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Network Working Group F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Updates: 3552 (if approved) I. Arce
Intended status: Best Current Practice Quarkslab
Expires: September 1, 2018 February 28, 2018
Security Considerations for Transient Numeric Identifiers Employed in
Network Protocols
draft-gont-numeric-ids-sec-considerations-02
Abstract
For more than 30 years, a large number of implementations of the TCP/
IP protocol suite have been subject to a variety of attacks, with
effects ranging from Denial of Service (DoS) or data injection, to
information leakage that could be exploited for pervasive monitoring.
The root of these issues has been, in many cases, the poor selection
of transient identifiers in such protocols, usually as a result of an
insufficient or misleading specifications. This document formally
updates RFC3552, such that RFCs are required to perform a security
and privacy analysis of the transient numeric identifiers they
specify.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on September 1, 2018.
Copyright Notice
Copyright (c) 2018 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
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(https://trustee.ietf.org/license-info) in effect on the date of
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This document may not be modified, and derivative works of it may not
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Issues with the Specification of Identifiers . . . . . . . . 4
4. Common Flaws in the Generation of Transient Identifiers . . . 5
5. Security and Privacy Requirements for Identifiers . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Network protocols employ a variety of transient numeric identifiers
for different protocol entities, ranging from DNS Transaction IDs
(TxIDs) to transport protocol numbers (e.g. TCP ports) or IPv6
Interface Identifiers (IIDs). These identifiers usually have
specific properties that must be satisfied such that they do not
result in negative interoperability implications (e.g. uniqueness
during a specified period of time), and associated failure severities
when such properties are not met.
For more than 30 years, a large number of implementations of the TCP/
IP protocol suite have been subject to a variety of attacks, with
effects ranging from Denial of Service (DoS) or data injection, to
information leakage that could be exploited for pervasive monitoring
[RFC7528]. The root of these issues has been, in many cases, the
poor selection of identifiers in such protocols, usually as a result
of an insufficient or misleading specification. While it is
generally trivial to identify an algorithm that can satisfy the
interoperability requirements for a given identifier, there exists
practical evidence that doing so without negatively affecting the
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security and/or privacy properties of the aforementioned protocols is
prone to error.
For example, implementations have been subject to security and/or
privacy issues resulting from:
o Predictable TCP sequence numbers
o Predictable transport protocol numbers
o Predictable IPv4 or IPv6 Fragment Identifiers
o Predictable IPv6 IIDs
o Predictable DNS TxIDs
Recent history indicates that when new protocols are standardized or
new protocol implementations are produced, the security and privacy
properties of the associated identifiers tend to be overlooked and
inappropriate algorithms to generate identifier values are either
suggested in the specification or selected by implementators. As a
result, we believe that advice in this area is warranted.
2. Terminology
Identifier:
A data object in a protocol specification that can be used to
definetely distinguish a protocol object (a datagram, network
interface, transport protocol endpoint, session, etc) from all
other objects of the same type, in a given context. Identifiers
are usually defined as a series of bits and represented using
integer values. We note that different identifiers may have
additional requirements or properties depending on their specific
use in a protocol. We use the term "identifier" as a generic term
to refer to any data object in a protocol specification that
satisfies the identification property stated above. Throughout
this document we refer as "transient network identifiers" (or
simply as "identifiers") to the identifiers being dynamically
selected by a protocol. Our use of "identifier" excludes static
values such as "Protocol Numbers" and the like.
Failure Severity:
The consequences of a failure to comply with the interoperability
requirements of a given identifier. Severity considers the worst
potential consequence of a failure, determined by the system
damage and/or time lost to repair the failure. In this document
we define two types of failure severity: "soft" and "hard".
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Hard Failure:
A hard failure is a non-recoverable condition in which a protocol
does not operate in the prescribed manner or it operates with
excessive degradation of service. For example, an established TCP
connection that is aborted due to an error condition constitutes,
from the point of view of the transport protocol, a hard failure,
since it enters a state from which normal operation cannot be
recovered.
Soft Failure:
A soft failure is a recoverable condition in which a protocol does
not operate in the prescribed manner but normal operation can be
resumed automatically in a short period of time. For example, a
simple packet-loss event that is subsequently recovered with a
retransmission can be considered a soft failure.
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. Issues with the Specification of Identifiers
While assessing protocol specifications and implementations regarding
the use of transient numeric identifiers, we found that most of the
issues discussed in this document arise as a result of one of the
following:
o Protocol specifications which under-specify the requirements for
their identifiers
o Protocol specifications that over-specify their identifiers
o Protocol implementations that simply fail to comply with the
specified requirements
A number of protocol implementations (too many of them) simply
overlook the security and privacy implications of identifiers.
Examples of them are the specification of TCP port numbers in
[RFC0793], the specification of TCP sequence numbers in [RFC0793], or
the specification of the DNS TxID in [RFC1035].
