XChat 1.8.7 and earlier, including default configurations of 1.4.2 and 1.4.3, allows remote attackers to execute arbitrary IRC commands as other clients via encoded characters in a PRIVMSG command that calls CTCP PING, which expands the characters in the client response when the percascii variable is set.
https://exchange.xforce.ibmcloud.com/vulnerabilities/7856
http://www.securityfocus.com/bid/3830
http://www.debian.org/security/2002/dsa-099
http://rhn.redhat.com/errata/RHSA-2002-005.html
http://online.securityfocus.com/advisories/3806
http://marc.info/?l=bugtraq&m=101060676210255&w=2
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000453