PANA Working Group
Internet Draft <M. Parthasarathy>
Category: Informational <Tahoe Networks>
Document: draft-ietf-pana-threats-eval-03.txt April 2003
Expires: September 2003
PANA Threat Analysis and security requirements
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The PANA (Protocol for carrying authentication for Network Access)
working group is developing methods for authenticating clients to the
access network using IP based protocols. This document discusses the
threats in general without referring to a specific authentication
protocol. The security requirements arising out of these threats will
be used as additional input to the PANA WG for designing the IP based
network access protocol.
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Table of Contents
1.0 Introduction..................................................2
2.0 Keywords......................................................2
3.0 Terminology and Definitions.................................3
4.0 Usage Scenarios.............................................4
5.0 Trust Relationships.........................................4
6.0 Threat Scenarios............................................5
6.1 PAA Discovery..............................................5
6.2 Authentication.............................................6
6.3 PaC leaving the network....................................9
6.4 Service theft..............................................9
6.5 Miscellaneous attacks.....................................10
7.0 Summary of Requirements....................................11
8.0 Security Considerations....................................12
9.0 Normative References.......................................12
10.0 Informative References....................................12
12.0 Acknowledgments.............................................13
13.0 Revision Log................................................13
14.0 Author's Addresses..........................................14
15.0 Full Copyright Statement....................................14
1.0 Introduction
The PANA (Protocol for carrying authentication for Network Access)
working group is developing methods for authenticating clients to the
access network using IP based protocols. This document discusses the
threats in general without referring to a specific authentication
protocol.
A client wishing to get access to the network must carry on multiple
steps. First, it needs to discover the IP address of the PANA
authentication agent (PAA) and then execute an authentication
protocol to authenticate itself to the network. Once the client is
authenticated, there might be other messages exchanged during the
lifetime of the network access. This document discusses the threats
in these steps without discussing any solutions. The requirements
arising out of these threats will be used as input to the PANA
working group.
2.0 Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
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3.0 Terminology and Definitions
Device
A network element (notebook computer, PDA, etc.) that requires
access to a provider's network.
Network Access Server (NAS)
Network device that provides access to the network.
PANA Client (PaC)
An entity in the edge subnet who is wishing to obtain network
access from a PANA authentication agent within a network. A PANA
client is associated with a device and a set of credentials to
prove its identity within the scope of PANA.
PANA Authentication Agent (PAA)
An entity whose responsibility is to authenticate the PANA client
(PaC) and grant network access service to the device.
Authentication Server (AS)
An entity that authenticates the PaC. It may be co-located with
PAA or part of the back-end infrastructure.
Device Identifier (DI)
The identifier used by the network as a handle to control and
police the network access of a client. Depending on the access
technology, identifier might contain any of IP address, link-layer
address, switch port number, etc. of a device. PANA authentication
agent keeps a table for binding device identifiers to the PANA
clients. At most one PANA client should be associated with a DI on
a PANA authentication agent.
Enforcement Point (EP)
A node that is capable of filtering packets sent by the PaC using
the DI information authorized by PAA.
Compound methods
Authentication protocol where, sequence of methods are used one
after an other or where methods are tunneled inside an
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independently established tunnel between the client and server
[TUN-EAP].
4.0 Usage Scenarios
PANA is expected to be used in environments where the nodes trust the
operator of the network to provide the service but do not trust the
other nodes in the network e.g., Public access networks, Hotel,
Airport. Note that the usage of word trust above does not mean trust
relationship. It just denotes the belief about how the nodes involved
in PANA behave in the future. In these environments, one may observe
the following.
o The link between PaC and PAA may be a shared medium e.g.
Ethernet or may not be a shared medium e.g. DSL network.
o All the PaCs may be authenticated to the access network at
layer 2 (3GPP2 CDMA network) and share a security association
with layer 2 authentication agent e.g., 802.11 Access point,
but still do not trust each other.
The scenarios mentioned above affect the threat model of PANA. This
document discusses the various threats in the context of the above
network access scenarios for a better understanding of the threats.
