wdiff rfc7430.alt-original rfc7430.txt

Network Working Group
Internet Engineering Task Force (IETF) M. Bagnulo
Internet-Draft
Request for Comments: 7430 UC3M
Intended status:
Category: Informational C. Paasch
Expires: September 28, 2015
ISSN: 2070-1721 UCLouvain
F. Gont
SI6 Networks / UTN-FRH
O. Bonaventure
UCLouvain
C. Raiciu
UPB
March 27,
July 2015

Analysis of MPTCP residual threats Residual Threats and possible fixes
draft-ietf-mptcp-attacks-04 Possible Fixes for Multipath TCP
(MPTCP)

Abstract

This documents performs an analysis of document analyzes the residual threats for MPTCP Multipath TCP (MPTCP)
and explores possible solutions to address them.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a maximum candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 28, 2015.
http://www.rfc-editor.org/info/rfc7430.

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Provisions Relating to IETF Documents
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. ADD_ADDR attack Attack . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack . . 10
3. DoS attack Attack on MP_JOIN . . . . . . . . . . . . . . . . . . . . 10 11
3.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack . . 11 12
4. SYN flooding amplification Flooding Amplification . . . . . . . . . . . . . . . . . 11 12
4.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack . . 12
5. Eavesdropper in the initial handshake Initial Handshake . . . . . . . . . . . . 12 13
5.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack . . 13
6. SYN/JOIN attack Attack . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack . . 14
7. Reccomendations Recommendations . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. MPTCP Security enhancements Improvements for MPTCP a Standards Track
Specification . . . . . . . . . . . . . . 15
8. Security considerations . . . . . . . . 14
7.2. Security Enhancements for MPTCP . . . . . . . . . . . . . 15
9. IANA
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. References . . 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
11.
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . 15
11.1. Normative References . . . . . . . . . . . 16
Acknowledgements . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 18

1. Introduction

This document provides a complement to the threat analysis for
Multipath TCP (MPTCP) [RFC6824] documented in RFC 6181 [RFC6181].
RFC 6181 provided a threat analysis for the general solution space of
extending TCP to operate with multiple IP addresses per connection.
Its main goal was to leverage previous experience acquired during the
design of other multi-address protocols, notably SHIM6 Shim6 [RFC5533],
SCTP [RFC4960] the
Stream Control Transmission Protocol (SCTP) [RFC4960], and MIPv6 Mobile
IPv6 (MIP6) [RFC6275] for designing MPTCP. Thus, RFC 6181 was
produced before the actual MPTCP specification (RFC6824) (RFC 6824) was
completed,
completed and documented a set of reccommendations recommendations that were
considered during the production of such that specification.

This document complements RFC 6181 with a vulnerability analysis of
the specific mechanisms specified in RFC 6824. The motivation for this
analysis is to identify possible security issues with MPTCP as
currently specified and propose security enhancements to address the
these identified security issues.

The goal of the security mechanisms defined in RFC 6824 were was to make
MPTCP no worse than currently available single-path TCP. We believe
that this goal is still valid, so we will perform our analysis on the
same grounds. This document describes all the threats identified
that are specific to MPTCP (as defined in RFC6824) RFC 6824) that are not
possible with (single-path) single-path TCP. This means that threats that are
common to TCP and MPTCP are not covered in this document.

Types of attackers: for all attacks considered

For each attack considered in this document, we identify the type of
attacker. We can classify the attackers based on their location as
follows:

o Off-path attacker. This is an attacker that does not need to be
located in any of the paths of the MPTCP session at any point in
time during the lifetime of the MPTCP session. This means that
the Off-path off-path attacker cannot eavesdrop any of the packets of the
MPTCP session.

o Partial time On-path Partial-time on-path attacker. This is an attacker that needs to
be in at least one of the paths during part but not during of the
entire lifetime of the
MPTCP session. session (but not the entire lifetime). The attacker can be
in the forward and/or backward directions, directions for the initial subflow
and/or other subflows. The specific needs of the attacker will be
made explicit in the attack description.

o On-path attacker. This attacker needs to be on at least one of
the paths during the whole duration of the MPTCP session. The
attacker can be in the forward and/or backward directions, directions for the
initial subflow and/or other subflows. The specific needs of the
attacker will be made explicit in the attack description.

