Network Working Group R. Sparks, Ed.

Network Working Group R. Sparks, Ed.
Request for Comments: 5393 Tekelec
Updates: 3261 S. Lawrence
Category: Standards Track Nortel Networks, Inc.
A. Hawrylyshen
Ditech Networks Inc.
B. Campen
Tekelec
December 2008

Addressing an Amplification Vulnerability
in Session Initiation Protocol (SIP) Forking Proxies

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (c) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.

Abstract

This document normatively updates RFC 3261, the Session Initiation
Protocol (SIP), to address a security vulnerability identified in SIP
proxy behavior. This vulnerability enables an attack against SIP
networks where a small number of legitimate, even authorized, SIP
requests can stimulate massive amounts of proxy-to-proxy traffic.

This document strengthens loop-detection requirements on SIP proxies
when they fork requests (that is, forward a request to more than one
destination). It also corrects and clarifies the description of the
loop-detection algorithm such proxies are required to implement.
Additionally, this document defines a Max-Breadth mechanism for
limiting the number of concurrent branches pursued for any given
request.

Sparks, et al. Standards Track [Page 1]

RFC 5393 Amplification Vulnerability in SIP December 2008

Table of Contents

1. Introduction ....................................................3
2. Conventions and Definitions .....................................3
3. Vulnerability: Leveraging Forking to Flood a Network ............3
4. Updates to RFC 3261 .............................................7
4.1. Strengthening the Requirement to Perform Loop Detection ....7
4.2. Correcting and Clarifying the RFC 3261
Loop-Detection Algorithm ...................................7
4.2.1. Update to Section 16.6 ..............................7
4.2.2. Update to Section 16.3 ..............................8
4.2.3. Impact of Loop Detection on Overall Network
Performance .........................................9
4.2.4. Note to Implementers ................................9
5. Max-Breadth ....................................................10
5.1. Overview ..................................................10
5.2. Examples ..................................................11
5.3. Formal Mechanism ..........................................12
5.3.1. Max-Breadth Header Field ...........................12
5.3.2. Terminology ........................................13
5.3.3. Proxy Behavior .....................................13
5.3.3.1. Reusing Max-Breadth .......................14
5.3.4. UAC Behavior .......................................14
5.3.5. UAS Behavior .......................................14
5.4. Implementer Notes .........................................14
5.4.1. Treatment of CANCEL ................................14
5.4.2. Reclamation of Max-Breadth on 2xx Responses ........14
5.4.3. Max-Breadth and Automaton UAs ......................14
5.5. Parallel and Sequential Forking ...........................15
5.6. Max-Breadth Split Weight Selection ........................15
5.7. Max-Breadth's Effect on Forking-Based
Amplification Attacks .....................................15
5.8. Max-Breadth Header Field ABNF Definition ..................16
6. IANA Considerations ............................................16
6.1. Max-Breadth Header Field ..................................16
6.2. 440 Max-Breadth Exceeded Response .........................16
7. Security Considerations ........................................16
7.1. Alternate Solutions That Were Considered and Rejected .....17
8. Acknowledgments ................................................19
9. References .....................................................19
9.1. Normative References ......................................19
9.2. Informative References ....................................19

Sparks, et al. Standards Track [Page 2]

RFC 5393 Amplification Vulnerability in SIP December 2008

1. Introduction

Interoperability testing uncovered a vulnerability in the behavior of
forking SIP proxies as defined in [RFC3261]. This vulnerability can
be leveraged to cause a small number of valid SIP requests to
generate an extremely large number of proxy-to-proxy messages. A
version of this attack demonstrates fewer than ten messages
stimulating potentially 2^71 messages.

This document specifies normative changes to the SIP protocol to
address this vulnerability. According to this update, when a SIP
proxy forks a request to more than one destination, it is required to
ensure it is not participating in a request loop.

This normative update alone is insufficient to protect against
crafted variations of the attack described here involving multiple
Addresses of Record (AORs). To further address the vulnerability,
this document defines the Max-Breadth mechanism to limit the total
number of concurrent branches caused by a forked SIP request. The
mechanism only limits concurrency. It does not limit the total
number of branches a request can traverse over its lifetime.

