Network Working Group H. Rafiee
INTERNET-DRAFT Huawei Technologies Duesseldorf GmbH
Intended Status: Informational Track C. Meinel
Hasso Plattner Institute
Expires: February 11, 2015 August 11, 2014
Possible Attack on Cryptographically Generated Addresses (CGA)
<draft-rafiee-6man-cga-attack-02.txt>
Abstract
This document describes the possible attacks with the use of
Cryptographically Generated Addresses.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Sec value vulnerability . . . . . . . . . . . . . . . . . . . 3
3. Key size Vulnerability . . . . . . . . . . . . . . . . . . . 5
4. Modifier can be zero . . . . . . . . . . . . . . . . . . . . 5
5. Variable length Prefixes . . . . . . . . . . . . . . . . . . 6
6. Use case Scenario for CGA attack . . . . . . . . . . . . . . 6
6.1. Duplicate Address Detection Process . . . . . . . . . . . 6
6.2. Nodes communications . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Cryptographically Generated Addresses (CGA) [RFC3972] is one of the
important options of Secure Neighbor Discovery (SeND) [RFC3971] in
IPv6 networks. CGA provides the node with the proof of IP address
ownership by finding a binding between the public key and the node's
IP address. Therefore, It can protect the nodes from network layer IP
spoofing attack and prevent forging the identity (if it is only based
on the IP address). However, CGA, itself is vulnerable to some types
of attacks such as DoS, replay attack (The use of timestamp would
mitigate this attack), etc [3]. The goal of this document is not to
focus on the well-known attacks but the CGA vulnerabilities that is
the result of the text in CGA specification to warn implementers
about these possible attacks.
2. Sec value vulnerability
CGA values are the fingerprint of public key. They are generated by
executing a hash function on public key and some other parameters.
Since the default algorithm for generating this hash is SHA-1, the
attacker node only needs to do brute force attacks against 59 bits.
CGA algorithm uses sec value (a value between 0 to 7) to increase the
brute force search space from 59 bits to maximum 171 bits (59+sec*16)
and as a result complicates the brute force attacks to break CGA.
Nevertheless, in practice, only sec value 0 and 1 can be used because
it takes hours to years to generate CGA sec value higher than 1 [2].
Unfortunately, in practice, it does not matter what sec value the
victim node chooses and the use of sec value only complicates the IP
address generation process for the victim node. This is because the
attacker will only use sec value 0 and SHA1 algorithm.
It is true that the attacker has a source IP address that is only in
3 bits differs from the victim node's source IP address. But since
the node only checks the target address and start the verification
process on the source address, this really doesn?t bother the
attacker and the attack is successful. The attacker node can persist
on his own IP address after a successful verification by CGA node and
either force CGA node to generate a new IP address and again the
attacker repeats this process or there will be duplicate addresses on
the network which cause many services in the victim network stop
working. This is because all the nodes verify this attacker node the
same way as the legitimate CGA node processed the verification. From
their aspects, these two nodes are the same.
The mentioned flaw occurs during verification processes in all
verifier nodes. The node needs to verify other nodes in two different
conditions -- during DAD process and during checking the neighbors'
reachability in cache. This means that the CGA security is only the
security of CGA sec value 0 that is 2^59 bits.
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The reason are as follow:
1- Section 5 RFC 3972
It uses the term ?the address?, The address refers to the source
address. (please refer to Number 4 to know the reason)
2- step 4, Section 5 RFC 3972 :The CGA verifier node ignores 3 bits
sec value in source address and 2 bits u and g
Based on NDP specification, the verifier node checks to see whether
or not the target address is the same as its own IP address. If it is
the same and the node supports CGA, then it starts CGA verification.
Based on step 4 section 5 RFC 3972, the CGA node compares the source
address (IID section) of the sender node to his own IID. The verifier
node ignores 3 bits sec value. So, the attacker can set the target
address to the real CGA address of the victim node disregard its sec
value and set the source address to his own CGA value that is only
different in the 3 leftmost bits. Since the verification is
successful, the attacker can spoof the IP address of CGA node.
3 - No comparison of source address and target address
Based on the Neighbor Discovery Protocol (NDP) specification on
section 7 RFC 4861 [RFC4861, RFC4862], there is nothing about to
compare the source IP address with the target address. In SeND
specification [RFC3971], there are rules for the sender node.
However, the verifier node never checks those rules. This is why the
attacker can ignore them. So, the attacker can create the SeND
message by using his own CGA address that differs only in sec value.
The attacker selects the victim node's source address as his own
target address and sends this message.
4- Section 5.1.2 RFC 3971
"If the interface has been configured to use CGA, the receiving node
MUST verify the source address of the packet by using the algorithm
described in Section 5 of [11]. The inputs to the algorithm are the
claimed address, as defined in the previous section, and the CGA
Parameters field.".
This is where I explained in Number 1 that "the address" refers to
the source IP address.
5- Section 7.2.3 RFC 4861
SeND only adds 4 options to NDP but does not change the initial way
to process the NDP messages.
"- The Target Address is a ?valid? unicast or anycast address
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assigned to the receiving interface [ADDRCONF],
- The Target Address is a unicast or anycast address for which the
node is offering proxy service, or
- The Target Address is a ?tentative? address on which Duplicate
Address Detection is being performed [ADDRCONF]."
