Wednesday, 21 December 2011

DEEPSEC 2011 edition of our "Hacking IPv6 Networks" training

Part of the materials used for the DEEPSEC 2011 edition of our "Hacking IPv6 Networks" training are now available on the SI6 Networks web site.

The materials can be accessed here. Please stay tuned as new editions of our trainings are scheduled.

Saturday, 17 December 2011

A Proposal for Generating Stable Privacy-Enhanced Addresses in IPv6

IPv6 hosts typically configure their IPv6 addresses by means of a mechanism known as StateLess Address AutoConfiguration (SLAAC). In SLAAC, a local router announces an IPv6 prefix to be used for address configuration, and the local hosts generate their IPv6 addresses by concatenating an Interface ID to the announced IPv6 prefix. For network technologies such as Ethernet, the Interface ID basically consists of the MAC address of the corresponding network interface card.

The aforementioned procedure typically results in stable addresses (i.e., the each interface card always gets the same IPv6 address). Such "stable" addresses are generally considered to simplify network management, since they simplify ACLs and logging. However, since IEEE identifiers are typically globally unique, the resulting IPv6 addresses can be leveraged to track and correlate the activity of a node, thus negatively affecting the privacy of users.
When a host moves from one network to another, the IPv6 prefix will change, but the Interface Identifier will be constant (since the MAC address does not change). As a result, it becomes trivial to track and correlate the activities of a node.
The "Privacy Extensions for Stateless Address Autoconfiguration in IPv6" (specified in RFC 4941) were introduced to mitigate the aforementioned problem, and basically result in temporary (and random) Interface Identifiers that are typically more difficult to leverage than those based on IEEE identifiers (e.g. Ethernet addresses). Such temporary addresses are generated in addition to the traditional autoconfiguration addresses (i.e., the stable addresses typically constructed with MAC addresses): the temporary addresses are employed for "outgoing" communications, while the stable addresses are used for performing "server" functions (i.e., receiving incoming connections).

Temporary addresses can be challenging in a number of areas.  For example, from a network-management point of view, they tend to increase the complexity of enforcing access controls and event logging.  As a result, some organizations disable the use of privacy addresses even at the expense of reduced privacy.
On the other hand, even when privacy addresses are enabled, the "stable" addresses are still used for performing "server" functions, and hence can still be leveraged to affect the privacy of users (albeit with increased difficulty), and can still be leveraged for the purpose of host-scanning.
IPv6 addresses based on IEEE identifiers (e.g. Ethernet addresses) can be easily predictable, particularly if all network interface cards in a subnet correspond to the same manufacturer: as soon as an attacker known one IPv6 address it can find the rest of the addresses by trying all possible combinations in the last three bytes of the IPv6 address.
Fernando Gont has published an IETF Internet-Draft entitled "A method for Generating Stable Privacy-Enhanced Addresses with IPv6 Stateless Address Autoconfiguration (SLAAC)", which proposes an algorithm for generating addresses that are stable within each subnet, but that result in different (and unpredictable) Interface Identifiers as hosts move from one network to another.  The aforementioned method is meant to be an alternative to generating Interface Identifiers based on IEEE identifiers, such that the same manageability benefits can be achieved without sacrificing the privacy of users.

The proposal is being discussed at the 6man working group of the IETF. Feedback from the community will be appreciated.

Thursday, 1 September 2011

Router Advertisement Guard (RA-Guard) Evasion


IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique for attack vectors based on ICMPv6 Router Advertisement messages. RFC6104 describes the problem statement of “Rogue IPv6 Router Advertisements”, and RFC6105 specifies the “IPv6 Router Advertisement Guard” functionality.
Surprisingly enough, RFC6104 (the problem statement document) focuses on mis-configured routers, while RFC6105 describes RA-Guard as a security mechanism!
The basic concept behind RA-Guard is that a layer-2 device filters ICMPv6 Router Advertisement messages, according to a number of different criteria. The most basic filtering criteria is that Router Advertisement messages are discarded by the layer-2 device unless they are received on a specified port of the layer-2 device. Clearly, the effectiveness of the RA-Guard mitigation relies on the ability of the layer-2 device of identifying ICMPv6 Router Advertisement messages.

