ext/common/ServerInstanceDir.h in Phusion Passenger gem before 4.0.6 for Ruby allows local users to gain privileges or possibly change the ownership of arbitrary directories via a symlink attack on a directory with a predictable name in /tmp/.

Michael Scherer reported that the passenger ruby gem, when used in standalone mode, does not use temporary files in a secure manner. In the lib/phusionpassenger/standalone/main.rb's createnginxcontroller function, passenger creates an nginx configuration file insecurely and starts nginx with that configuration file:

@tempdir = "/tmp/passenger-standalone.#{$$}" @configfilename = "#{@tempdir}/config"

If a local attacker were able to create a temporary directory that passenger uses and supply a custom nginx configuration file they could start an nginx instance with their own configuration file. This could result in a denial of service condition for a legitimate service or, if passenger were executed as root (in order to have nginx listen on port 80, for instance), this could lead to a local root compromise.