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.