We can also classify the attackers based on their actions as follows:

o Eavesdropper. The attacker is able to capture some of the packets
of the MPTCP session to perform the attack, but it is not capable
of changing, discarding discarding, or delaying any packet of the MPTCP
session. The attacker can be in the forward and/or backward
directions,
directions for the initial subflow and/or other subflows. The
specific needs of the attacker will be made explicit in the attack
description.

o Active attacker. The attacker is able to change, discard discard, or
delay some of the packets of the MPTCP session. The attacker can
be in the forward and/or backward directions, directions for the initial
subflow and/or other subflows. The specific needs of the attacker
will be made explicit in the attack description.

In this document, we consider the following possible combinations of
attackers:

o an On-path on-path eavesdropper

o an On-path on-path active attacker

o an Off-path off-path active attacker

o a Partial-time On-path partial-time on-path eavesdropper

o a Partial-time On-path partial-time on-path active attacker

In the rest of the document document, we describe different attacks that are
possible against the MPTCP protocol specified in RFC6824 RFC 6824 and we propose
possible security enhancements to address them.

2. ADD_ADDR attack Attack

Summary of the attack:

Type of attack: MPTCP session hijack enabling Man-in-the-Middle. a man-in-the-middle
(MitM) attack

Type of attacker: Off-path, off-path active attacker. attacker

Description:

In this attack, the attacker uses the ADD_ADDR option defined in
RFC6824 RFC
6824 to hijack an ongoing MPTCP session and enables enable himself to perform
a Man-in-the-Middle man-in-the-middle attack on the MPTCP session.

Consider the following scenario. Host A with address IPA has one
MPTCP session with Host B with address IPB. The MPTCP subflow
between IPA and IPB is using port PA on host Host A and port PB on host Host B.
The tokens for the MPTCP session are TA and TB for Host A and Host B B,
respectively. Host C is the attacker. It owns address IPC. The
attack is executed as follows:

1. Host C sends a forged packet with source address IPA, destination
address IPB, source port PA PA, and destination port PB. The packet
has the ACK flag set. The TCP sequence number for the segment is
i
i, and the ACK sequence number is j. We will assume all these
are
valid, valid; later, we discuss what the attacker needs to figure these ones
later on.
them out. The packet contains the ADD_ADDR option. The ADD_ADDR
option announces IPC as an alternative address for the
connection. It also contains an eight bit 8-bit address identifier
which that
does not bring provide any strong security benefit.

2. Host B receives the ADD_ADDR message and it replies by sending a TCP
SYN packet. (Note: the

Note: The MPTCP specification [RFC6824] states that the host
receiving the ADD_ADDR option may initiate a new subflow. If
the host is configured so that it does not initiate a new
subflow
subflow, the attack will not succeed. For example, on the
current Linux implementation, the server does not create
subflows. Only the client does so.) so.

The source address for the packet is IPB, IPB; the destination address
for the packet is IPC, IPC; the source port is
PB' PB'; and the
destination port is PA' (It (it is not required that PA=PA' nor that
PB=PB'). The sequence number for this packet is the new initial
sequence number for this subflow. The ACK sequence number is not
relevant as the ACK flag is not set. The packet carries an
MP_JOIN option and it carries the token TA. It also carries a random nonce
generated by Host B called RB.

3. Host C receives the SYN+MP_JOIN packet from Host B, B and it alters it
in the following way. It changes the source address to IPC and
the destination address to IPA. It sends the modified packet to
Host A, impersonating Host B.

4. Host A receives the SYN+MP_JOIN message and it replies with a
SYN/ACK+MP_JOIN SYN/
ACK+MP_JOIN message. The packet has source address IPA and
destination address IPC, as well as all the other needed
parameters. In particular, Host A computes a valid HMAC Hashed
Message Authentication Code (HMAC) and places it in the MP_JOIN
option.

