Wagtail: Denial of service via unbounded filter specs in the image preview

Impact

An authenticated admin user can trigger expensive rendition processing with purposefully crafted filter specs resulting in potentially service degradation.

The vulnerability is not exploitable by an ordinary site visitor without access to the Wagtail admin.

Patches

Patched versions have been released as Wagtail 7.0.8, 7.3.3, 7.4.2.

Workarounds

For sites that cannot easily upgrade to a current supported version, the vulnerability can be patched by adding the following code to urls.py URL pattern declarations to override the vulnerable view.

from django.core.exceptions import PermissionDenied
from django.http import HttpResponseBadRequest
from django.shortcuts import get_object_or_404
from django.urls import path
from wagtail.admin import urls as wagtailadmin_urls
from wagtail.images.exceptions import InvalidFilterSpecError
from wagtail.images.permissions import permission_policy
from wagtail.images.views import preview


def patched_preview(request, image_id, filter_spec):
    image = get_object_or_404(get_image_model(), id=image_id)

    if not permission_policy.user_has_permission_for_instance(request.user, "change", image):
        raise PermissionDenied

    try:
        filter_obj = Filter(spec=filter_spec)
    except InvalidFilterSpecError:
        return HttpResponseBadRequest("Invalid filter spec", content_type="text/plain")

    allowed = {"original", "width", "height", "min", "max", "fill"}
    if any(operation.method not in allowed for operation in filter_obj.operations):
        return HttpResponseBadRequest("Invalid filter spec", content_type="text/plain")

    return preview(request, image_id, filter_spec)


urlpatterns = [
    # Example where the CMS admin is at /admin/.
    # Add this before the Wagtail admin URLs registration, with the same sub-path.
    path("admin/images//preview//", patched_preview)
    path("admin/", include(wagtailadmin_urls)),
]