Cisco has closed a backdoor that could have allowed hackers with access to the networking giant’s systems to take control of customers’ Umbrella Virtual Appliance devices.
Cisco Umbrella is a cloud-based Secure Internet Gateway (SIG) designed to provide visibility and protection for devices on and outside the corporate network. Virtual appliances allow organizations to map internal IPs to internal Active Directory users and computers, and forward external DNS queries from the network to an Umbrella data center.
The vulnerability, discovered by David Coomber and tracked as CVE-2017-6679, is related to an undocumented SSH tunnel between the Umbrella Virtual Appliance and a terminating server in Cisco’s data centers. This encrypted channel is designed to allow Cisco support personnel to troubleshoot customer installations and it provides unrestricted access.
In Umbrella Virtual Appliance 2.0.3 and prior versions this tunnel is always enabled and accessing it does not require explicit permission from the customer. A connection does however require valid keys that are only provided to privileged Cisco Umbrella support staff.
An attacker who can access Cisco’s terminating server could use this SSH tunnel as a backdoor into an organization’s on-premises devices. The backdoor provides full control over a virtual appliance.
Cisco has classified this as a vulnerability, which it has rated “medium severity” with a CVSS score of 6.4, due to the fact that it involves an undocumented entry method into customers’ network devices.
“While Cisco has NO indications that our remote support SSH hubs have ever been compromised, Cisco has made significant changes to the behavior of the remote support tunnel capability to further secure the feature,” the company said in a service notification published earlier this month.
In Umbrella Virtual Appliance versions prior to 2.1.0, customers could prevent access to the SSH tunnel by blocking the relevant firewall ports. However, starting with version 2.1.0, customers need to explicitly approve the creation of the SSH tunnel between the appliance and Cisco’s data centers.
Organizations can now also configure the duration of the tunnel, disable it at any time, and they need to provide configuration parameters out-of-band to Cisco support staff before the tunnel can be established.