Patched code sample

import typing as t


class SessionMixin(t.MutableMapping[str, t.Any]):
    """Expands a basic dictionary with session attributes."""

    def __init__(self, initial: t.Optional[t.Dict[str, t.Any]] = None) -> None:
        if initial:
            self.update(initial)

    def _get_interface(self) -> "SessionInterface":
        raise NotImplementedError

    #: Some implementations can detect changes to the session and set
    #: this when that happens. The default :class:`Session` will set
    #: this to ``True`` if ``__setitem__`` or ``__delitem__`` is
    #: called.
    modified = False

    #: The session was accessed at all. This is used by the default
    #: implementation to add a ``Vary: Cookie`` header.
    accessed = False

    def __getitem__(self, key: str) -> t.Any:
        self.accessed = True
        return self._get_interface().get_data(self)[key]

    def __setitem__(self, key: str, value: t.Any) -> None:
        self._get_interface().get_data(self)[key] = value
        self.modified = True

    def __delitem__(self, key: str) -> None:
        del self._get_interface().get_data(self)[key]
        self.modified = True

    # The fix for CVE-2024-27205 involved adding the `self.accessed = True`
    # line to the __contains__, __iter__, and __len__ methods. Previously,
    # these operations did not mark the session as accessed, preventing the
    # Vary: Cookie header from being set.

    def __contains__(self, item: str) -> bool:
        """Check if a key is in the session."""
        self.accessed = True
        return item in self._get_interface().get_data(self)

    def __iter__(self) -> t.Iterator[str]:
        self.accessed = True
        return iter(self._get_interface().get_data(self))

    def __len__(self) -> int:
        self.accessed = True
        return len(self._get_interface().get_data(self))

    def __repr__(self) -> str:
        return f"<{type(self).__name__} {self._get_interface().get_data(self)!r}>"