DoS style attack on noVNC server can lead to service interruption or disruption
---

### Summary ###
There is currently no limit to the number of noVNC or SPICE console
sessions that can be established by a single user. The console host has
limited resources and an attacker launching many sessions may be able to
exhaust the available resources, resulting in a Denial of Service (DoS)
condition.

### Affected Services / Software ###
Horizon, Nova, noVNC proxy, SPICE console, Grizzly, Havana
 
### Discussion ###
Currently with a single user token, no restrictions are enforced on the
number or frequency of noVNC or SPICE console sessions that may be
established.  While a user can only access their own virtual machine
instances, resources can be exhausted on the console proxy host by
creating an excessive number of simultaneous console sessions.  This can
result in timeouts for subsequent connection requests to instances using
the same console proxy.  Not only would this prevent the user from
accessing their own instances, but other legitimate users would also be
deprived of console access.  Further, other services running on the
noVNC proxy and Compute hosts may degrade in responsiveness.

By taking advantage of this lack of restrictions around noVNC or SPICE
console connections, a single user could cause the console proxy
endpoint to become unresponsive, resulting in a Denial Of Service (DoS)
style attack.  It should be noted that there is no amplification effect.

### Recommended Actions ###
For current stable OpenStack releases (Grizzly, Havana), users need to
workaround this vulnerability by using rate-limiting proxies to cover
access to the noVNC proxy service.  Rate-limiting is a common mechanism
to prevent DoS and Brute-Force attacks.

For example, if you are using a proxy such as Repose, enable the rate
limiting feature by following these steps:

  https://repose.atlassian.net/wiki/display/REPOSE/Rate+Limiting+Filter

Future OpenStack releases are looking to add the ability to restrict
noVNC and SPICE console connections.

### Contacts / References ###
This OSSN : https://wiki.openstack.org/wiki/OSSN/OSSN-0008
Original LaunchPad Bug : https://bugs.launchpad.net/nova/+bug/1227575
OpenStack Security ML : openstack-security@lists.openstack.org
OpenStack Security Group : https://launchpad.net/~openstack-ossg