However, trying to filter layer-3 packets at layer-2 can be tricky.

Router Advertisement Guard (RA Guard) Evasion Vulnerability 

While there is currently no legitimate for IPv6 Extension Headers in ICMPv6 Router Advertisement messages, implementations allow the use of Extension Headers included in these messages, by simple ignoring the received options. Some implementations of RA-Guard (notably that of Cisco Systems) try to identify ICMPv6 Router Advertisement messages by looking at the “Next Header” field of the fixed IPv6 header, rather than following the entire header chain. As a result, these implementations fail to identify any ICMPv6 Router Advertisement messages that include any Extension Headers (for example, Hop by Hop Options header, Destination Options Header, etc.).

The following figure illustrates the structure of ICMPv6 Router Advertisement messages that implements this RA-Guard evasion technique:

While this attack vector is effective with RA-Guard implementations that fail to process the entire header chain (e.g., Cisco's), it could be easily mitigated by simply having the RA-Guard implementation process the entire header chain.

However, a more sophisticated attack vector can be employed by leveraging the IPv6 fragmentation (i.e., the IPv6 Fragment Header).

The basic idea is that if the forged ICMPv6 Router Advertisement is fragmented into at least two fragments, the layer-2 device implementing “RA-Guard” would be unable to identify the attack packet, and would those would fail do discard it.

A simple implementation of this attack vector would be an original ICMPv6 Router Advertisement message preceded with a Destination Options Header, that results in two fragments. The following figure illustrates the “original” attack packet, prior to fragmentation, and the two resulting fragments which are actually sent as part of the attack.

It should be noted that the length of the he Destination Options Header (its "Hdr Ext Len" field) is present only in the first fragment (but not in the second).  Therefore, it would be impossible for a device processing only the second fragment to locate the ICMPv6 header contained in that fragment, since it is unknown how many bytes should be “skipped” to get to the next header following the Destination Options Header.

Thus, by leveraging the use of the Fragment Header together with the use of the Destination Options header, an attacker could conceal the type and contents of the ICMPv6 message he is sending (an ICMPv6 Router Advertisement in this example).  
A layer-2 device could, however, at least detect that that an ICMPv6 message (or some type) is being sent, since the “Next Header” field of the Destination Options header contained in the first fragment is set to “58” (ICMPv6).
It is be possible for an attacker to take this idea further, such that it is impossible for the RA-Guard implementation to even detect that the attacker is sending an ICMPv6 message in the first place. This can be achieved with an original ICMPv6 Router Advertisement message preceded with two Destination Options Headers, that results in two fragments. The following figure illustrates the “original” attack packet, prior to fragmentation, and the two resulting packets which are actually sent as part of the attack.

In this variant, the “Next Header” field of the Destination Options header contained in the first fragment is set to “60” (Destination Options header), and thus it is impossible for a device processing only the first fragment to detect that an ICMPv6 message is being sent in the first place.

The second fragment presents the same challenges as the second fragment of the previous variant.  That is, it would be impossible for a device processing only the second fragment to locate the second Destination Options header (and hence the ICMPv6 header), since the length of the first Destination Options header (i.e., its “Hdr Ext Len” field) is present in the first fragment (rather than the second).


Fernando Gont has published (on behalf of CPNI) two IETF Internet-Drafts, that propose mitigations for these RA-Guard evasion vulnerabilities. One of the internet-drafts proposes to modify the protocol specifications such that use of IPv6 Extension Headers with Neighbor Discovery messages is forbidden (when SEND is not employed). The other internet-draft proposes an improved filtering policy that could be readily employed by network administrators and operators to mitigate these RA-Guard evasion techniques.

These two internet-drafts have been presented at the 6man meeting and the v6ops meeting that took place at IETF 81, and are currently being discussed by the 6man and the v6ops working groups of the IETF.

Some Conclusions 

In IPv4,  options are limited to 40 bytes (resulting in a maximum IPv4 header of 60 bytes). On the other hand, IPv6 can easily employ 64K of options. This is just a simple example of how a protocol feature can be a two-sided knife: the larger option space available in IPv6 makes in much more difficult to enforce layer-2 ACLs.