Addressing an Amplification Vulnerability in Session Initiation Protocol (SIP) Forking Proxies
draft-ietf-sip-fork-loop-fix-08
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5393.
|
|
|---|---|---|---|
| Authors | Robert Sparks , Byron Campen , Scott Lawrence , Alan Hawrylyshen | ||
| Last updated | 2018-12-20 (Latest revision 2008-10-29) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 5393 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Cullen Fluffy Jennings | ||
| Send notices to | (None) |
draft-ietf-sip-fork-loop-fix-08
Network Working Group R. Sparks, Ed.
Internet-Draft Tekelec
Updates: 3261 (if approved) S. Lawrence
Intended status: Standards Track Nortel Networks, Inc.
Expires: May 1, 2009 A. Hawrylyshen
Ditech Networks Inc.
B. Campen
Tekelec
October 28, 2008
Addressing an Amplification Vulnerability in Session Initiation Protocol
(SIP) Forking Proxies
draft-ietf-sip-fork-loop-fix-08
Status of this Memo
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This Internet-Draft will expire on May 1, 2009.
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.
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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.
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Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Vulnerability: Leveraging Forking to Flood a Network . . . . . 5
4. Updates to RFC 3261 . . . . . . . . . . . . . . . . . . . . . 9
4.1. Strengthening the Requirement to Perform Loop-detection . 9
4.2. Correcting and Clarifying the RFC 3261 Loop-detection
Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Update to section 16.6 . . . . . . . . . . . . . . . . 10
4.2.2. Update to Section 16.3 . . . . . . . . . . . . . . . . 11
4.2.3. Impact of Loop-detection on Overall Network
Performance . . . . . . . . . . . . . . . . . . . . . 11
4.2.4. Note to Implementors . . . . . . . . . . . . . . . . . 11
5. Max-Breadth . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.3. Formal Mechanism . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. "Max-Breadth" Header . . . . . . . . . . . . . . . . . 15
5.3.2. Terminology . . . . . . . . . . . . . . . . . . . . . 15
5.3.3. Proxy Behavior . . . . . . . . . . . . . . . . . . . . 15
5.3.4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 16
5.3.5. UAS behavior . . . . . . . . . . . . . . . . . . . . . 16
5.4. Implementor Notes . . . . . . . . . . . . . . . . . . . . 17
5.4.1. Treatment of CANCEL . . . . . . . . . . . . . . . . . 17
5.4.2. Reclamation of Max-Breadth on 2xx Responses . . . . . 17
5.4.3. Max-Breadth and Automaton UAs . . . . . . . . . . . . 17
5.5. Parallel and Sequential Forking . . . . . . . . . . . . . 17
5.6. Max-Breadth Split Weight Selection . . . . . . . . . . . . 18
5.7. Max-Breadth's Effect on Forking-based Amplification
Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.8. Max-Breadth Header Field ABNF Definition . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6.1. Max-Breadth Header Field . . . . . . . . . . . . . . . . . 18
6.2. 440 Max-Breadth Exceeded response . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.1. Alternate solutions that were considered and rejected . . 20
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. -06 to -07 . . . . . . . . . . . . . . . . . . . . . . . . 21
9.2. -05 to -06 . . . . . . . . . . . . . . . . . . . . . . . . 22
9.3. -04 to -05 . . . . . . . . . . . . . . . . . . . . . . . . 22
9.4. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 22
9.5. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 22
9.6. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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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 2^71 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
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 which use a small number of resources,
including most unintentional self-inflicted variations 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 willbe 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.
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
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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):
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 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
/\ /\ /\ /\ /\ /\ /\ /\
.
.
.
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Figure 1: 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
2^71-1 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
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 2^70 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 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.
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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-
detection, the total number of stimulated messages in the first
scenario described is reduced to 14, and in the variation involving
one server, the number of stimulated messages is 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
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).
Forwarded Requests vs. Number of Participating AORs
___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
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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.
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 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 will protect the network to the extent it
was designed to do (always keep the constant multiplier due to
exhausting Max-Forwards while not forking in mind.) 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
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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 remainder of this section's description assumes the existance 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
acheived.
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).
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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. 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 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.
