A race condition in the nginx module in Phusion Passenger 3.x through 5.x before 5.3.2 allows local escalation of privileges when a non-standard passengerinstanceregistrydir with insufficiently strict permissions is configured. Replacing a file with a symlink after the file was created, but before it was chowned, leads to the target of the link being chowned via the path. Targeting sensitive files such as root's crontab file allows privilege escalation.

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.