SA-CONTRIB-2026-247: Critical severity drupal/imageapi_optimize_webp vulnerability

This module enables you to generate and serve WebP derivatives of image style images, overriding Drupal core's image style download controller to serve them. The module serves WebP image derivative responses with the HTTP header Cache-Control: public for all file schemes other than the private file system, including the temporary file system and access-controlled custom schemes. This can lead to some users getting cached versions of files with information they should not be able to access. For example, files may be cached by Varnish or a CDN. This vulnerability is mitigated by the fact that it only affects sites that serve image derivatives from a custom or temporary file scheme with access control, behind a shared cache that honors the Cache-Control header. Access checks are still enforced on requests that reach the site itself. An attacker would also need to know the path of a previously requested file still in cache. This is the same vulnerability class fixed in Drupal core by SA-CORE-2025-008 and SA-CORE-2026-010; the module's controller override reintroduced the behavior.