Potential exposure of private data via missing Vary: Authorization in UpdateCacheMiddleware

An issue was discovered in Django 5.2 before 5.2.15 and 6.0 before 6.0.6.

`django.middleware.cache.UpdateCacheMiddleware` in Django does not add `Authorization` to the `Vary` response header for requests bearing that header without `Cache-Control: public`, which allows remote attackers to read private cached responses via unauthenticated requests to the same URL.

Earlier, unsupported Django series (such as 5.0.x, 4.1.x, and 3.2.x) were not evaluated and may also be affected.

Django would like to thank Shai Berger for reporting this issue.