CVE-2011-0702: Low severity feh vulnerability

A Debian bug report [1],[2] indicated that feh is vulnerable to an arbitrary file overwrite flaw. If a user could guess the PID of the feh process and create a symlink in /tmp, they could cause the overwrite of any file that the user running feh has write access to via wget overwriting the file. In src/imlib.c we have:

235 char fehhttploadimage(char url) 236 { 237 char tmpname; 238 char basename; 239 char path = NULL; 240 241 if (opt.keephttp) { 242 if (opt.outputdir) 243 path = opt.outputdir; 244 else 245 path = ""; 246 } else 247 path = "/tmp/"; 248 249 basename = strrchr(url, '/') + 1; 250 tmpname = fehuniquefilename(path, basename); ... 455 execlp("wget", "wget", "--cache=off", "-O", tmpname, url, quiet, NULL);

and in src/utils.c we have:

162 char fehuniquefilename(char path, char basename) 163 { ... 170 171 / Massive paranoia ;) / 172 if (i > 999998) 173 i = 1; 174 175 ppid = getpid(); 176 snprintf(cppid, sizeof(cppid), "%06ld", (long) ppid); 177 178 / make sure file doesn't exist / 179 do { 180 snprintf(num, sizeof(num), "%06ld", i++); 181 tmpname = estrjoin("", path, "feh", cppid, "", num, "", basename, NULL); 182 } 18 while (stat(tmpname, &st) == 0); 184 return(tmpname); 185 }

fehuniquefilename() has some attempt at randomness, but I don't think it's enough. i always starts at 1, so unless the same process is creating multiple temporary files, you're almost guaranteed to have that extra randomness being '1'; it's also just as easy to create a bunch of symlinks to account for the "randomness" provided you know the PID. Using mkstemp would be a much better solution.

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=612035 [2] https://bugs.launchpad.net/ubuntu/+source/feh/+bug/607328