Shopware is an open commerce platform based on Symfony Framework and Vue. The Symfony Session Handler pops the Session Cookie and assigns it to the Response. Since Shopware 6.5.8.0, the 404 pages are cached to improve the performance of 404 pages. So the cached Response which contains a Session Cookie when the Browser accessing the 404 page, has no cookies yet. The Symfony Session Handler is in use, when no explicit Session configuration has been done. When Redis is in use for Sessions using the PHP Redis extension, this exploiting code is not used. Shopware version 6.5.8.7 contains a patch for this issue. As a workaround, use Redis for Sessions, as this does not trigger the exploit code.
The code uses a cache that contains sensitive information, but the cache can be read by an actor outside of the intended control sphere.