Cotonti: Cross-Site Request Forgery in the administration rights handler

Cotonti 1.0.0 (master branch, commit f43f1fc3) is vulnerable to Cross-Site Request Forgery in the administration rights handler. In system/admin/admin.rights.php, the rights update action ('a=update') modifies group access rights (including via cot_auth_add_group) without calling cot_check_xg() to validate the anti-CSRF token. A remote attacker who lures an authenticated administrator into visiting a malicious page can force the browser to submit a forged request that grants elevated permissions to an attacker-controlled group, escalating privileges to administrator. Because Cotonti administrators can modify templates and configuration, this can be further leveraged toward remote code execution.