rxvt-unicode is a clone of the well known terminal emulator rxvt.
A vulnerability has been discovered in rxvt-unicode. Please review the CVE identifiers referenced below for details.
in the Perl background extension, when an attacker can control the data written to the user's terminal and certain options are set. The "background" extension is automatically loaded if certain X resources are set such as 'transparent' (see the full list at the top of src/perl/background[1]). So it is possible to be using this extension without realising it.
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-9.30"