Impact

Instances of HeavySelect2Mixin subclasses like the ModelSelect2MultipleWidget and ModelSelect2Widget can secret access tokens across requests. This can allow users to access restricted querysets and restricted data.

Patches

The problem has been patched in version 8.4.1 and all following versions.

Workarounds

This vulnerability is limited use cases where instances of widget classes are created during app loading (not during a request).

Example of affected code:

class MyForm(forms.ModelForm):
    class Meta:
        widgets = {"my_select_field": Select2ModelWidget()}

Django allows you to pass just the widget class (not the instance). This can be used to mitigate the session request leak.

Example of affected code:

class MyForm(forms.ModelForm):
    class Meta:
        widgets = {"my_select_field": Select2ModelWidget}

References

Thanks to @neartik for reporting this issue. I will address it later. I had to delete your issue, to avoid exploitation of this security issue.