ruby-rack vulnerabilities

USN-4561-1 fixed vulnerabilities in Rack. This update provides the
corresponding update for Ubuntu 16.04 LTS, Ubuntu 20.04 LTS and Ubuntu 20.10.

Original advisory details:

It was discovered that Rack incorrectly handled certain paths. An attacker
could possibly use this issue to obtain sensitive information. This issue
only affected Ubuntu 16.04 LTS, Ubuntu 18.04 LTS and Ubuntu 20.04 LTS.
(CVE-2020-8161)

It was discovered that Rack incorrectly validated cookies. An attacker
could possibly use this issue to forge a secure cookie. (CVE-2020-8184)