django CMS: Page cache ignores plugin-declared Vary headers (disclosure & poisoning)

django CMS is a content management system powered by Django. Prior to 5.0.8 and in 5.1.0a1, the django CMS page cache in cms/cache/page.py ignores request headers declared by plugins through get_vary_cache_on(). The _page_cache_key function includes the cache prefix, site, language, path, and timezone but not the declared header values. Although set_page_cache adds those names to the response Vary header, get_page_cache retrieves the first stored variant under the same header-agnostic key. When CMS_PAGE_CACHE is enabled and a plugin varies content on a header such as Country-Code, one visitor can receive another visitor’s request-specific content, and an unauthenticated attacker can prime the cache with attacker-chosen content. This issue is fixed in versions 5.0.8 and 5.1.0.