Shopware SSO referer trust leading to an arbitrary redirect target

Summary

Shopware is an open commerce platform. Versions 6.7.3.0 through 6.7.10.0 have an open redirect in Shopware's public SSO entry point at `GET /api/oauth/sso/auth`. When the endpoint is reached without the expected SSO session state, the application falls back to the request's `Referer` header and uses that value as the redirect destination. In the validated behavior, the server does not restrict that fallback target to same-origin URLs, does not require a relative path, and does not reject dangerous schemes such as `javascript:`. As a result, an unauthenticated request can turn this endpoint into a reusable redirect primitive whose destination is fully controlled by attacker-supplied request metadata. The security problem is not limited to a harmless navigation mismatch. The endpoint sits under `/api/oauth/`, which gives the redirect a trustworthy application-controlled origin and makes it suitable for phishing chains, branded redirect abuse, and cases where client software automatically follows redirects issued by a trusted host. The attached evidence also shows that the response is not only an HTTP `302` with a user-controlled `Location` header. The HTML body contains a matching meta refresh tag and redirect link built from the same attacker-controlled value. In the validated proof, the endpoint redirects to `https://attacker.example/poc` when that URL is supplied through `Referer`, and it also reflects `javascript:alert(1)` into `Location` and the HTML redirect body without any scheme filtering. This report therefore stays conservative and claims an open redirect with arbitrary redirect targets, while noting that the lack of scheme restrictions makes the behavior materially worse than a same-scheme external redirect. Version 6.7.10.1 fixes the issue.