The remote Debian host is missing a security-related update.
Orange Tsai discovered a path traversal flaw in ruby-sprockets, a Rack-based asset packaging system. A remote attacker can take advantage of this flaw to read arbitrary files outside an application's root directory via specially crafted requests, when the Sprockets server is used in production.
Upgrade the ruby-sprockets packages. For the stable distribution (stretch), this problem has been fixed in version 3.7.0-1+deb9u1.