One document matched: draft-ietf-sip-fork-loop-fix-05.txt
Differences from draft-ietf-sip-fork-loop-fix-04.txt
Network Working Group R. Sparks, Ed.
Internet-Draft Estacado Systems
Updates: 3261 (if approved) S. Lawrence
Intended status: Standards Track Pingtel Corp.
Expires: September 8, 2007 A. Hawrylyshen
Ditech Networks Inc.
March 7, 2007
Addressing an Amplification Vulnerability in Session Initiation Protocol
(SIP) Forking Proxies
draft-ietf-sip-fork-loop-fix-05
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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
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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.
Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Vulnerability: Leveraging Forking to Flood a Network . . . . . 3
4. Normative changes to RFC 3261 . . . . . . . . . . . . . . . . 5
4.1. Strengthening the requirement to perform loop-detection . 5
4.2. Correcting and clarifying the RFC 3261 loop-detection
algorithm . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2.1. Update to section 16.6 . . . . . . . . . . . . . . . . 6
4.2.2. Update to section 16.3 . . . . . . . . . . . . . . . . 7
4.2.3. Note to Implementers . . . . . . . . . . . . . . . . . 7
5. Impact on overall network performance . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. -03 to -04 (addressing WGLC comments) . . . . . . . . . . 10
9.2. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 10
9.3. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. 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].
2. 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 2^70 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.
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.
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):
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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 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 2.
|
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 2: Attack request propagation
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
2^70 requests. The actual number of messages may be much larger if
the time to process the entire tree worth of requests is longer than
Timer C at either proxy. In this case, a storm of 408s, and/or a
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storm of CANCELs will also be propagating through the tree along with
the INVITEs. 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 2^69 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 are
compared using URI equality, the following registration should be
sufficient to set this attack up 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 increases, it would have taken 3 trillion years to
complete the processing of the single INVITE 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.
4. Normative changes 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.
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When a SIP proxy forks a particular request to more than one
destination, 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 in 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
[I-D.ietf-sip-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 is sufficient to protect the network. 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).
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 XXXX (RFC-Editor:
replace XXXX with the RFC number of this document) 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 first part 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
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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 RFC 3261).
4. Loop Detection Check
Proxies required to perform loop-detection by RFC-XXXX (RFC-Editor:
replace XXXX with the RFC number of this document) 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-XXXX. 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-XXXX 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. 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 which 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
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in [RFC3309] 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,
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 other'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,
parameters values with and without quoted strings must not cause an
implementation to fail.
5. Impact 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 case 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.
6. IANA Considerations
None.
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
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an exponentially growing message exchange attack.
Alternative solutions that were discussed included
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. The invite 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.
Controlling the number of concurrent requests: Bounding the total
number branches to which the original request can be forwarded
simultaneously limits the impact of the attack at any given point
in time. Proposals for limiting mechanisms where considered, but
no consensus to adopt them currently exists.
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 [I-D.ietf-sip-outbound]. Mechanisms like this may be
considered again in the future, but are currently insufficiently
developed to address the present threat.
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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 a too extreme
solution.
8. Acknowledgments
Thanks go to the implementors 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. Change Log
RFC Editor - Remove this section before publication
9.1. -03 to -04 (addressing WGLC comments)
Changed the hash recommendation per list consensus
Reintroduced Call-ID and CSeq (list discussion rediscovered one
use for them in avoiding repeated hash collisions)
9.2. -02 to -03
Closed Open Issue 1 "Why are we including all of the Route headers
values?". The text has been modified to include only those values
used in processing the request.
Closed Open Issues 2 and 3 "Why did 3261 include Call-ID To-tag,
and From-tag and CSeq?" and "Why did 3261 include Proxy-Require
and Proxy-Authorization?". The group has not been able to
identify why these fields would be included in the hash generally,
and successful interoperability tests have not included them.
Since they were not included in the text for -02, the text for
this version was not affected.
Removed the word "cryptographic" from the hash description in the
non-normative note to implementers (per list discussion) and added
characterization of the properties the hash chosen should have.
9.3. -01 to -02
Integrated several editorial fixes suggested by Jonathan Rosenberg
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Noted that the reduction of the attack to a single registration
against a single URI as documented in previous versions, is, in
fact, going to be effective against implementations conforming to
the standards before this repair.
Re-incorporated motivation from the original maxforwards-problem
draft into the security considerations section based on feedback
from Cullen Jennings
Introduced replacement text for the loop detection algorithm
description in RFC 3261, fixing the bug 648 (the topmost Via value
must not be included in the second part) and clarifying the
algorithm. Removed several other fields suggested by 3261 and
placed open issues around their presence.
Added a Notes to Implementors section capturing the "common way"
text and pointing to the interoperability issues that have been
observed with loop detection at previous SIPits
10. References
10.1. 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.
10.2. Informative References
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-08 (work in progress), March 2007.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
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Authors' Addresses
Robert Sparks (editor)
Estacado Systems
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA
Email: RjS@nostrum.com
Scott Lawrence
Pingtel Corp.
400 West Cummings Park
Suite 2200
Woburn, MA 01801
USA
Phone: +1 781 938 5306
Email: slawrence@pingtel.com
Alan Hawrylyshen
Ditech Networks Inc.
1167 Kensington Rd NW
Suite 200
Calgary, Alberta T2N 1X7
Canada
Phone: +1 403 806 3366
Email: ahawrylyshen@ditechnetworks.com
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