string": b"",
+        "method": "GET",
+        "path": "/",
+    }
+    request = StarletteRequest(scope)
+    # Should not raise
+    validate_oauth_state_cookie(request, state_value)
+
+
 def test_state_cookie_lifetime_default(monkeypatch):
     """Test that _state_cookie_lifetime defaults to 180 seconds (3 minutes)."""
     monkeypatch.delenv("CHAINLIT_STATE_COOKIE_LIFETIME", raising=False)