5. Host C receives the SYN/ACK+MP_JOIN message and it changes the
source address to IPC and the destination address to IPB. It
sends the modified packet to IPB IPB, impersonating Host A.

6. Host B receives the SYN/ACK+MP_JOIN message. Host B verifies the
HMAC of the MP_JOIN option and confirms its validity. It replies
with an ACK+MP_JOIN packet. The packet has source address IPB
and destination address IPC, as well as all the other needed
parameters. The returned MP_JOIN option contains a valid HMAC
computed by Host B.

7. Host C receives the ACK+MP_JOIN message from B and it alters it in
the following way. It changes the source address to IPC and the
destination address to IPA. It sends the modified packet to Host A
A, impersonating Host B.

8. Host A receives the ACK+MP_JOIN message and creates the new
subflow. At this point point, the attacker has managed to place itself
as a MitM for one subflow for the existing MPTCP session. It
should be noted that there still exists the subflow between
address addresses IPA and IPB
that does not flow through the attacker, attacker still exists, so the
attacker has not completely intercepted all the packets in the
communication (yet). If the attacker wishes to completely
intercept the MPTCP session session, it can do the following additional
step.

9. Host C sends two TCP RST messages. One TCP RST packet is sent to
Host B, with source address IPA and IPA, destination address IPB IPB, and
source and destination ports PA and PB, respectively. The other
TCP RST message is sent to Host A, with source address IPB and IPB,
destination address IPA IPA, and source and destination ports PB and
PA, respectively. Both RST messages must contain a valid
sequence number. Note that figuring the sequence numbers to be
used here for subflow A is the same difficulty as being able to
send the initial ADD_ADDR option with valid Sequence sequence number and
ACK value. If there are more subflows, then the attacker needs
to find the Sequence Number sequence number and ACK for each subflow. At this point
point, the attacker has managed to fully hijack the MPTCP
session.

Information required by the attacker to perform the described attack:

In order to perform this attack the attacker needs to guess or know
the following pieces of information: (The information. The attacker need needs this
information for one of the subflows belonging to the MPTCP session.) session.

o the four-tuple {Client-side IP Address, Client-side Port, Server-
side Address, Servcer-side Server-side Port} that identifies the target TCP
connection

o a valid sequence number for the subflow

o a valid ACK sequence number for the subflow

o a valid address identifier for IPC

TCP connections are uniquely identified by the four-tuple {Source
Address, Source Port, Destination Address, Destination Port}. Thus,
in order to attack a TCP connection, an attacker needs to know or be
able to guess each of the values in that four-tuple. Assuming the
two peers of the target TCP connection are known, the Source Address
and the Destination Address can be assumed to be known.

We note that in

Note: In order to be able to successfully perform this attack, the
attacker needs to be able to send packets with a forged source
address. This means that the attacker cannot be located in a
network where techniques like ingress filtering [RFC2827] or
source address validation [RFC7039] are deployed. However,
ingress filtering is not as widely implemented as one would expect, expect
and hence cannot be relied upon as a mitigation for this kind of
attack.

Assuming the attacker knows the application protocol for which the
TCP connection is being employed, the server-side port can also be
assumed to be known. Finally, the client-side port will generally
not be known, known and will need to be guessed by the attacker. The
chances of an attacker guessing the client-side port will depend on
the ephemeral port range employed by the client, client and whether or not
the client implements port randomization [RFC6056].

Assuming TCP sequence number randomization is in place (see e.g. e.g.,
[RFC6528]), an attacker would have to blindly guess a valid TCP
sequence number. That is,

RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT =<
SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

As a result, the chances of an attacker to succeed succeeding will depend on the
TCP receive window size at the target TCP peer.

We note that automatic

Note: Automatic TCP buffer tuning mechanisms have been become common
for popular TCP implementations, and hence implementations; hence, very large TCP window
sizes of values up to 2 MB could end up being employed by such TCP
implementations.