The mechanisms in this update will protect against variations of the
attack described here that use a small number of resources, including
most unintentional self-inflicted variations that occur through
accidental misconfiguration. However, an attacker with access to a
sufficient number of distinct resources will still be able to
stimulate a very large number of messages. The number of concurrent
messages will be limited by the Max-Breadth mechanism, so the entire
set will be spread out over a long period of time, giving operators
better opportunity to detect the attack and take corrective measures
outside the protocol. Future protocol work is needed to prevent this
form of the attack.

2. Conventions and Definitions

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 RFC 2119 [RFC2119].

3. Vulnerability: Leveraging Forking to Flood a Network

This section describes setting up an attack with a simplifying
assumption: that two accounts on each of two different RFC 3261
compliant proxy/registrar servers that do not perform loop detection
are available to an attacker. This assumption is not necessary for
the attack but makes representing the scenario simpler. The same
attack can be realized with a single account on a single server.

Sparks, et al. Standards Track [Page 3]

RFC 5393 Amplification Vulnerability in SIP December 2008

Consider two proxy/registrar services, P1 and P2, and four Addresses
of Record, a@P1, b@P1, a@P2, and b@P2. Using normal REGISTER
requests, establish bindings to these AORs as follows (non-essential
details elided):

REGISTER sip:P1 SIP/2.0
To: <sip:a@P1>
Contact: <sip:a@P2>, <sip:b@P2>

REGISTER sip:P1 SIP/2.0
To: <sip:b@P1>
Contact: <sip:a@P2>, <sip:b@P2>

REGISTER sip:P2 SIP/2.0
To: <sip:a@P2>
Contact: <sip:a@P1>, <sip:b@P1>

REGISTER sip:P2 SIP/2.0
To: <sip:b@P2>
Contact: <sip:a@P1>, <sip:b@P1>

With these bindings in place, introduce an INVITE request to any of
the four AORs, say a@P1. This request will fork to two requests
handled by P2, which will fork to four requests handled by P1, which
will fork to eight messages handled by P2, and so on. This message
flow is represented in Figure 1.

|
a@P1
/ \
/ \
/ \
/ \
a@P2 b@P2
/ \ / \
/ \ / \
/ \ / \
a@P1 b@P1 a@P1 b@P1
/ \ / \ / \ / \
a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2
/\ /\ /\ /\ /\ /\ /\ /\
.
.
.

Figure 1: Attack Request Propagation

Sparks, et al. Standards Track [Page 4]

RFC 5393 Amplification Vulnerability in SIP December 2008

Requests will continue to propagate down this tree until Max-Forwards
reaches zero. If the endpoint and two proxies involved follow RFC
3261 recommendations, the tree will be 70 rows deep, representing
2^71-1 requests. The actual number of messages may be much larger if
the time to process the entire tree's worth of requests is longer
than Timer C at either proxy. In this case, a storm of 408 responses
and/or a storm of CANCEL requests will also be propagating through
the tree along with the INVITE requests. Remember that there are
only two proxies involved in this scenario - each having to hold the
state for all the transactions it sees (at least 2^70 simultaneously
active transactions near the end of the scenario).

The attack can be simplified to one account at one server if the
service can be convinced that contacts with varying attributes
(parameters, schemes, embedded headers) are sufficiently distinct,
and these parameters are not used as part of AOR comparisons when
forwarding a new request. Since RFC 3261 mandates that all URI
parameters must be removed from a URI before looking it up in a
location service and that the URIs from the Contact header field are
compared using URI equality, the following registration should be
sufficient to set up this attack using a single REGISTER request to a
single account:

REGISTER sip:P1 SIP/2.0
To: <sip:a@P1>
Contact: <sip:a@P1;unknown-param=whack>,<sip:a@P1;unknown-param=thud>

This attack was realized in practice during one of the SIP
Interoperability Test (SIPit) sessions. The scenario was extended to
include more than two proxies, and the participating proxies all
limited Max-Forwards to be no larger than 20. After a handful of
messages to construct the attack, the participating proxies began
bombarding each other. Extrapolating from the several hours the
experiment was allowed to run, the scenario would have completed in
just under 10 days. Had the proxies used the RFC 3261 recommended
Max-Forwards value of 70, and assuming they performed linearly as the
state they held increased, it would have taken 3 trillion years to
complete the processing of the single INVITE request that initiated
the attack. It is interesting to note that a few proxies rebooted
during the scenario and rejoined in the attack when they restarted
(as long as they maintained registration state across reboots). This
points out that if this attack were launched on the Internet at
large, it might require coordination among all the affected elements
to stop it.

