One of the recurring myths of IPv6 is its very large address space somehow confers a higher degree of security. The theory goes something like this: there is so much more of the IPv6 address space to test in order to find out what is connected to the network, it would take too long to scan the entire space looking for devices. The first problem with this myth is it simply is not true—it is quite possible to scan the entire IPv6 address space rather quickly, probing enough addresses to perform a tree-based search to find attached devices. The second problem is this assumes the only modes of attack available in IPv4 will directly carry across to IPv6. But every protocol has its own set of tradeoffs, and therefore its own set of attack surfaces.
Assume, for instance, you follow the “quick and easy” way of configuring IPv6 addresses on devices as they are deployed in your network. The usual process for building an IPv6 address for an interface is to take the prefix, learned from the advertisement of a locally attached router, and the MAC address of one of the locally attached interfaces, combining them into an IPv6 address (SLAAC). The size of the IPv6 address space proves very convenient here, as it allows the MAC address, which is presumably unique, to be used in building a (presumably unique) IPv6 address.
According to RFC7721, this process opens several new attack surfaces that did not exist in IPv4, primarily because the device has exposed more information about itself through the IPv6 address. First, the IPv6 address now contains at least some part of the OUI for the device. This OUI can be directly converted to a device manufacturer using web pages such as this one. In fact, in many situations you can determine where and when a device was manufactured, and often what class of device it is. This kind of information gives attackers an “inside track” on determining what kinds of attacks might be successful against the device.
Second, if the IPv6 address is calculated based on a local MAC address, the host bits of the IPv6 address of a host will remain the same regardless of where it is connected to the network. For instance, I may normally connect my laptop to a port in a desk in the Raleigh area. When I visit Sunnyvale, however, I will likely connect my laptop to a port in a conference room there. If I connect to the same web site from both locations, the site can infer I am using the same laptop from the host bits of the IPv6 address. Across time, an attacker can track my activities regardless of where I am physically located, allowing them to correlate my activities. Using the common lower bits, an attacker can also infer my location at any point in time.
Third, knowing what network adapters an organization is likely to use reduces the amount of raw address space that must be scanned to find active devices. If you know an organization uses Juniper routers, and you are trying to find all their routers in a data center or IX fabric, you don’t really need to scan the entire IPv6 address space. All you need to do is probe those addresses which would be formed using SLAAC with OUI’s formed from Juniper MAC addresses.
Beyond RFC7721, many devices also return their MAC address when responding to ICMPv6 probes in the time exceeded response. This directly exposes information about the host, so the attacker does not need to infer information from SLAAC-derived MAC addresses.
What can be done about these sorts of attacks?
The primary solution is to use semantically opaque identifiers when building IPv6 addresses using SLAAC—perhaps even using a cryptographic hash to create the base identifiers from which IPv6 addresses are created. The bottom line is, though, that you should examine the vendor documentation for each kind of system you deploy—especially infrastructure devices—as well as using packet capture tools to understand what kinds of information your IPv6 addresses may be leaking and how to prevent it.