According to [RFC0793], the Acknowledgement Number acknowledgement number is considered
valid as long as it does not acknowledge the receipt of data that has
not yet been sent. That is, the following expression must be true:

SEG.ACK <= SND.NXT

However, for implementations that support [RFC5961], the following
(stricter) validation check is enforced:

SND.UNA - SND.MAX.WND MAX.SND.WND <= SEG.ACK <= SND.NXT

Finally, in order for the address identifier to be valid, the only
requirement is that it needs to be different than from the ones already
being used by Host A in that MPTCP session, so a random identifier is
likely to work.

Given that a large number of factors affect the chances of an
attacker of successfully performing the aforementioned off-path attacks,
we provide two general expressions for the expected number of packets
the attacker needs to send to succeed in the attack: one for MPTCP
implementations that support [RFC5961], [RFC5961] and another for MPTCP
implementations that do not.

Implementations that do not support RFC 5961 5961:

Packets = (2^32/(RCV_WND)) * 2 * EPH_PORT_SIZE/2 * 1/MSS

Where the new : parameters are:

Packets:
Maximum number of packets required to successfully perform an off-
path (blind) attack.

RCV_WND:
TCP receive window size (RCV.WND) at the target node.

EPH_PORT_SIZE:
Number of ports comprising the ephemeral port range at the
"client" system.

MSS:
Maximum Segment Size, assuming the attacker will send full
segments to maximize the chances to get of getting a hit.

Notes:
The value "2^32" represents the size of the TCP sequence number
space.

The value "2" accounts for 2 two different ACK numbers (separated by
2^31) that should be employed to make sure the ACK number is
valid.

The following table contains some sample results for the number of
required packets, based on different values of RCV_WND and
EPH_PORT_SIZE for a an MSS of 1500 bytes.

+-------------+---------+---------+--------+---------+
| Ports \ Win | 16 KB | 128 KB | 256 KB | 2048 KB |
+-------------+---------+---------+--------+---------+
| 4000 | 699050 | 87381 | 43690 | 5461 |
+-------------+---------+---------+--------+---------+
| 10000 | 1747626 | 218453 | 109226 | 13653 |
+-------------+---------+---------+--------+---------+
| 50000 | 8738133 | 1092266 | 546133 | 68266 |
+-------------+---------+---------+--------+---------+

Table 1: Max. Maximum Number of Packets for Successful Attack

Implementations that do support RFC 5961 5961:

Packets = (2^32/(RCV_WND)) * (2^32/(2 * SND_MAX_WND)) *
EPH_PORT_SIZE/2 * 1/MSS

Where:

Packets:
Maximum number of packets required to successfully perform an off-
path (blind) attack.

RCV_WND:
TCP receive window size (RCV.WND) at the target MPTCP endpoint.

SND_MAX_WND:
Maximum TCP send window size ever employed by the target MPTCP
end-point (SND.MAX.WND).
endpoint (MAX.SND.WND).

EPH_PORT_SIZE:
Number of ports comprising the ephemeral port range at the
"client" system.

Notes:
The value "2^32" represents the size of the TCP sequence number
space.

The parameter "SND_MAX_WND" "MAX.SND.WND" is specified in [RFC5961].

The value "2*SND_MAX_WND" "2 * SND_MAX_WND" results from the expresion expression "SND.NXT -
SND.UNA - MAX.SND.WND", assuming that, for connections that
perform bulk data transfers, "SND.NXT - SND.UNA == MAX.SND.WND".
If an attacker targets a TCP endpoint that is not actively
transferring data, "2 * SND_MAX_WND" would become "SND_MAX_WND"
(and hence a successful attack would typically require more
packets).

The following table contains some sample results for the number of
required packets, based on different values of RCV_WND, SND_MAX_WND,
and EPH_PORT_SIZE. For these implementations, only a limited number
of sample results are provided, just as provided (as an indication of how [RFC5961]
increases the difficulty of performing these attacks. attacks).

+-------------+-------------+-----------+-----------+---------+
| Ports \ Win | 16 KB | 128 KB | 256 KB | 2048 KB |
+-------------+-------------+-----------+-----------+---------+
| 4000 | 45812984490 | 715827882 | 178956970 | 2796202 |
+-------------+-------------+-----------+-----------+---------+

Table 2: Max. Maximum Number of Packets for Successful Attack

Note:
In the aforementioned table, all values are computed with RCV_WND
equal to SND_MAX_WND.

