During the spawning of a malicious Passenger-managed application, SpawningKit in Phusion Passenger 5.3.x before 5.3.2 allows such applications to replace key files or directories in the spawning communication directory with symlinks. This then could result in arbitrary reads and writes, which in turn can result in information disclosure and privilege escalation.
blog.phusion.nl/2018/06/12/passenger-5-3-2-various-security-fixes
blog.phusion.nl/passenger-5-3-2
github.com/phusion/passenger
github.com/phusion/passenger/commit/fd3717a3cd357aa0e80e1e81d4dc94a1eaf928f1
github.com/rubysec/ruby-advisory-db/blob/master/gems/passenger/CVE-2018-12026.yml
nvd.nist.gov/vuln/detail/CVE-2018-12026
security.gentoo.org/glsa/201807-02