It is usually the new CGA victim node that needs to process Duplicate
Address Detection (DAD). In other words, the attacker has a valid
unicast target address that is similar to what is chosen by the
victim CGA node. There is no place in the standard NDP specifications
to explain that the target address should be compared to the source
address. The reason is because sometimes the target address is
temporary. Unfortunately in SeND specification, this check was not
done too because it only follows ND specification.
6- Section 7.2 RFC 3972: SeND cannot also protect the node against
this problem
The author of CGA specification was aware of this problem:
"For each valid CGA Parameters data structure, there are 4*(Sec+1)
different CGAs that match the value. This is because decrementing the
Sec value in the three leftmost bits of the interface identifier does
not invalidate the address, and the verifier ignores the values of
the ?u? and ?g? bits. In SEND, this does not have any security or
implementation implications."
But the assumption was that SeND can protect the nodes from this
attack by the use of a valid certification. But unfortunately,
certification is only to authorize routers and not for all nodes in
the network. The other solution is to use Microsoft Active Directory
(AD). But in practice not all places use or will use this approach.
3. Key size Vulnerability
The lower limit for key size is 384 bits. The attacker can choose the
lowest size public key so that he can better play with the public key
bits and easier and faster generates the similar hash of the CGA
node.
4. Modifier can be zero
The attacker does not need to generate a really good random value.
Since for him it is only important to match the hash value. This is
especially true for the scenario where the attacker needs to do brute
force attacks against all 64 bits and sec value is not ignored.
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5. Variable length Prefixes
The assumption in CGA algorithm is that the subnet prefix is 64 bits.
This really makes the verification process easier and straight
forward. But if any networks wants to have a variable length prefix,
then CGA verifier node must know which part of this address is IID
and which part is prefix. If it can receive this information from an
authorized router, then there might be no risk for the verifier node.
But if this value supposed to receive from the sender node, then the
problem would be where to add such information. If it is as a new
option in CGA, then the attacker can spoof this value and sign it
with its own private key and claim different prefix. But if this
value is as a part of IID, then the problem would be the number of
bits required to carry the prefix length. This process will decrease
the number of bits to carry the CGA value and will lead to reducing
CGA security. (59- number of bits to carry the prefix length)
6. Use case Scenario for CGA attack
In the following subsections, some of these attacks are explained in
more detail.
6.1. Duplicate Address Detection Process
When a node generates his IP address, it process the DAD in order to
avoid collision on the network. The attacker might be able to
generate the CGA value the same of the legitimate CGA node and claim
the ownership of that IP address. The CGA nodes only tries 3 times
and then it gives up.
6.2. Nodes communications
When two nodes want to start communication, they try to find the IP
address of eachother by sending multicast NS/NA messages. The
attacker can offline generates the value of victim which differs only
in 3 bits sec value and then impersonate the victim node and try to
communicate with other node. This attack is likely possible
especially when the attacker can send this response faster than the
real node or the real node is offline at the time of this request by
other node.
7. Security Considerations
-
8. IANA Considerations
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-
9. Appendix
- CGA multicore attack
This is where you can find CGA attacks (multicore).
http://www.hpi.uni-potsdam.de/meinel/security_tech/ipv6_security/ipv6ssl.html
10. Acknowledgements
The author would like to acknowledge Fabian Braeunlein, one of a
bachelor student at Hasso Plattner Institute who assists us, during
this busy moments, for writing the attacking codes.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3972] Aura, T., "Cryptographically Generated Addresses
(CGA)," RFC 3972, March 2005.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., Soliman,
H., "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., Jinmei, T., "IPv6
Stateless Address Autoconfiguration", RFC 4862, September
2007.
[1] AlSa'deh, A., Rafiee, H., Meinel, C., "Cryptographically
Generated Addresses (CGAs): Possible Attacks and Proposed
Mitigation Approaches," in proceedings of 12th IEEE International
Conference on Computer and Information Technology (IEEE CIT'12),
pp.332-339, 2012.
[2] Bos, J., Oezen, O., Hubaux, J., "Analysis and Optimization of
Cryptographically Generated Addresses", In Proceedings of the
12th International Conference on Information Security (2009),
ACM, pp. 17 ? 32.
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[ugbits] Carpenter, B., Jiang, S., "Significance of IPv6
Interface Identifiers", RFC 7136, February 2014.
[variableprefix] Carpenter, B., Chown, T, Gont, F.,
Jiang, S., Petrescu, A., Yourtchenko, A.," Analysis
of the 64-bit Boundary in IPv6 Addressing",
http://tools.ietf.org/html/draft-ietf-6man-why64 ,
April 2014
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Authors' Addresses
Hosnieh Rafiee
HUAWEI TECHNOLOGIES Duesseldorf GmbH
Riesstrasse 25, 80992,
Munich, Germany
Phone: +49 (0)162 204 74 58
Email: hosnieh.rafiee@huawei.com
Christoph Meinel
Hasso-Plattner-Institute
Prof.-Dr.-Helmert-Str. 2-3
Potsdam, Germany
Email: meinel@hpi.uni-potsdam.de
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