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

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]).