Fix for CVE-2015-3222 which allows for root escalation via syscheck

[awiddersheim]

Affected versions: 2.7 - 2.8.1

Beginning is OSSEC 2.7 (d88cf1c) a feature was added to syscheck, which
is the daemon that monitors file changes on a system, called
report_changes. This feature is only available on *NIX systems. It's
purpose is to help determine what about a file has changed. The logic to
do accomplish this is as follows which can be found in
src/syscheck/seechanges.c:

252 /* Run diff */
253 date_of_change = File_DateofChange(old_location);
254 snprintf(diff_cmd, 2048, "diff \"%s\" \"%s\" > \"%s/local/%s/diff.%d\" "
255     "2>/dev/null",
256     tmp_location, old_location,
257     DIFF_DIR_PATH, filename + 1, (int)date_of_change);
258 if (system(diff_cmd) != 256) {
259     merror("%s: ERROR: Unable to run diff for %s",
260            ARGV0,  filename);
261     return (NULL);
262 }

Above, on line 258, the system() call is used to shell out to the
system's "diff" command. The raw filename is passed in as an argument
which presents an attacker with the possibility to run arbitrary code.
Since the syscheck daemon runs as the root user so it can inspect any
file on the system for changes, any code run using this vulnerability
will also be run as the root user.

An example attack might be creating a file called foo-$(touch bar)
which should create another file bar.

Again, this vulnerability exists only on *NIX systems and is contingent
on the following criteria:

1. A vulnerable version is in use.
2. The OSSEC agent is configured to use syscheck to monitor the file
   system for changes.
3. The list of directories monitored by syscheck includes those writable
   by underprivileged users.
4. The report_changes option is enabled for any of those directories.

The fix for this is to create temporary trusted file names that symlink
back to the original files before calling system() and running the
system's diff command.