Steve Kemp discovered a vulnerability in xonix, a game, where an external program was invoked while retaining setgid privileges. A local attacker could exploit this vulnerability to gain gid "games".
For the current stable distribution (woody) this problem will be fixed in version 1.4-19woody1.
For the unstable distribution (sid), this problem will be fixed soon.
We recommend that you update your xonix package.
MD5 checksums of the listed files are available in the original advisory.