D-Bus is a well-known IPC (Inter-Process Communication) protocol (and daemon) that helps applications to talk to each other. The use of D-Bus is great because it allows to implement discovery and signaling.

The D-Bus session is by default addressed by environment variable DBUS_SESSION_BUS_ADDRESS. Using systemd variable DBUS_SESSION_BUS_ADDRESS is automatically set for user sessions. D-Bus usage is linked to permissions.

D-Bus has already had several security issues (mostly DoS issues), to allow applications to keep talking to each other. It is important to protect against this type of attack to keep the system more stable.

Domain Object Recommendations
Platform-DBus-1 Security model Use D-Bus as IPC.
Platform-DBus-2 Security model Apply D-BUS security patches: D-Bus CVE