Loop detection, as specified in this document, at any of the proxies
in the scenarios described so far would have stopped the attack
immediately. (If all the proxies involved implemented this loop

Sparks, et al. Standards Track [Page 5]

RFC 5393 Amplification Vulnerability in SIP December 2008

detection, the total number of stimulated messages in the first
scenario described would be reduced to 14; in the variation involving
one server, the number of stimulated messages would be reduced to
10.) However, there is a variant of the attack that uses multiple
AORs where loop detection alone is insufficient protection. In this
variation, each participating AOR forks to all the other
participating AORs. For small numbers of participating AORs (10, for
example), paths through the resulting tree will not loop until very
large numbers of messages have been generated. Acquiring a
sufficient number of AORs to launch such an attack on networks
currently available is quite feasible.

In this scenario, requests will often take many hops to complete a
loop, and there are a very large number of different loops that will
occur during the attack. In fact, if N is the number of
participating AORs, and provided N is less than or equal to Max-
Forwards, the amount of traffic generated by the attack is greater
than N!, even if all proxies involved are performing loop detection.

Suppose we have a set of N AORs, all of which are set up to fork to
the entire set. For clarity, assume AOR 1 is where the attack
begins. Every permutation of the remaining N-1 AORs will play out,
defining (N-1)! distinct paths, without repeating any AOR. Then,
each of these paths will fork N ways one last time, and a loop will
be detected on each of these branches. These final branches alone
total N! requests ((N-1)! paths, with N forks at the end of each
path).

___N____Requests_
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 64 |
| 5 | 325 |
| 6 | 1956 |
| 7 | 13699 |
| 8 | 109600 |
| 9 | 986409 |
| 10 | 9864100 |

Forwarded Requests vs. Number of Participating AORs

In a network where all proxies are performing loop detection, an
attacker is still afforded rapidly increasing returns on the number
of AORs they are able to leverage. The Max-Breadth mechanism defined
in this document is designed to limit the effectiveness of this
variation of the attack.

Sparks, et al. Standards Track [Page 6]

RFC 5393 Amplification Vulnerability in SIP December 2008

In all of the scenarios, it is important to notice that at each
forking proxy, an additional branch could be added pointing to a
single victim (that might not even be a SIP-aware element), resulting
in a massive amount of traffic being directed towards the victim from
potentially as many sources as there are AORs participating in the
attack.

4. Updates to RFC 3261

4.1. Strengthening the Requirement to Perform Loop Detection

The following requirements mitigate the risk of a proxy falling
victim to the attack described in this document.

When a SIP proxy forks a particular request to more than one
location, it MUST ensure that request is not looping through this
proxy. It is RECOMMENDED that proxies meet this requirement by
performing the loop-detection steps defined in this document.

The requirement to use this document's refinement of the loop-
detection algorithm from RFC 3261 is set at should-strength to allow
for future Standards-Track mechanisms that will allow a proxy to
determine it is not looping. For example, a proxy forking to
destinations established using the sip-outbound mechanism [OUTBOUND]
would know those branches will not loop.

A SIP proxy forwarding a request to only one location MAY perform
loop detection but is not required to. When forwarding to only one
location, the amplification risk being exploited is not present, and
the Max-Forwards mechanism will protect the network to the extent it
was designed (always keep in mind the constant multiplier due to
exhausting Max-Forwards while not forking). A proxy is not required
to perform loop detection when forwarding a request to a single
location even if it happened to have previously forked that request
(and performed loop detection) in its progression through the
network.

4.2. Correcting and Clarifying the RFC 3261 Loop-Detection Algorithm

4.2.1. Update to Section 16.6

This section replaces all of item 8 in Section 16.6 of RFC 3261 (item
8 begins on page 105 and ends on page 106 of RFC 3261).

Sparks, et al. Standards Track [Page 7]

RFC 5393 Amplification Vulnerability in SIP December 2008

8. Add a Via Header Field Value

The proxy MUST insert a Via header field value into the copy before
the existing Via header field values. The construction of this value
follows the same guidelines of Section 8.1.1.7. This implies that
the proxy will compute its own branch parameter, which will be
globally unique for that branch, and will contain the requisite magic
cookie. Note that following only the guidelines in Section 8.1.1.7
will result in a branch parameter that will be different for
different instances of a spiraled or looped request through a proxy.

