GHSA-fwjf-m4qw-9f2x

### Summary The CMS page cache key ignores the request headers that plugins declare via `get_vary_cache_on()`. The header is added to the response `Vary` header, but the CMS's own cache key does not incorporate the header values, so the first visitor's variant is served to all subsequent visitors regardless of their header values.

### Details `_page_cache_key` (in `cms/cache/page.py`) keys only on cache prefix, site, language, path and timezone. `set_page_cache` collects the plugin-declared vary headers and calls `patch_vary_headers(response, ...)` (affecting only the emitted `Vary` header), but stores and retrieves the cached page under the header-agnostic key. `get_page_cache` therefore returns whichever variant was cached first.

### Impact - **Information disclosure:** when a plugin varies its output on a request header (e.g. `Country-Code`), the variant rendered for the first anonymous visitor is served to everyone until the entry expires, leaking request-specific content across users. - **Cache poisoning:** an unauthenticated attacker can prime the anonymous page cache with content rendered from attacker-chosen header values, which is then served to subsequent visitors.

Applies only when `CMS_PAGE_CACHE` is enabled and at least one plugin implements `get_vary_cache_on()`.

### Patches Fixed in 5.0.8: the page cache now folds the request's values implements `get_vary_cache_on()`.

### Patches Fixed in 5.0.8: the page cache now folds the request's values for plugin-declared vary headers into the content key. The set of vary headers is persisted on write and looked up first on read (mirroring Django's `learn_cache_key`/`get_cache_key`); a missing header-list entry degrades to a cache miss, never a wrong-variant hit.

### Workarounds Disable `CMS_PAGE_CACHE`, or avoid plugins that rely on `get_vary_cache_on()`, until upgraded.

### Credits Reported by the security team at the University of Sydney ([@reporter]).