rxvt-unicode (urxvt) is a clone of the rxvt terminal emulator.
rxvt-unicode does not properly handle OSC escape sequences, including those used to read and write X window properties.
A remote attacker could entice a user to run a specially crafted file using rxvt-unicode, possibly resulting in execution of arbitrary code with the privileges of the process or a Denial of Service condition.
There is no known workaround at this time.
All rxvt-unicode users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=x11-terms/rxvt-unicode-9.20"