5.0 Trust Relationships
PANA authentication involves a client, PANA agent (PAA),
Authentication server (AS) and an Enforcement point (EP). The
communication paths involved between the various entities are as
follows.
1) The path between PaC and PAA
2) The path between PaC and EP
3) The path between PAA and EP
4) The path between PAA and AS
This document discusses the threats involved in path (1) and (2).
If PAA and EP are co-located, the path between PAA and EP (3) can be
considered secure. Even when they are not co-located, the network
operators can setup a security association between PAA and EP to
secure the traffic between them. Hence it is assumed that path (3) is
secure and not discussed further in the document.
The authentication server (AS) could be co-located in the same
network as PAA or with the back-end system. If it is co-located, then
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the path (4) can be considered secure. If it is not co-located, there
are various threats possible in this path. [RAD-EAP] discusses the
possible threats in this path. This implies that if RADIUS is used as
the protocol between PAA and AS, then all the vulnerabilities that
are mentioned in [RAD-EAP] are applicable to PANA. As it is beyond
the scope of PANA to address these threats, this document does not
discuss this further.
There is no pre-existing trust relationship between PaC and EP. When
PaC is successfully authenticated, it will further enable key
derivation between PaC and EP, which can be used to secure the link.
For example, EAP keying framework [EAP-KEY], defines a three party
EAP exchange where the clients derive the transient sessions keys to
secure the link between PaC and NAS in their final step. Similarly,
PANA will provide the ability to derive keys between PaC and EP that
can be used to secure the link further. This is further discussed in
section 6.4 below.
6.0 Threat Scenarios
The PANA authentication client (PaC) needs to discover the PAA first.
This involves either sending solicitations or waiting for
advertisements. Once it has discovered the PAA, it will lead to
authentication exchange with PAA. Once the access is granted, PaC
will most likely exchange data with other nodes in the Internet. All
of these are vulnerable to denial of service (DoS), man-in-the-middle
(MITM) and service theft attacks.
The threats are grouped by the various stages the client goes through
to gain access to the network. Section 6.1 discusses the threats
related to PAA discovery. Section 6.2 discusses about the
authentication itself. Section 6.3 discusses about the threats
involved while leaving the network. Section 6.4 discusses about
service theft. Section 6.5 discusses the miscellaneous threats.
6.1 PAA Discovery
PaC is in the process of discovering the PAA. The agents like PAA are
discovered by sending solicitations or receiving advertisements.
Following are the possible threats.
T6.1.1: A malicious node can pretend to be a PAA by sending a spoofed
advertisement.
In existing dial-up networks, the clients authenticate to the network
but generally do not verify the authenticity of the messages coming
from Network Access Server (NAS). This mostly works because the link
between the device and the NAS is not shared with other nodes
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(assuming that nobody tampers with the physical link), and clients
trust the NAS and the phone network to provide the service, without
which the network operator will not make any profit. Since in this
environment, nodes in the network cannot directly communicate with
each other and may assume that the other end of the point-to-point
link is the PAA, spoofing attacks are not present.
In environments where the link is shared, any node can pretend to be
a PAA (including the nodes that are authenticated at layer 2). Hence,
the threat is still present in such networks. It is difficult to
protect the discovery process, as there is no a priori trust
relationship between PAA and PaC. It is also possible that EP might
be able to filter out the packets coming from PaC that resembles PAA
packets.
The advertisement may be used to include other information like
supported authentication methods etc., besides the discovery of the
PAA itself. This can lead to a bidding down attack, as a malicious
node can send a spoofed advertisement with capabilities that indicate
less secure authentication methods than what the real PAA supports,
thereby fooling the PaC into negotiating a less secure authentication
method than what would otherwise be available. This is best avoided
by limiting the amount of information sent during the PAA discovery
process.
Requirement 1
PANA MUST not assume that the discovery process is protected. Since,
it is difficult to protect the discovery process, the information
exchanged during the discovery process SHOULD be limited.
6.2 Authentication
This section discusses the threats specific to the authentication
protocol. Section 6.2.1 discusses the possible threat associated with
success/failure indications that are transmitted to PaC at the end of
the authentication. Section 6.2.2 discusses the man-in-the-middle
attack when compound methods are used. Section 6.2.3 discusses the
replay attack and section 6.2.4 discusses about the device identifier
attack.