2.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack

1. To include the token of the connection in the ADD_ADDR option.
This would make it harder for the attacker to launch the attack,
since he the attacker needs to either eavesdrop the token (so this
can no longer be a blind attack) or to guess it, but a random 32 bit
32-bit number is not so easy to guess. However, this would imply
that any eavesdropper that is able to see the token, token would be able
to launch this attack. This solution then increases the
vulnerability window against eavesdroppers from the initial 3-way
handshake for the MPTCP session to any exchange of the ADD_ADDR
messages.

2. To include the HMAC of the address contained in the ADD_ADDR
option. The key used for the HMAC is the concatenation of the
key of the receiver and the key of the sender (in the same way
they are used for generating the HMAC of the MP_JOIN message).
This makes it much more secure, since it requires the attacker to
have both keys (either by eavesdropping it in the first exchange
or by guessing it). Because this solution relies on the key used
in the MPTCP session, the protection of this solution would
increase if new key generation methods are defined for MPTCP
(e.g.
(e.g., using SSL Secure Socket Layer (SSL) keys as has been
proposed).

3. To include the destination address of the SYN packet in the HMAC
of the MP_JOIN message. As the attacker requires to change changing the
destination address to perform the described attack, protecting
it would prevent the attack. It wouldn't allow hosts behind NATs
to be reached by an address in the ADD_ADDR option, even with
static NAT bindings (like a web server at home).

Out of

Of the options described above, option 2 is reccommended recommended as it
achieves a higher security level while preserving the required
functionality (i.e. (i.e., NAT compatibility).

3. DoS attack Attack on MP_JOIN

Summary of the attack:

Type of attack: MPTCP Denial-of-Service denial-of-service attack, preventing the
hosts from creating new subflows. subflows

Type of attacker: Off-path, off-path active attacker

Description:

As currently specified, the initial SYN+MP_JOIN message of the 3-way
handshake for additional subflows creates state in the host receiving
the message. This is because the SYN+MP_JOIN contains the 32-bit
token that allows the receiver to identify the MPTCP-session MPTCP session and the
32-bit random nonce, nonce used in the HMAC calculation. As this
information is not resent re-sent in the third ACK of the 3-way handshake, a
host must create state upon reception of a SYN+MP_JOIN.

Assume that there exists an MPTCP-session MPTCP session exists between host Host A and host Host B, with token Ta
tokens TA and Tb. TB. An attacker, sending a SYN+MP_JOIN to host Host B, with
the valid token Tb, TB, will trigger the creation of state on host Host B.
The number of these half-open connections a host can store per
MPTCP-session MPTCP
session is limited by a certain number, number and it is
implementation-dependent. implementation-
dependent. The attacker can simply exhaust this limit by sending
multiple SYN+MP_JOINs with different 5-tuples. The (possibly forged)
source address of the attack packets will typically correspond to an
address that is not in use, or else else, the SYN/ACK sent by Host B would
elicit a RST from the impersonated node, thus removing the
corresponding state at Host B. Further discussion of traditional SYN-flood SYN
flooding attacks and common mitigations can be found in
[RFC4987] [RFC4987].

This effectively prevents the host Host A from sending any more SYN+MP_JOINs
to host Host B, as the number of acceptable half-open connections per MPTCP-session
MPTCP session on host Host B has been exhausted.

The attacker needs to know the token Tb TB in order to perform the
described attack. This can be achieved if it is a Partial-time On- partial-time on-
path eavesdropper , observing the 3-way handshake of the establishment
of an additional subflow between host Host A and host Host B. If the attacker
is never on-path, it has to guess the 32-bit token.

