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[Bug 1227575] Re: DoS style attack on noVNC server can lead to service interruption or disruption

To: yahoo-eng-team@xxxxxxxxxxxxxxxxxxx
From: Nathan Kinder <nkinder@xxxxxxxxxx>
Date: Sun, 09 Mar 2014 16:55:41 -0000
Reply-to: Bug 1227575 <1227575@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

I've adjusted the "Summary" section as suggested by Rob. I've gone
ahead and published the following OSSN to the following locations:

https://wiki.openstack.org/wiki/OSSN/OSSN-0008
openstack@xxxxxxxxxxxxxxxxxxx
openstack-dev@xxxxxxxxxxxxxxxxxxx

Thanks everyone for the reviews and contributions to this OSSN!

--------------------------------------------------------------------------
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@xxxxxxxxxxxxxxxxxxx
OpenStack Security Group : https://launchpad.net/~openstack-ossg

** Changed in: ossn
Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1227575

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

Status in OpenStack Compute (Nova):
In Progress
Status in OpenStack Security Notes:
Fix Released

Bug description:
There is no limiting on the number of VNC sessions that can be created for a single user's VNC token.
Any attempt to create multiple (say hundreds or thousands) of websocket connections to the VNC server
results in many connection timeouts. Due to these connection timeout error, other users trying to access their
VM's VNC console cannot do so.

A sample script that tries to create 100,000 connections to Nova noVNC proxy, shows timeout errors
Script: http://paste.openstack.org/show/47254/

Script output.... connections get timed out after a while
-------------------
....
..

Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
Received 'RFB 003.008
'
Creating Connection
Receiving...
timed out
Creating Connection
Receiving...
timed out
Creating Connection
Receiving...
timed out
Creating Connection
Receiving...
timed out
Creating Connection
Receiving...
timed out
--------------------

Impact:
1. Many of the sessions timeout. Any attempt to open other sessions also intermittently fail.
This can cause serious problems to users already having a running VNC session or trying to create new sessions.

2. The overall performance and response times of other nova services running on the novnc host, using tcp protocol
also gets affected after the connection timeout errors.

For example:
Before running the sumulate thousands of connections program:
$ time nova get-vnc-console c1b093a3-f53b-4282-b89c-e68f0fa1b6e5 novnc
+-------+---------------------------------------------------------------------------------+
| Type | Url |
+-------+---------------------------------------------------------------------------------+
| novnc | http://10.2.3.102:6080/vnc_auto.html?token=e776dd33-422f-4b56-9f98-e317410d0212 |
+-------+---------------------------------------------------------------------------------+

real 0m0.751s
user 0m0.376s
sys 0m0.084s

rohit@precise-dev-102:~/tools/websocket-client-0.7.0$

After running the program, the response time is quite high:
$ time nova get-vnc-console c1b093a3-f53b-4282-b89c-e68f0fa1b6e5 novnc

+-------+---------------------------------------------------------------------------------+
| Type | Url |
+-------+---------------------------------------------------------------------------------+
| novnc | http://10.2.3.102:6080/vnc_auto.html?token=6865d675-d852-478b-b1ee-457b092f11b9 |
+-------+---------------------------------------------------------------------------------+

real 3m9.231s
user 0m0.424s
sys 0m0.108s

Possible solutions:
1. Allow just 1 session per instance, and raise a new exception, say, VNCSessionAlreadyExists to reject multiple
connections for the same token, and return an error code to the user.
2. Make the number of sessions allowed per instance configurable, limited by some count of sessions.

However, both of these solutions may need to override and modify the do_proxy() method of websockify's WebSocketProxy class,
which can lead to maintenance issues.

Another possible solution would be to implement some kind of callback function in websockify, to which we can pass the token
for reconnection. This would first need contribution to the websockify project code, and then update Nova.

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