6.2.1 Success or Failure Indications
An attacker can send false authentication success or failure to the
PaC. By sending false failure, the attacker can prevent the client
from accessing the network. By sending false success, the attacker
can prematurely end the authentication exchange effectively denying
service for the PaC.
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If the link is not shared, it may be hard to launch this attack as
the attacker needs to inject this packet at the right time and the
PaC can always reject packets coming from any source address other
than the PAA.
If the link is shared, it is easy to spoof these packets. If layer 2
provides per-packet encryption with pair-wise keys, it might make it
hard for the attacker to guess the success/failure packet that the
client would accept. Even if the node is already authenticated at
layer 2, it can still pretend to be a PAA and spoof the success or
failure.
This attack is possible because the success or failure indication is
not protected using a security association between PaC and PAA. In
order to avoid this attack, PaC and PAA should mutually authenticate
each other. In the process of mutually authenticating each other,
they should be able to derive keys to protect the success/failure
indications.
Requirement 2
PaC and PAA MUST mutually authenticate to each other using methods
that can derive keys, which in turn can protect the success and
failure indications.
6.2.2 MITM attack
A malicious node can claim to be PAA to the real PaC and claim to be
PaC to the real PAA. This is a MITM attack where the PaC is fooled to
think that it is communicating with real PAA and the real PAA is
fooled to think that it is communicating with real PaC.
An instance of MITM attack, when compound authentication methods are
used is described in [TUN-EAP]. In these attacks, the server first
authenticates to the client. As the client has not proven its
identity yet, it acts as the man-in-the-middle, tunneling the
identity of the legitimate client to gain access to the network. The
attack is possible because there is no verification that the same
entities participated among the compound methods. It is not possible
to do such verification, if compound methods are used without being
able to create cryptographic binding among them. This implies that
PANA will be vulnerable to such attacks if compound methods are used
without being able to cryptographically bind them.
Requirement 3
When compound authentication methods are used in PANA, the methods
MUST be cryptographically bound.
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6.2.3 Replay Attack
A malicious node can replay the messages that caused authentication
failure or success at a later time to create false failures or
success. The attacker can also potentially replay other messages of
the PANA protocol to deny service to the PaC.
This threat is absent if the link is not a shared medium. If the link
is shared, then the attacker can replay old messages to deny service
to the client.
If the packets are encrypted at layer 2 using pair-wise keys, it will
make it hard for the attacker to learn the unencrypted (i.e.,
original) packet that needs to be replayed. Even if layer 2 provides
replay protection, the attacker can still replay the PANA messages
(layer 3) for denying service to the client.
Requirement 4
PANA MUST be resistant to replay attacks.
6.2.4 Device Identifier attack
When the client is successfully authenticated, PAA sends access
control information to EP for granting access to the network. The
access control information typically contains the device identifier
of the PaC, which is obtained from the IP headers and MAC headers of
the packets exchanged during the authentication process. The attacker
can gain unauthorized access into the network using the following
steps.
. An attacker pretends to be a PAA and sends advertisements. PaC
gets fooled and starts exchanging packets with the attacker.
. The attacker modifies the IP source address on the packet,
adjusts the UDP/TCP checksum and forwards the packet to the real
PAA. It does the same on return packets also.
. When the real PaC is successfully authenticated, the attacker
gains access to the network as the packets contained the IP
address (and potentially the MAC address also) of the attacker.
This threat is absent if the link is not a shared medium. If the
layer 2 provides per-packet protection, then it is not possible to
change the MAC address and hence this threat may be absent in such
cases if EP filters both on IP and MAC address. If the link is
shared, it is easy to launch this attack.
Requirement 5
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PANA MUST be able to protect the device identifier transmitted from
the PaC to PAA.
6.3 PaC leaving the network
When the PaC leaves the network, it needs to inform the PAA before
disconnecting from the network so that the resources used by PaC can
be accounted properly. PAA may also choose to revoke the access any
time if it deems necessary. Following are the possible threats.