Proxies required to perform loop detection by RFC 5393 have an
additional constraint on the value they place in the Via header
field. Such proxies SHOULD create a branch value separable into two
parts in any implementation-dependent way.

The remainder of this section's description assumes the existence of
these two parts. If a proxy chooses to employ some other mechanism,
it is the implementer's responsibility to verify that the detection
properties defined by the requirements placed on these two parts are
achieved.

The first part of the branch value MUST satisfy the constraints of
Section 8.1.1.7. The second part is used to perform loop detection
and distinguish loops from spirals.

This second part MUST vary with any field used by the location
service logic in determining where to retarget or forward this
request. This is necessary to distinguish looped requests from
spirals by allowing the proxy to recognize if none of the values
affecting the processing of the request have changed. Hence, the
second part MUST depend at least on the received Request-URI and any
Route header field values used when processing the received request.
Implementers need to take care to include all fields used by the
location service logic in that particular implementation.

This second part MUST NOT vary with the request method. CANCEL and
non-200 ACK requests MUST have the same branch parameter value as the
corresponding request they cancel or acknowledge. This branch
parameter value is used in correlating those requests at the server
handling them (see Sections 17.2.3 and 9.2).

4.2.2. Update to Section 16.3

This section replaces all of item 4 in Section 16.3 of RFC 3261 (item
4 appears on page 95 of RFC 3261).

Sparks, et al. Standards Track [Page 8]

RFC 5393 Amplification Vulnerability in SIP December 2008

4. Loop-Detection Check

Proxies required to perform loop detection by RFC 5393 MUST perform
the following loop-detection test before forwarding a request. Each
Via header field value in the request whose sent-by value matches a
value placed into previous requests by this proxy MUST be inspected
for the "second part" defined in Section 4.2.1 of RFC 5393. This
second part will not be present if the message was not forked when
that Via header field value was added. If the second field is
present, the proxy MUST perform the second-part calculation described
in Section 4.2.1 of RFC 5393 on this request and compare the result
to the value from the Via header field. If these values are equal,
the request has looped and the proxy MUST reject the request with a
482 (Loop Detected) response. If the values differ, the request is
spiraling and processing continues to the next step.

4.2.3. Impact of Loop Detection on Overall Network Performance

These requirements and the recommendation to use the loop-detection
mechanisms in this document make the favorable trade of exponential
message growth for work that is, at worst, order n^2 as a message
crosses n proxies. Specifically, this work is order m*n where m is
the number of proxies in the path that fork the request to more than
one location. In practice, m is expected to be small.

The loop-detection algorithm expressed in this document requires a
proxy to inspect each Via element in a received request. In the
worst case, where a message crosses N proxies, each of which loop
detect, proxy k does k inspections, and the overall number of
inspections spread across the proxies handling this request is the
sum of k from k=1 to k=N which is N(N+1)/2.

4.2.4. Note to Implementers

A common way to create the second part of the branch parameter value
when forking a request is to compute a hash over the concatenation of
the Request-URI, any Route header field values used during processing
the request, and any other values used by the location service logic
while processing this request. The hash should be chosen so that
there is a low probability that two distinct sets of these parameters
will collide. Because the maximum number of inputs that need to be
compared is 70, the chance of a collision is low even with a
relatively small hash value, such as 32 bits. CRC-32c as specified
in [RFC4960] is a specific acceptable function, as is MD5 [RFC1321].
Note that MD5 is being chosen purely for non-cryptographic
properties. An attacker who can control the inputs in order to
produce a hash collision can attack the connection in a variety of
other ways. When forming the second part using a hash,

Sparks, et al. Standards Track [Page 9]

RFC 5393 Amplification Vulnerability in SIP December 2008

implementations SHOULD include at least one field in the input to the
hash that varies between different transactions attempting to reach
the same destination to avoid repeated failure should the hash
collide. The Call-ID and CSeq fields would be good inputs for this
purpose.

A common point of failure to interoperate at SIPit events has been
due to parsers objecting to the contents of another element's Via
header field values when inspecting the Via stack for loops.
Implementers need to take care to avoid making assumptions about the
format of another element's Via header field value beyond the basic
constraints placed on that format by RFC 3261. In particular,
parsing a header field value with unknown parameter names, parameters
with no values, or parameter values with or without quoted strings
must not cause an implementation to fail.

