Admin with session cookie gets cached page for anonymous users

Problem/Motivation

In some cases on our client (logged in as admin) sees a cached version of the anonymous user page. Only after clearing caches the page shows normally (uncached with admin toolbar)

Steps to reproduce

When a user with an expired or invalid session token requests a page it gets cached in varnish (due to being an anonymous user). Then when a user with a valid session token request the page, he gets the cached page.

Proposed resolution

This is a flaw in the varnish vcl. A change in the vcl_hash is needed.

Add following line after the set req.http.X-SESS line (untested):
set req.http.X-Bin = "user:" + req.http.X-SESS;

Remaining tasks

Fix the issue or propose a better solution.

User interface changes

API changes

Data model changes