3.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack

The third packet of the 3-way handshake could be extended to contain also
contain the 32-bit token and the random nonce that has been sent in
the SYN+MP_JOIN. Further, host Host B will have to generate its own
random nonce in a reproducible fashion (e.g., a Hash hash of the 5-tuple +
initial sequence-number sequence number + local secret). This will allow host Host B to
reply to a SYN+MP_JOIN without having to create state. Upon the
reception of the third ACK, host Host B can then verify the correctness of
the HMAC and create the state.

4. SYN flooding amplification Flooding Amplification

Summary of the attack:

Type of attack: The attacker can use the uses SYN+MP_JOIN messages to amplify
the SYN flooding attack.

Type of attacker: Off-path, off-path active attacker

Description:

SYN flooding attacks [RFC4987] use SYN messages to exhaust the
server's resources and prevent new TCP connections. A common
mitigation is the use of SYN cookies [RFC4987] that allow the stateless
processing of the initial SYN message.

With MPTCP, the initial SYN can be processed in a stateless fashion
using the aforementioned SYN cookies. However, as we described in the
previous section, as currently specified, the SYN+MP_JOIN messages are
not processed in a stateless manner. This opens a new attack vector.
The attacker can now open a an MPTCP session by sending a regular SYN
and creating the associated state but then send sending as many
SYN+MP_JOIN messages as supported by the server with different
combinations of source address and source port combinations, port, consuming the
server's resources without having to create state in the attacker.
This is an amplification attack, where the cost on the attacker side
is only the cost of the state associated with the initial SYN while
the cost on the server side is the state for the initial SYN plus all
the state associated to with all the following SYN+MP_JOIN. SYN+MP_JOINs.

4.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack

1. The solution described for the previous DoS attack on MP_JOIN
would also prevent this attack.

2. Limiting the number of half open half-open subflows to a low number (e.g. 3 (e.g.,
three subflows) would also limit the impact of this attack.

5. Eavesdropper in the initial handshake Initial Handshake

Summary of the attack attack:

Type of attack: An eavesdropper present in the initial handshake
where the keys are exchanged can hijack the MPTCP session at any
time in the future.

Type of attacker: a Partial-time On-path partial-time on-path eavesdropper

Description:

In this case, the attacker is present along the path when the initial
3-way handshake takes place, place and therefore is able to learn the keys
used in the MPTCP session. This allows the attacker to move away
from the MPTCP session path and still be able to hijack the MPTCP
session in the future. This vulnerability was readily identified at
the moment of the design of
when designing the MPTCP security solution [RFC6181], and the threat
was considered acceptable.

5.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack

There are many techniques that can be used to prevent this attack attack,
and each of them represents different tradeoffs. trade-offs. At this point, we
limit ourselves to enumerate them and provide useful pointers.

1. Use of hash-chains. hash chains. The use of hash chains for MPTCP has been
explored in [hash-chains] [HASH-CHAINS].

2. Use of SSL keys for MPTCP security as described in
[I-D.paasch-mptcp-ssl] [MPTCP-SSL].

3. Use of Cryptographically-Generated Cryptographically Generated Addresses (CGAs) for MPTCP
security. CGAs [RFC3972] have been used in the past to secure
multi addressed
multi-addressed protocols like SHIM6 Shim6 [RFC5533].

4. Use of TCPCrypt [I-D.bittau-tcp-crypt] tcpcrypt [TCPCRYPT].

5. Use of DNSSEC. DNSSEC has been proposed to secure the Mobile IP
protocol [dnssec] [DNSSEC].

6. SYN/JOIN attack Attack

Summary of the attack attack:

Type of attack: An attacker that can intercept the SYN/JOIN
message can alter the source address being added.

Type of attacker: a Partial-time On-path partial-time on-path eavesdropper
Description:

The attacker is present along the path when the SYN/JOIN exchange
takes place, and this place. This allows the attacker to add any new address it
wants to by simply substituting the source address of the SYN/JOIN
packet for one it chooses. This vulnerability was readily identified
at the moment of the design of
when designing the MPTCP security solution [RFC6181], and the threat
was considered acceptable.