Removing, obfuscating, or in any other way modifying the branch
parameter values in Via header fields in a received request before
forwarding it removes the ability for the node that placed that
branch parameter into the message to perform loop detection. If two
elements in a loop modify branch parameters this way, a loop can
never be detected.

5. Max-Breadth

5.1. Overview

The Max-Breadth mechanism defined here limits the total number of
concurrent branches caused by a forked SIP request. With this
mechanism, all proxyable requests are assigned a positive integral
Max-Breadth value, which denotes the maximum number of concurrent
branches this request may spawn through parallel forking as it is
forwarded from its current point. When a proxy forwards a request,
its Max-Breadth value is divided among the outgoing requests. In
turn, each of the forwarded requests has a limit on how many
concurrent branches it may spawn. As branches complete, their
portion of the Max-Breadth value becomes available for subsequent
branches, if needed. If there is insufficient Max-Breadth to carry
out a desired parallel fork, a proxy can return the 440 (Max-Breadth
Exceeded) response defined in this document.

This mechanism operates independently from Max-Forwards. Max-
Forwards limits the depth of the tree a request may traverse as it is
forwarded from its origination point to each destination it is forked
to. As Section 3 shows, the number of branches in a tree of even
limited depth can be made large (exponential with depth) by
leveraging forking. Each such branch has a pair of SIP transaction

Sparks, et al. Standards Track [Page 10]

RFC 5393 Amplification Vulnerability in SIP December 2008

state machines associated with it. The Max-Breadth mechanism limits
the number of branches that are active (those that have running
transaction state machines) at any given point in time.

Max-Breadth does not prevent forking. It only limits the number of
concurrent parallel forked branches. In particular, a Max-Breadth of
1 restricts a request to pure serial forking rather than restricting
it from being forked at all.

A client receiving a 440 (Max-Breadth Exceeded) response can infer
that its request did not reach all possible destinations. Recovery
options are similar to those when receiving a 483 (Too Many Hops)
response, and include affecting the routing decisions through
whatever mechanisms are appropriate to result in a less broad search,
or refining the request itself before submission to make the search
space smaller.

5.2. Examples

UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 30 | |
|-------------------->| Max-Forwards: 69 | |
| |------------------->| |
| | INVITE | |
| | Max-Breadth: 30 | |
| | Max-Forwards: 69 | |
| |--------------------------------------->|
| | | |

Parallel Forking

UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 60 | |
|-------------------->| Max-Forwards: 69 | |
| |------------------->| |
| | some error response| |
| |<-------------------| |
| | INVITE | |
| | Max-Breadth: 60 | |
| | Max-Forwards: 69 | |
| |--------------------------------------->|
| | | |

Sequential Forking

Sparks, et al. Standards Track [Page 11]

RFC 5393 Amplification Vulnerability in SIP December 2008

UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 60 | INVITE |
|-------------------->| Max-Forwards: 69 | Max-Breadth: 60 |
| |------------------->| Max-Forwards: 68 |
| | |------------------>|
| | | |
| | | |
| | | |

No Forking

MB == Max-Breadth MF == Max-Forwards

| MB: 4
| MF: 5
MB: 2 P MB: 2
MF: 4 / \ MF: 4
+---------------+ +------------------+
MB: 1 P MB: 1 MB: 1 P MB: 1
MF: 3 / \ MF: 3 MF: 3 / \ MF: 3
+---+ +-------+ +----+ +-------+
P P P P
MB: 1 | MB: 1 | MB: 1 | MB: 1 |
MF: 2 | MF: 2 | MF: 2 | MF: 2 |
P P P P
MB: 1 | MB: 1 | MB: 1 | MB: 1 |
MF: 1 | MF: 1 | MF: 1 | MF: 1 |
P P P P
.
.
.

Max-Breadth and Max-Forwards Working Together

5.3. Formal Mechanism

5.3.1. Max-Breadth Header Field

The Max-Breadth header field takes a single positive integer as its
value. The Max-Breadth header field value takes no parameters.

