Designated CVE-2018-0125, the critical bug is located in the web interface of Cisco's RV132W ADSL2+ Wireless-N VPN Router and RV134W VDSL2 Wireless-AC VPN Routers. Firmware version 184.108.40.206 fixes this vulnerability.
The flaw stems from "incomplete input validation on user-controlled input in an HTTP request to the targeted device," Cisco warns in an advisory, and "could allow an unauthenticated, remote attacker to execute arbitrary code and gain full control of an affected system, including issuing commands with root privileges." Moreover, attackers can also trigger the aforementioned (DoS) condition by causing an affected system to reload.
Cisco also issued fixes for three high-severity and 15 medium-severity flaws, while also updating an advisory related to a critical vulnerability found in its Adaptive Security Appliance.