6.1. Possible security enhancements Security Enhancements to prevent this attack Prevent This Attack

It should be noted that this vulnerability is fundamental due to the
NAT support requirement. In other words, MPTCP must work through
NATs in order to be deployable in the current Internet. NAT behavior
is unfortunately indistinguishable from this attack. It is
impossible to secure the source address, since doing so would prevent
MPTCP to work though from working through NATs. This basically implies that the
solution cannot rely on securing the address. A more promising
approach would be then to look into securing the payload exchanged, exchanged and
thus limiting the impact that the attack would have in the
communication (e.g.
TCPCrypt [I-D.bittau-tcp-crypt] (e.g., tcpcrypt [TCPCRYPT] or similar).

7. Reccomendations

Current Recommendations

The current MPTCP specification [RFC6824] is experimental. Experimental. There is
an ongoing effort to move it to Standards track. Track. We believe that the
work on MPTCP security should follow two threads:

o The work on improving MPTCP security so that is enough to the MPTCP
specification [RFC6824] can become a
Standard Standards Track document.

o The work on analyzing possible additional security enhancements to
provide a more secure version of MPTCP.

We will expand on these two next. in the following subsections.

7.1. MPTCP security Security Improvements for a Standard Standards Track specification. Specification

We believe that in order for MPTCP to progress to Standard Standards Track,
the ADD_ADDR attack must be addressed. We believe that the solution
that should be adopted in order to deal with this attack is to
include an HMAC to the ADD_ADDR message (with the address being added
used as input to the HMAC, HMAC as well as the key). This would make the
ADD_ADDR message as secure as the JOIN message. In addition, this
implies that if we implement a more secure way to create the key used
in the MPTCP connection, then the security of both the MP_JOIN and
the ADD_ADDR messages is automatically improved (since both use the
same key in the HMAC).

We believe that this is enough for MPTCP to progress as a Standard
track document, Standards
Track document because the security level is similar to single path
TCP, as results from single-path
TCP per our previous analysis. Moreover, the security level achieved
with these changes is exactly the same as other
Standard Standards Track
documents. In particular, this would be the same security level as
SCTP with dynamic addresses as defined in [RFC5061]. The Security Consideration
Considerations section of RFC5061 RFC 5061 (which is a
Standard Standards Track
document) reads:

The addition and or deletion of an IP address to an existing
association does provide an additional mechanism by which existing
associations can be hijacked. Therefore, this document requires
the use of the authentication mechanism defined in [RFC4895] to
limit the ability of an attacker to hijack an association.

Hijacking an association by using the addition and deletion of an
IP address is only possible for an attacker who is able to
intercept the initial two packets of the association setup when
the SCTP-AUTH extension is used without pre-shared keys. If such
a threat is considered a possibility, then the [RFC4895] extension
must
MUST be used with a preconfigured shared endpoint pair key to
mitigate this threat.

This is the same security level that would be achieved by MPTCP plus with
the addition of the ADD_ADDR security measure reccommended.

7.1. recommended in this
document.

7.2. Security enhancements Enhancements for MPTCP

We also believe that is worthwhile exploring to explore alternatives to secure
MPTCP. As we identified earlier, the problem is of securing JOIN
messages is fundamentally incompatible with NAT support, so it is
likely that a solution to this problem involves the protection of the
data itself. Exploring the integration of MPTCP and approaches like
TCPCrypt [I-D.bittau-tcp-crypt] or
tcpcrypt [TCPCRYPT] and exploring integration with SSL seem
promising venues.
promising.

8. Security considerations Considerations

This whole document is about security considerations for MPTCP.

9. IANA Considerations

There are no IANA considerations in this memo.

11. References

11.1.

9.1. Normative References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981. 1981,
<http://www.rfc-editor.org/info/rfc793>.

[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005. 2005,
<http://www.rfc-editor.org/info/rfc3972>.

[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <http://www.rfc-editor.org/info/rfc4895>.

[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/
RFC5061, September 2007,
<http://www.rfc-editor.org/info/rfc5061>.

[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961, DOI
10.17487/RFC5961, August
2010. 2010,
<http://www.rfc-editor.org/info/rfc5961>.

[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056, DOI
10.17487/RFC6056, January
2011. 2011,
<http://www.rfc-editor.org/info/rfc6056>.