Sparks, et al. Standards Track [Page 12]

RFC 5393 Amplification Vulnerability in SIP December 2008

5.3.2. Terminology

For each "response context" (see Section 16 of [RFC3261]) in a proxy,
this mechanism defines two positive integral values: Incoming Max-
Breadth and Outgoing Max-Breadth. Incoming Max-Breadth is the value
in the Max-Breadth header field in the request that formed the
response context. Outgoing Max-Breadth is the sum of the Max-Breadth
header field values in all forwarded requests in the response context
that have not received a final response.

5.3.3. Proxy Behavior

If a SIP proxy receives a request with no Max-Breadth header field
value, it MUST add one, with a value that is RECOMMENDED to be 60.
Proxies MUST have a maximum allowable Incoming Max-Breadth value,
which is RECOMMENDED to be 60. If this maximum is exceeded in a
received request, the proxy MUST overwrite it with a value that
SHOULD be no greater than its allowable maximum.

All proxied requests MUST contain a single Max-Breadth header field
value.

SIP proxies MUST NOT allow the Outgoing Max-Breadth to exceed the
Incoming Max-Breadth in a given response context.

If a SIP proxy determines a response context has insufficient
Incoming Max-Breadth to carry out a desired parallel fork, and the
proxy is unwilling/unable to compensate by forking serially or
sending a redirect, that proxy MUST return a 440 (Max-Breadth
Exceeded) response.

Notice that these requirements mean a proxy receiving a request with
a Max-Breadth of 1 can only fork serially, but it is not required to
fork at all -- it can return a 440 instead. Thus, this mechanism is
not a tool a user agent can use to force all proxies in the path of a
request to fork serially.

A SIP proxy MAY distribute Max-Breadth in an arbitrary fashion
between active branches. A proxy SHOULD NOT use a smaller amount of
Max-Breadth than was present in the original request unless the
Incoming Max-Breadth exceeded the proxy's maximum acceptable value.
A proxy MUST NOT decrement Max-Breadth for each hop or otherwise use
it to restrict the "depth" of a request's propagation.

Sparks, et al. Standards Track [Page 13]

RFC 5393 Amplification Vulnerability in SIP December 2008

5.3.3.1. Reusing Max-Breadth

Because forwarded requests that have received a final response do not
count towards the Outgoing Max-Breadth, whenever a final response
arrives, the Max-Breadth that was used on that branch becomes
available for reuse. Proxies SHOULD be prepared to reuse this Max-
Breadth in cases where there may be elements left in the target-set.

5.3.4. UAC Behavior

A User Agent Client (UAC) MAY place a Max-Breadth header field value
in outgoing requests. If so, this value is RECOMMENDED to be 60.

5.3.5. UAS Behavior

This mechanism does not affect User Agent Server (UAS) behavior. A
UAS receiving a request with a Max-Breadth header field will ignore
that field while processing the request.

5.4. Implementer Notes

5.4.1. Treatment of CANCEL

Since CANCEL requests are never proxied, a Max-Breadth header field
value is meaningless in a CANCEL request. Sending a CANCEL in no way
affects the Outgoing Max-Breadth in the associated INVITE response
context. Receiving a CANCEL in no way affects the Incoming Max-
Breadth of the associated INVITE response context.

5.4.2. Reclamation of Max-Breadth on 2xx Responses

Whether 2xx responses free up Max-Breadth is mostly a moot issue,
since proxies are forbidden to start new branches in this case. But,
there is one caveat. A proxy may receive multiple 2xx responses for
a single forwarded INVITE request. Also, [RFC2543] implementations
may send back a 6xx followed by a 2xx on the same branch.
Implementations that subtract from the Outgoing Max-Breadth when they
receive a 2xx response to an INVITE request must be careful to avoid
bugs caused by subtracting multiple times for a single branch.

5.4.3. Max-Breadth and Automaton UAs

Designers of automaton user agents (UAs) (including B2BUAs, gateways,
exploders, and any other element that programmatically sends requests
as a result of incoming SIP traffic) should consider whether Max-
Breadth limitations should be placed on outgoing requests. For
example, it is reasonable to design B2BUAs to carry the Max-Breadth
value from incoming requests into requests that are sent as a result.

Sparks, et al. Standards Track [Page 14]

RFC 5393 Amplification Vulnerability in SIP December 2008

Also, it is reasonable to place Max-Breadth constraints on sets of
requests sent by exploders when they may be leveraged in an
amplification attack.