T6.3.1: A malicious node can pretend to be a PAA and revoke the
access to PaC.
T6.3.2: A malicious node can pretend to be a real PaC and transmit a
disconnect message.
This threat is absent if the link between PaC and PAA is not a shared
medium.
If the link is shared, any node on the link can spoof the disconnect
message. Even if the layer 2 has per-packet authentication, the
attacker can pretend to be a PaC e.g. by spoofing the IP address, and
disconnect from the network. Similarly, any node can pretend to be a
PAA and revoke the access to the PaC.
In some link layers, e.g., 802.11, disassociate and de-authenticate
messages are not protected (even with 802.11i). In such link layers,
protecting PANA messages may not be very useful as the attacker can
attack using the link layer mechanisms rather than PANA.
Requirement 6
PANA MUST be able to protect disconnect and revocation messages.
6.4 Service theft
An attacker can gain unauthorized access into the network by stealing
the service from another client. Once the PaC is successfully
authenticated, EP will have filters in place to prevent unauthorized
access into the network. The filters will be based on something that
will be carried on every packet. For example, the filter could be
based on IP and MAC address where the packets will be dropped unless
the packets coming with certain IP address match the MAC address
also. Following are the possible threats.
T6.4.1: Attacker can spoof both the IP and MAC address of an
authorized client to gain unauthorized access. Attacker can launch
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this attack easily by just sniffing the wire for IP and MAC address.
This lets the attacker use the network without any authorization thus
getting a free service.
These threats are absent in links that are not shared as simple
ingress filtering can prevent one node from impersonating as another
node.
If the link between PaC and PAA is shared, it is easy to launch this
attack. If layer 2 provides per-packet protection using pair-wise
keys, it can prevent the attacker from gaining unauthorized access
using the layer 2 identifier of some other node.
PANA MUST be able to prevent service theft. In some cases e.g. non-
shared links, it is sufficient to provide access control information
like IP address, MAC address, etc., to EP, which in turn can prevent
unauthorized users from gaining access to the network by policing the
packets for matching addresses. In the case of shared links, this
information is not sufficient to prevent service theft. PANA MUST be
able to bootstrap a shared secret between the PaC and PAA which can
be further used to setup a security association (e.g., IPsec) between
PaC and EP to prevent service theft on shared links.
Requirement 7
PANA MUST be able to generate sufficient access control information
like IP address, MAC address etc. for EP to prevent service theft.
PANA MUST be able to bootstrap a shared secret between the PaC and
PAA which can be further used to setup a security association (e.g.,
IPsec) between PaC and EP to prevent service theft on shared links.
6.5 Miscellaneous attacks
T6.5.1: There are various forms of DoS attacks that can be launched
on the PAA or AS. A few are mentioned below. AS it is hard to protect
against, there is no specific requirement on DoS attack. Protocol
should be designed carefully to mitigate or prevent such attacks.
. Attacker can bombard the PAA with lots of authentication
requests. If PAA and AS are not collocated, PAA may have to
allocate resources to store some state about PaC locally before
it receives the response from the backend AS. This can deplete
memory resources on PAA.
. The attacker can force the PAA or AS to make a public key
computation with minimal effort, that can deplete the CPU
resources of the PAA or AS.
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T6.5.2: PaC acquires IP address before PANA authentication begins
using methods like e.g., DHCP in IPv4 and auto-configuration in IPv6
[PANAREQ]. If IP addresses are assigned before authentication, it
opens up the possibility of DoS attack where malicious nodes can
deplete the IP addresses by assigning multiple IP addresses. If
stateless auto-configuration [ADDRCONF] is used, the attacker can
respond to duplicate address detection probes sent by any node on the
network effectively not allowing the node to configure a link local
address. If stateful mechanism is used in IPv6 e.g., DHCPv6, then
this attack is still possible. Address depletion attack is not
specific to PANA, but a known attack in DHCP [DHCP-AUTH]. If PANA
assumes that the client has an IP address already, it opens up the
network to the DoS attack.
Requirement 8
PANA should not assume that the PaC has acquired an IP address before
PANA begins.
