Preparation

Before we release details that may lead to reproduction of the issue, there are a few things that you can do. Both as a user, and as a systems administrator.

For everyone

Install your updates, automatically and quickly

Make sure your firewall is up to date and running

For the SysAdmins

Turn on IPv4 port-based DHCP filtering in your switches

Turn on port-based DHCPv6 filtering in your switches

Turn on port-based Router Advertisement filtering in your switches

Make sure you have host-based firewalls installed and enabled

Make sure you can quickly and automatically push updates to your host-based firewalls.

Make sure clients are on different networks/VLAN from your infrastructure

Make sure your networking infrastructure cannot be reached from normal clients

The above advice are the only ones we can give right now. We will update this section with proper firewall rules for some platforms, and more detailed advice once the updates have been pushed by vendors.

The cause

Many systems accept link-settings from a link-local RouterAdvertisement packet, that doesn't contain any route. This allows a link-local attacker to change interface level settings. More specifically, the Hop Limit

Result of the attack

Hop Limit is reduced to 1. This means that IPv6 packets originating from the device will be discarded after the first routing hop. This effectively disables IPv6 connectivity beyond the local network.

The root cause:

The ignored lines of RFC3756:

As an example, one possible approach to mitigate this threat is to ignore very small hop limits. The nodes could implement a configurable minimum hop limit, and ignore attempts to set it below said limit.

The obeyed lines of RFC4861:

The IP Hop Limit field has a value of 255, i.e., the packet could not possibly have been forwarded by a router.

ICMP Checksum is valid.

ICMP Code is 0.

ICMP length (derived from the IP length) is 16 or more octets.

All included options have a length that is greater than zero.

And most importantly:

If the received Cur Hop Limit value is non-zero, the host SHOULD set its CurHopLimit variable to the received value.

By implementing this, we are allowed to send a RA packet, without any options, no route, no nothing. And the client will accept the value, validate the small basics, and apply the hop limit to the interface

This then causes all outgoing traffic from the host to have a hop limit of 1, leading to an effective "Game Over".

By implementing the suggestions in RFC3756, this situation would have been avoidable. In fact, careful reading of RFC4861 suggests that a Router Advertisement without options, and with a lifetime != 0, can be considered an invalid RA packet, and should be discarded.