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.
Exploitability
AV:NAC:MAu:NImpact
C:PI:NA:N4.3/AV:N/AC:M/Au:N/C:P/I:N/A:N