get_input_file (char const *filename, char const *outname, mode_t file_type)
{
    bool elsewhere = strcmp (filename, outname) != 0;
    char const *cs;
    char *diffbuf;
    char *getbuf;

    if (inerrno == -1)
      inerrno = stat_file (filename, &instat);

    /* Perhaps look for RCS or SCCS versions.  */
    if (S_ISREG (file_type)
	&& patch_get
	&& invc != 0
	&& (inerrno
	    || (! elsewhere
		&& (/* No one can write to it.  */
		    (instat.st_mode & (S_IWUSR|S_IWGRP|S_IWOTH)) == 0
		    /* Only the owner (who's not me) can write to it.  */
		    || ((instat.st_mode & (S_IWGRP|S_IWOTH)) == 0
			&& instat.st_uid != geteuid ()))))
	&& (invc = !! (cs = (version_controller
			     (filename, elsewhere,
			      inerrno ? (struct stat *) 0 : &instat,
			      &getbuf, &diffbuf))))) {

	    if (!inerrno) {
		if (!elsewhere
		    && (instat.st_mode & (S_IWUSR|S_IWGRP|S_IWOTH)) != 0)
		    /* Somebody can write to it.  */
		  fatal ("File %s seems to be locked by somebody else under %s",
			 quotearg (filename), cs);
		if (diffbuf)
		  {
		    /* It might be checked out unlocked.  See if it's safe to
		       check out the default version locked.  */

		    if (verbosity == VERBOSE)
		      say ("Comparing file %s to default %s version...\n",
			   quotearg (filename), cs);

		    if (systemic (diffbuf) != 0)
		      {
			say ("warning: Patching file %s, which does not match default %s version\n",
			     quotearg (filename), cs);
			cs = 0;
		      }
		  }
		if (dry_run)
		  cs = 0;
	    }

	    if (cs && version_get (filename, cs, ! inerrno, elsewhere, getbuf,
				   &instat))
	      inerrno = 0;

	    free (getbuf);
	    free (diffbuf);
      }

    if (inerrno)
      {
	instat.st_mode = S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH;
	instat.st_size = 0;
      }
    else if (! ((S_ISREG (file_type) || S_ISLNK (file_type))
	        && (file_type & S_IFMT) == (instat.st_mode & S_IFMT)))
      {
	say ("File %s is not a %s -- refusing to patch\n",
	     quotearg (filename),
	     S_ISLNK (file_type) ? "symbolic link" : "regular file");
	return false;
      }
    return true;
}

The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.