7.0 Summary of Requirements
1. PANA MUST not assume that the discovery process is protected.
Since, it is difficult to protect the discovery process, the
information exchanged during the discovery process SHOULD be
limited.
2. PaC and PAA MUST mutually authenticate to each other using
methods that can derive keys, which in turn can protect the
success and failure indications.
3. When compound authentication methods are used in PANA, the
methods MUST be cryptographically bound.
4. PANA MUST be resistant to replay attacks.
5. PANA MUST be able to protect the device identifier
transmitted from the PaC to PAA.
6. PANA MUST be able to protect disconnect and revocation
messages.
7. PANA MUST be able to generate sufficient access control
information like IP address, MAC address etc. for EP to
prevent service theft. PANA MUST be able to bootstrap a
shared secret between the PaC and PAA which can be further
used to setup a security association (e.g., IPsec) between
PaC and EP to prevent service theft on shared links.
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8. PANA should not assume that the PaC has acquired an IP
address before PANA begins.
8.0 Security Considerations
This document discusses various threats with IP based network access
protocol.
9.0 Normative References
1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2. [PANAUS] Yoshihiro Ohba et. al, "Problem Space and Usage Scenarios
for PANA", draft-ietf-pana-usage-scenarios-03.txt
3. [PANAREQ] A. Yegin et al., "Protocol for Carrying Authentication
for Network Access (PANA) Requirements and Terminology", draft-
ietf-pana-requirements-04.txt
4. [KEYWORDS] S. Bradner, "Key words for use in RFCS to indicate
requirement levels", RFC 2119, March 1997
10.0 Informative References
5. Bernard Aboba, "Pros and Cons of Upper Layer Network Access",
BURP BOF.
6. Arunesh Mishra, William A. Arbaugh, "An Initial Security Analysis
of the IEEE 802.1X Standard"
7. IEEE. Standard for port based network access control. IEEE Draft
P802.1X
8. [RADIUS] C. Rigney et.al, "Remote Authentication Dial In User
Service".
9. [EAP-KEY] Bernard Aboba et. al, "EAP keying framework", Work in
Progress.
10. [ADDRCONF] Susan Thomson et. al "IPv6 Stateless Address
Autoconfiguration", RFC2462.
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11. [DHCP-AUTH] R. Droms, et. al "Authentication for DHCP messages",
RFC3118.
12. [RAD-EAP] Bernard Aboba, et. al "Radius support for Extensible
authentication protocol", draft-aboba-radius-rfc2869bis-08.txt
13. [TUN-EAP] J. Puthenkulam et. al "The compound authentication
binding problem", draft-puthenkulam-eap-binding-01.txt
12.0 Acknowledgments
The author would like to thank the following people (in no specific
order) for providing comments: Alper Yegin, Basavaraj Patil, Pekka
Nikkander, Bernard Aboba, Francis Dupont, Michael Thomas, Yoshihiro
Ohba, Gabriel Montenegro, Tschofenig Hannes and Bill Sommerfeld.
13.0 Revision Log
Changes between 02 and 03
- Changed Requirement 1 to include text about weak
authentication suites.
- Clarified text in a few places.
- Rearranged the order of definitions in terminology
section.
- Removed some confusing text with respect to IPsec from
the Service theft section.
Changes between revision 01 and 02
- Renamed the section "Assumptions" to "Trust relationships"
and added more text to clarify the relationship between PaC
and EP.
- Added more text for threats in PAA AS path
- Merged the "Type of Attacks" section into "Threat Scenarios"
- Removed the requirement on DoS attack
- Reworded most of the requirements
Changes between revision 00 and 01.
- Removed unused terms from section 3.0
- Removed identity protection as a threat after feedback from
Atlanta IETF55 meeting.
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- Renamed the section "Attacks on Normal Data communication"
to "Service theft". Removed confidentiality as a requirement
from that section.
- Added a new threat "Device Identifier attack".
14.0 Author's Addresses
Mohan Parthasarathy
Tahoe Networks
3052 Orchard Drive
San Jose, CA 95134
Phone: 408-944-8220
Email: mohanp@tahoenetworks.com
15.0 Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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