Summary
Mechanize (rubygem) < v2.8.5 leaks the Authorization header after a redirect to a different port on the same site.
Mitigation
Upgrade to Mechanize v2.8.5 or later.
Notes
See https://curl.se/docs/CVE-2022-27776.html for a similar vulnerability in curl.
Cookies are shared with a server at a different port on the same site, per https://datatracker.ietf.org/doc/html/rfc6265#section-8.5 which states in part:
Cookies do not provide isolation by port. If a cookie is readable
by a service running on one port, the cookie is also readable by a
service running on another port of the same server. If a cookie is
writable by a service on one port, the cookie is also writable by a
service running on another port of the same server. For this
reason, servers SHOULD NOT both run mutually distrusting services on
different ports of the same host and use cookies to store security-
sensitive information.