5.5. Parallel and Sequential Forking

Inherent in the definition of this mechanism is the ability of a
proxy to reclaim apportioned Max-Breadth while forking sequentially.
The limitation on outgoing Max-Breadth is applied to concurrent
branches only.

For example, if a proxy receives a request with a Max-Breadth of 4
and has 8 targets to forward it to, that proxy may parallel fork to 4
of these targets initially (each with a Max-Breadth of 1, totaling an
Outgoing Max-Breadth of 4). If one of these transactions completes
with a failure response, the outgoing Max-Breadth drops to 3,
allowing the proxy to forward to one of the 4 remaining targets
(again, with a Max-Breadth of 1).

5.6. Max-Breadth Split Weight Selection

There are a variety of mechanisms for controlling the weight of each
fork branch. Fork branches that are given more Max-Breadth are more
likely to complete quickly (because it is less likely that a proxy
down the line will be forced to fork sequentially). By the same
token, if it is known that a given branch will not fork later on, a
Max-Breadth of 1 may be assigned with no ill effect. This would be
appropriate, for example, if a proxy knows the branch is using the
SIP outbound extension [OUTBOUND].

5.7. Max-Breadth's Effect on Forking-Based Amplification Attacks

Max-Breadth limits the total number of active branches spawned by a
given request at any one time, while placing no constraint on the
distance (measured in hops) that the request can propagate. (i.e.,
receiving a request with a Max-Breadth of 1 means that any forking
must be sequential, not that forking is forbidden)

This limits the effectiveness of any amplification attack that
leverages forking because the amount of state/bandwidth needed to
process the traffic at any given point in time is capped.

Sparks, et al. Standards Track [Page 15]

RFC 5393 Amplification Vulnerability in SIP December 2008

5.8. Max-Breadth Header Field ABNF Definition

This specification extends the grammar for the Session Initiation
Protocol by adding an extension-header. The ABNF [RFC5234]
definition is as follows.

Max-Breadth = "Max-Breadth" HCOLON 1*DIGIT

6. IANA Considerations

This specification registers a new SIP header field and a new SIP
response according to the processes defined in [RFC3261].

6.1. Max-Breadth Header Field

This information appears in the Header Fields sub-registry of the SIP
Parameters registry.

RFC 5393 (this specification)

Header Field Name: Max-Breadth

Compact Form: none

6.2. 440 Max-Breadth Exceeded Response

This information appears in the Response Codes sub-registry of the
SIP Parameters registry.

Response code: 440

Default Reason Phrase: Max-Breadth Exceeded

7. Security Considerations

This document is entirely about documenting and addressing a
vulnerability in SIP proxies as defined by RFC 3261 that can lead to
an exponentially growing message exchange attack.

The Max-Breadth mechanism defined here does not decrease the
aggregate traffic caused by the forking-loop attack. It only serves
to spread the traffic caused by the attack over a longer period by
limiting the number of concurrent branches that are being processed
at the same time. An attacker could pump multiple requests into a
network that uses the Max-Breadth mechanism and gradually build
traffic to unreasonable levels. Deployments should monitor carefully
and react to gradual increases in the number of concurrent
outstanding transactions related to a given resource to protect

Sparks, et al. Standards Track [Page 16]

RFC 5393 Amplification Vulnerability in SIP December 2008

against this possibility. Operators should anticipate being able to
temporarily disable any resources identified as being used in such an
attack. A rapid increase in outstanding concurrent transactions
system-wide may be an indication of the presence of this kind of
attack across many resources. Deployments in which it is feasible
for an attacker to obtain a very large number of resources are
particularly at risk. If detecting and intervening in each instance
of the attack is insufficient to reduce the load, overload may occur.

Implementers and operators are encouraged to follow the
recommendations being developed for handling overload conditions (see
[REQS] and [DESIGN]).

Designers of protocol gateways should consider the implications of
this kind of attack carefully. As an example, if a message transits
from a SIP network into the Public Switched Telephone Network (PSTN)
and subsequently back into a SIP network, and information about the
history of the request on either side of the protocol translation is
lost, it becomes possible to construct loops that neither Max-
Forwards nor loop detection can protect against. This, combined with
forking amplification on the SIP side of the loop, will result in an
attack as described in this document that the mechanisms here will
not abate, not even to the point of limiting the number of concurrent
messages in the attack. These considerations are particularly
important for designers of gateways from SIP to SIP (as found in
B2BUAs, for example). Many existing B2BUA implementations are under
some pressure to hide as much information about the two sides
communicating with them as possible. Implementers of such
implementations may be tempted to remove the data that might be used
by the loop-detection, Max-Forwards, or Max-Breadth mechanisms at
other points in the network, taking on the responsibility for
detecting loops (or forms of this attack). However, if two such
implementations are involved in the attack, neither will be able to
detect it.

