The labeled-response module in UnrealIRCd 6.0.0 before 6.2.7.1 does not escape the IRCv3 "label" message tag when echoing it into replies, which allows any user on a network with two or more linked servers to inject server-to-server commands that are executed with server authority.
Add the following line to the configuration file of all servers until the patch or upgrade is applied: blacklist-module "labeled-response";
Upgrade to UnrealIRCd 6.2.7.1 or later. Alternatively, apply the hot-patch to fix the issue without restart: ./unrealircd hot-patch bundle-20261107