Not rated as a security issue upstream, however letting you know anyway:
http://www.inspircd.org/2015/04/16/v2019-released.html

This release fixes the issues discovered since 2.0.18, containing multiple
important stability and correctness related improvements, including a fix for a
bug which allowed malformed DNS records to cause netsplits on a network.
Triggering this issue is non-trivial and it may not occur in all circumstances,
but all users are advised to upgrade. If that���s not possible, disabling DNS
lookups for connecting users (nouserdns setting) mitigates the problem.

server:irc inspircd 2.0.18