mpp writes Payment-Receipt and Cache-Control before the wrapped application runs, letting a consumer's own Cache-Control expose paid responses to shared caches

Summary

Use of Cache Containing Sensitive Information in ZenHive mpp allows a shared HTTP cache to store a paid response and serve it to clients that never paid.

MPP.Plug.verify_credential in lib/mpp/plug.ex sets payment-receipt and cache-control: private on the connection before the wrapped application runs, and registers no register_before_send/2 callback. Plug.Conn.put_resp_header/3 replaces an existing header, so a mounting application that sets its own cache-control on the paid resource (for example public, max-age=3600) silently overrides the private the library relies on, and a CDN or reverse proxy can then store the paid 200 together with its Payment-Receipt and serve both to unpaid clients. The library-level guarantee is therefore defeatable by the application it protects. For the same reason a downstream non-2xx response still carried Payment-Receipt, issuing a receipt for a response that delivered no resource.

This issue affects mpp: from 0.1.0 before 0.16.2.

Impact

An unpaid client requesting the same URL through the shared cache receives the cached paid response, including the Payment-Receipt header issued to the paying client. Operators lose the revenue for every cache hit, and the receipt identifying a paid transaction is disclosed to third parties.

Workarounds

In the application mounted behind MPP.Plug, stop setting Cache-Control on paid routes so the library's private survives to the response, or set a directive that is itself safe for a shared cache (private, or no-store). Where the application must set its own caching policy, configure the CDN or reverse proxy not to cache responses carrying a Payment-Receipt header.

Configurations

Reachable only where the application mounted behind MPP.Plug sets its own Cache-Control on the paid resource, overriding the private the library wrote, and a shared HTTP cache (CDN or reverse proxy) sits in front of that route. An application that sets no Cache-Control of its own keeps the library's private and is not affected.