Non-registered IRC users using (1) ircd-hybrid 7.0.1 and earlier, (2) ircd-ratbox 1.5.1 and earlier, or (3) ircd-ratbox 2.0rc6 and earlier do not have a rate-limit imposed, which could allow remote attackers to cause a denial of service by repeatedly making requests, which are slowly dequeued.
https://exchange.xforce.ibmcloud.com/vulnerabilities/16457