{"id":"EEF-CVE-2026-89186","summary":"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","details":"## Summary\n\nUse 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.\n\n`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.\n\nThis issue affects mpp: from 0.1.0 before 0.16.2.\n\n## Impact\n\nAn 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.\n\n## Workarounds\n\nIn 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.\n\n## Configurations\n\nReachable 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.","aliases":["CVE-2026-89186","GHSA-82qh-vrvm-gqvc"],"modified":"2026-09-16T08:40:58.908314385Z","published":"2026-09-16T08:24:15.058Z","database_specific":{"cpe_ids":["cpe:2.3:a:ZenHive:mpp:*:*:*:*:*:*:*:*"],"cwe_ids":["CWE-524"],"capec_ids":["CAPEC-204"]},"references":[{"type":"ADVISORY","url":"https://github.com/ZenHive/mpp/security/advisories/GHSA-82qh-vrvm-gqvc"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-89186.html"},{"type":"WEB","url":"https://github.com/ZenHive/mpp/commit/2d4d1d94aae7790ae0623063961adbeef171fa71"},{"type":"FIX","url":"https://github.com/ZenHive/mpp/commit/2fd91a5ecbd0b0ad2a4ac202b79659e8126dbc0b"},{"type":"PACKAGE","url":"https://hex.pm/packages/mpp"}],"affected":[{"package":{"name":"mpp","ecosystem":"Hex","purl":"pkg:hex/mpp"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.0"},{"fixed":"0.16.2"}]}],"versions":["0.1.0","0.1.1","0.10.0","0.11.0","0.12.0","0.13.0","0.14.0","0.15.0","0.16.0","0.16.1","0.2.0","0.3.0","0.3.1","0.3.2","0.4.0","0.6.0","0.6.1","0.6.2","0.6.3","0.6.4","0.7.0","0.8.0","0.9.0"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-89186.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/ZenHive/mpp","events":[{"introduced":"2d4d1d94aae7790ae0623063961adbeef171fa71"},{"fixed":"2fd91a5ecbd0b0ad2a4ac202b79659e8126dbc0b"}]}],"versions":["v0.16.1","v0.16.0","v0.15.0","v0.14.0","v0.13.0","v0.12.0","v0.11.0","v0.10.0","v0.9.0","v0.8.0","v0.7.0","v0.6.4","v0.6.3","v0.6.1","v0.6.0","v0.5.1","v0.4.0","v0.3.0","v0.2.0","v0.1.0"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-89186.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N"}],"credits":[{"name":"E.FU","type":"FINDER"},{"name":"E.FU","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}