ircu 2.10.12.05 and earlier allows remote attackers to discover the hidden IP address of arbitrary +x users via a series of /silence commands with (1) CIDR mask arguments or (2) certain other arguments that represent groups of IP addresses, then monitoring CTCP ping replies.
https://exchange.xforce.ibmcloud.com/vulnerabilities/35997
http://www.securityfocus.com/archive/1/476285/100/0/threaded