A privilege escalation vulnerability in the Secure Shell (SSH) subsystem in the StarOS operating system for Cisco ASR 5000 Series, ASR 5500 Series, ASR 5700 Series devices, and Cisco Virtualized Packet Core could allow an authenticated, remote attacker to gain unrestricted, root shell access. The vulnerability is due to missing input validation of parameters passed during SSH or SFTP login. An attacker could exploit this vulnerability by providing crafted user input to the SSH or SFTP command-line interface (CLI) during SSH or SFTP login. An exploit could allow an authenticated attacker to gain root privileges access on the router. Note: Only traffic directed to the affected system can be used to exploit this vulnerability. This vulnerability can be triggered via both IPv4 and IPv6 traffic. An established TCP connection toward port 22, the SSH default port, is needed to perform the attack. The attacker must have valid credentials to login to the system via SSH or SFTP. The following products have been confirmed to be vulnerable: Cisco ASR 5000/5500/5700 Series devices running StarOS after 17.7.0 and prior to 18.7.4, 19.5, and 20.2.3 with SSH configured are vulnerable. Cisco Virtualized Packet Core – Single Instance (VPC-SI) and Distributed Instance (VPC-DI) devices running StarOS prior to N4.2.7 (19.3.v7) and N4.7 (20.2.v0) with SSH configured are vulnerable. Cisco Bug IDs: CSCva65853.