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.