eventapp/lib/gcloud.rb in the ISUCON5 qualifier portal (aka eventapp) web application before 2015-10-30 makes improper popen calls, which allows remote attackers to execute arbitrary commands via an HTTP request that includes shell metacharacters in an argument to a "gcloud compute" command.
Improper Neutralization of Special Elements used in an OS Command The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command before it is sent to a downstream component.