[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 2012.

[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061, September
2007.

[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007.
2012, <http://www.rfc-editor.org/info/rfc6528>.

[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013.

11.2. 2013,
<http://www.rfc-editor.org/info/rfc6824>.

9.2. Informative References

[RFC6181]

[DNSSEC] Kukec, A., Bagnulo, M., "Threat Analysis Ayaz, S., Bauer, C., and W. Eddy,
"ROAM-DNSSEC: Route Optimization for TCP Extensions Aeronautical Mobility
using DNSSEC", 4th ACM International Workshop on Mobility
in the Evolving Internet Architecture (MobiArch), 2009.

[HASH-CHAINS]
Diez, J., Bagnulo, M., Valera, F., and I. Vidal, "Security
for
Multipath Operation multipath TCP: a constructive approach", International
Journal of Internet Protocol Technology, Vol. 6, No. 3,
2011.

[MPTCP-SSL]
Paasch, C. and O. Bonaventure, "Securing the MultiPath TCP
handshake with Multiple Addresses", external keys", Work in Progress, draft-
paasch-mptcp-ssl-00, October 2012.

[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 6181,
March 2011. 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.

[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
June 2009.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol", 2009, <http://www.rfc-editor.org/info/rfc5533>.

[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for
Multipath Operation with Multiple Addresses", RFC
4960, September 2007. 6181,
DOI 10.17487/RFC6181, March 2011,
<http://www.rfc-editor.org/info/rfc6181>.

[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 2011.

[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
2011, <http://www.rfc-editor.org/info/rfc6275>.

[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, DOI 10.17487/RFC7039, October 2013.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.

[I-D.paasch-mptcp-ssl]
Paasch, C. and O. Bonaventure, "Securing the MultiPath TCP
handshake with external keys", draft-paasch-mptcp-ssl-00
(work in progress), October 2012.

[I-D.bittau-tcp-crypt] 2013,
<http://www.rfc-editor.org/info/rfc7039>.

[TCPCRYPT]
Bittau, A., Boneh, D., Hamburg, M., Handley, M., Mazieres,
D., and Q. Slack, "Cryptographic protection of TCP Streams
(tcpcrypt)", draft-bittau-tcp-crypt-04 (work Work in progress), Progress, draft-bittau-tcp-crypt-04,
February 2014.

[hash-chains]
Diez, J., Bagnulo, M., Valera, F., and I. Vidal, "Security
for multipath TCP: a constructive approach", International
Journal of Internet Protocol Technology 6, 2011.

[dnssec] Kukec, A., Bagnulo, M., Ayaz, S., Bauer, C., and W. Eddy,
"OAM-DNSSEC: Route Optimization for Aeronautical Mobility
using DNSSEC", 4th ACM International Workshop on Mobility
in the Evolving Internet Architecture MobiArch 2009, 2009.

10. Acknowledgments

Acknowledgements

We would like to thank Mark Handley for his comments on the attacks
and countermeasures discussed in this document. thanks We would also like
to thank to Alissa
Copper, Cooper, Phil Eardley, Yoshifumi Nishida, Barry
Leiba, Stephen Farrell, and Stefan Winter for the their comments and review.
reviews.

Marcelo Bagnulo,
Christophe Christoph Paasch, Oliver Bonaventure Bonaventure, and Costin
Raiciu are partially funded by the EU Trilogy 2 project.

Authors' Addresses

Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Spain

Phone: 34 91 6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es

Christoph Paasch
UCLouvain
Place Sainte Barbe, 2
Louvain-la-Neuve, 1348
Belgium

Email: christoph.paasch@uclouvain.be christoph.paasch@gmail.com

Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com

Olivier Bonaventure
UCLouvain
Place Sainte Barbe, 2
Louvain-la-Neuve, 1348
Belgium

Email: olivier.bonaventure@uclouvain.be
Costin Raiciu
Universitatea Politehnica Bucuresti
Splaiul Independentei 313a
Bucuresti
Romania

Email: costin.raiciu@cs.pub.ro