7.1. Alternate Solutions That Were Considered and Rejected

Alternative solutions that were discussed include:

Doing nothing - rely on suing the offender: While systems that have
accounts have logs that can be mined to locate abusers, it isn't
clear that this provides a credible deterrent or defense against
the attack described in this document. Systems that don't
recognize the situation and take corrective/preventative action
are likely to experience failure of a magnitude that precludes
retrieval of the records documenting the setup of the attack. (In
one scenario, the registrations can occur in a radically different
time period than the INVITE transaction. The INVITE request

Sparks, et al. Standards Track [Page 17]

RFC 5393 Amplification Vulnerability in SIP December 2008

itself may have come from an innocent). It's even possible that
the scenario may be set up unintentionally. Furthermore, for some
existing deployments, the cost and audit ability of an account is
simply an email address. Finding someone to punish may be
impossible. Finally, there are individuals who will not respond
to any threat of legal action, and the effect of even a single
successful instance of this kind of attack would be devastating to
a service provider.

Putting a smaller cap on Max-Forwards: The effect of the attack is
exponential with respect to the initial Max-Forwards value.
Turning this value down limits the effect of the attack. This
comes at the expense of severely limiting the reach of requests in
the network, possibly to the point that existing architectures
will begin to fail.

Disallowing registration bindings to arbitrary contacts: The way
registration binding is currently defined is a key part of the
success of the kind of attack documented here. The alternative of
limiting registration bindings to allow only binding to the
network element performing the registration, perhaps to the
extreme of ignoring bits provided in the Contact in favor of
transport artifacts observed in the registration request, has been
discussed (particularly in the context of the mechanisms being
defined in [OUTBOUND]). Mechanisms like this may be considered
again in the future, but are currently insufficiently developed to
address the present threat.

Deprecate forking: This attack does not exist in a system that
relies entirely on redirection and initiation of new requests by
the original endpoint. Removing such a large architectural
component from the system at this time was deemed too extreme a
solution.

Don't reclaim breadth: An alternative design of the Max-Breadth
mechanism that was considered and rejected was to not allow the
breadth from completed branches to be reused (see
Section 5.3.3.1). Under this alternative, an introduced request
would cause, at most, the initial value of Max-Breadth
transactions to be generated in the network. While that approach
limits any variant of the amplification vulnerability described
here to a constant multiplier, it would dramatically change the
potential reach of requests, and there is belief that it would
break existing deployments.

Sparks, et al. Standards Track [Page 18]

RFC 5393 Amplification Vulnerability in SIP December 2008

8. Acknowledgments

Thanks go to the implementers that subjected their code to this
scenario and helped analyze the results at SIPit 17. Eric Rescorla
provided guidance and text for the hash recommendation note.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

9.2. Informative References

[DESIGN] Hilt, V., "Design Considerations for Session Initiation
Protocol (SIP) Overload Control", Work in Progress,
July 2008.

[OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
Work in Progress, October 2008.

[REQS] Rosenberg, J., "Requirements for Management of Overload
in the Session Initiation Protocol", Work in Progress,
July 2008.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[RFC2543] Handley, M., Schulzrinne, H., Schooler, E., and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
March 1999.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.

Sparks, et al. Standards Track [Page 19]

RFC 5393 Amplification Vulnerability in SIP December 2008

Authors' Addresses

Robert Sparks (editor)
Tekelec
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA

EMail: [email protected]

Scott Lawrence
Nortel Networks, Inc.
600 Technology Park
Billerica, MA 01821
USA

Phone: +1 978 288 5508
EMail: [email protected]

Alan Hawrylyshen
Ditech Networks Inc.
823 E. Middlefield Rd
Mountain View, CA 94043
USA

Phone: +1 650 623 1300
EMail: [email protected]

Byron Campen
Tekelec
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA

EMail: [email protected]

Sparks, et al. Standards Track [Page 20]