client/cl_parse.c in the id3 Quake 3 Engine 1.32c and the Icculus Quake 3 Engine (ioquake3) revision 810 and earlier allows remote malicious servers to overwrite arbitrary write-protected cvars variables on the client, such as cl_allowdownload for Automatic Downloading and fs_homepath for the quake3 path, via a string of cvar names and values sent from the server. NOTE: this can be combined with another vulnerability to overwrite arbitrary files.
https://exchange.xforce.ibmcloud.com/vulnerabilities/27486
https://exchange.xforce.ibmcloud.com/vulnerabilities/26889
http://www.vupen.com/english/advisories/2006/2569
http://www.securityfocus.com/archive/1/438660/100/0/threaded
http://www.securityfocus.com/archive/1/438515/100/0/threaded
http://securityreason.com/securityalert/1171