agent/Core/Controller/SendRequest.cpp
in Phusion Passenger before 4.0.60 and 5.0.x before 5.0.22, when used in Apache integration mode or in standalone mode without a filtering proxy, allows remote attackers to spoof headers passed to applications by using an _
(underscore) character instead of a -
(dash) character in an HTTP header, as demonstrated by an X_User
header.