4.2.4. Note to Implementors
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
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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,
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 elements 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 parameters 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 they 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.
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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 may be
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
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 it 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
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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
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
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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
The Max-Breadth header takes a single positive integer as its value.
The Max-Breadth header takes no parameters.
5.3.2. Terminology
For each "response context" (see [RFC3261] Sec 16) in a proxy, this
mechanism defines two positive integral values; Incoming Max-Breadth
and Outgoing Max-Breadth. Incoming Max-Breadth is the value of the
Max-Breadth header field value in the request that formed the
response context. Outgoing Max-Breadth is the sum of the Max-Breadth
of 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
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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.
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 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 UAS behavior. A UAS receiving a
request with a Max-Breadth header field will ignore that field while
processing the request.
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5.4. Implementor 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
effects the Outgoing Max-Breadth in the associated INVITE response
context. Receiving a CANCEL in no way effects 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. For INVITE, we may receive multiple 2xx for a
single branch. Also, 2543 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 an INVITE/2xx 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 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 over into requests that are sent as a result.
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).
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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 [I-D.ietf-sip-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. (ie,
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.
5.8. Max-Breadth Header Field ABNF Definition
This specification extends the grammar for the Session Initiation
Protocol by adding the following extension-header:
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 should appear in the header sub-registry under
http://www.iana.org/assignments/sip-parameters.
RFC XXXX (this specification)
Header Field Name: Max-Breadth
Compact Form: none
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6.2. 440 Max-Breadth Exceeded response
This information should appear in the response-code sub-registry
under http://www.iana.org/assignments/sip-parameters.
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
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
[I-D.ietf-sipping-overload-reqs] and
[I-D.ietf-sipping-overload-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 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.
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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 the responsibility for detecting loops (or forms of this
attack) on themselves. 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 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.
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
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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 a too extreme
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 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.
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. -06 to -07
Cleaning up some things based on WGLC and review for publication
request (like refreshing references)
Added a sentence to the overview discussing what a client might do
if it got a 440
Reinforced that a UAC will ignore a Max-Breadth header
Updated the reference to CRC32C - from 3309 to 4960
Integrated fixes from Jan Kolomaznik's review
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9.2. -05 to -06
Integrated Max-Breadth based on working group discussion of the
secdir review
Added a paragraph pointing out that removing or modifying other
node's branch parameters defeats their ability to loop detect
Moved the total number of messages from O(2^70) to O(2^71) based
on an observation by Jan Kolomaznik. To see this, note that the
total number of requests is the sum from i=0 to Max-Forwards of
2^i which is 2^(Max-Forwards+1) - 1. The point of the text
doesn't change - (the point being that the number is _big_).
Made the new 4xx concrete (choosing 440)
Added a sentence reinforcing that if you forward to only one
branch, you still potentially have a constant multiplier of
messages in the network as Max-Forwards runs out (based on
feedback from Thomas Cross.)
9.3. -04 to -05
Boilerplate update, editorial nits fixed
9.4. -03 to -04
Addressed 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)
9.5. -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.
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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.
9.6. -01 to -02
Integrated several editorial fixes suggested by Jonathan Rosenberg
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-15 (work in progress), June 2008.
[I-D.ietf-sipping-overload-design]
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Hilt, V., "Design Considerations for Session Initiation
Protocol (SIP) Overload Control",
draft-ietf-sipping-overload-design-00 (work in progress),
October 2008.
[I-D.ietf-sipping-overload-reqs]
Rosenberg, J., "Requirements for Management of Overload in
the Session Initiation Protocol",
draft-ietf-sipping-overload-reqs-05 (work in progress),
July 2008.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
Authors' Addresses
Robert Sparks (editor)
Tekelec
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA
Email: RjS@nostrum.com
Scott Lawrence
Nortel Networks, Inc.
600 Technology Park
Billerica, MA 01821
USA
Phone: +1 978 248 5508
Email: scott.lawrence@nortel.com
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Alan Hawrylyshen
Ditech Networks Inc.
823 E. Middlefield Rd
Mountain View, CA 94043
Canada
Phone: +1 650 623 1300
Email: alan.ietf@polyphase.ca
Byron Campen
Tekelec
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA
Email: bcampen@estacado.net
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