On the other hand, there are a number of protocol specifications that
over-specify some of their associated protocol identifiers. For
example, [RFC4291] essentially results in link-layer addresses being
embedded in the IPv6 Interface Identifiers (IIDs) when the
interoperability requirement of uniqueness could be achieved in other
ways that do not result in negative security and privacy implications
[RFC7721]. Similarly, [RFC2460] suggests the use of a global counter
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for the generation of Fragment Identification values, when the
interoperability properties of uniqueness per {Src IP, Dst IP} could
be achieved with other algorithms that do not result in negative
security and privacy implications.
Finally, there are protocol implementations that simply fail to
comply with existing protocol specifications. For example, some
popular operating systems (notably Microsoft Windows) still fail to
implement randomization of transport protocol ephemeral ports, as
specified in [RFC6056].
By requiring protocol specifications to clearly specify the
interoperability requirements for the transient numeric identifiers
they specify, the constraints in the possible algorithms to generate
them, as well as possible over-specification of such identifiers,
become evident. Furthermore, requiring specifications to include a
security and privacy analysis of the transient numeric identifiers
they specify prevents the corresponding considerations from being
overlooked at the time a protocol is specified.
4. Common Flaws in the Generation of Transient Identifiers
This section briefly notes common flaws associated with the
generation of transient numeric identifiers. Such common flaws
include, but are not limited to:
o Employing trivial algorithms (e.g. global counters) that result in
predictable identifiers
o Employing the same identifier across contexts in which constancy
is not required
o Re-using identifiers across different protocols or layers of the
protocol stack
o Initializing counters or timers to constant values, when such
initialization is not required
o Employing the same increment space across different contexts
o Use of flawed PRNGs.
Employing trivial algorithms for generating the identifiers means
that any node that is able to sample the aforementioned identifier
can easily predict future identifiers employed by the victim node.
For example, the algorithm for Fragment Identification selection in
[RFC2460] and the algorithm for TCP ISN selection in [RFC0793].
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When one identifier is employed across contexts where such constancy
is not needed, activity correlation is made made possible. For
example, [RFC4291] essentially results in link-layer addresses being
embedded in the IPv6 Interface Identifiers (IIDs) when the
interoperability requirement of uniqueness could be achieved in other
ways. Employing an identifier that is constant across networks
allows for node tracking across networks.
Re-using identifiers across different layers or protocols ties the
security and privacy of the protocol re-using the identifier to the
security and privacy properties of such identifier (over which the
protocol re-using the identifier may have no control regarding its
generation). Besides, when re-using an identifier across protocols
from different layer, this breaks the goal of layers of isolating the
properties of a layer from that of another layer. The reuse of link-
layer addresses in IPv6 addresses specified in [RFC4291] is one
example of that.
At times, a protocol needs to convey order information (whether
sequence, timing, etc.). In many cases, there is no reason for the
corresponding counter or timer to be initialized to any specific
value e.g. at system bootstrap. For example, an implementations that
employs a counter for the Fragment Identifier [RFC2460] that gets
initialized to zero upon system bootstrapping will leak the amount of
fragmented traffic that this node has transmitted. Similarly, a node
that updates a timer to zero when bootstrapping will reveal the
"uptime" of the node.
When a node that implements a per-context linear function may share
the increment space among different contexts (please see the "Simple
Hash-Based Algorithm" in [I-D.gont-predictable-numeric-ids]).
Sharing the same increment space allows an attacker that can sample
identifiers in other context to e.g. learn how many identifiers have
been generated between two sampled values. [Sanfilippo1998a] and
[Sanfilippo1998b] employ shared increment spaces to leak the amount
of fragmented traffic that has been transmitted by a target node.
Finally, some implementations have been found to emply flawed PRNGs.
See e.g.[Klein2007].
5. Security and Privacy Requirements for Identifiers
Protocol specifications that specify transient numeric identifiers
MUST:
1. Clearly specify the interoperability requirements for the
aforementioned identifiers.
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2. Provide a security and privacy analysis of the aforementioned
identifiers.
3. Recommend an algorithm for generating the aforementioned
identifiers that mitigates security and privacy issues, such as
those discussed in [I-D.gont-predictable-numeric-ids].
6. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
7. Security Considerations
The entire document is about the security and privacy implications of
transient numeric identifiers, and formally updates [RFC3552] such
that the "Security Considerations" sections of RFCs are required to
perform a security and privacy analysis of the numeric identifiers
they specify.
8. Acknowledgements
This document is based on the document
[I-D.gont-predictable-numeric-ids] co-authored by Fernando Gont and
Ivan Arce. Thus, the authors would like to thank (in alphabetical
order) Steven Bellovin, Joseph Lorenzo Hall, Gre Norcie, for
providing valuable comments on that document.
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
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[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>.
[RFC7098] Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
Flow Label for Load Balancing in Server Farms", RFC 7098,
DOI 10.17487/RFC7098, January 2014,
<https://www.rfc-editor.org/info/rfc7098>.
9.2. Informative References
[I-D.gont-predictable-numeric-ids]
Gont, F. and I. Arce, "Security and Privacy Implications
of Numeric Identifiers Employed in Network Protocols",
draft-gont-predictable-numeric-ids-01 (work in progress),
July 2017.
[Klein2007]
Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
Predictable IP ID Vulnerability", 2007,
<http://www.trusteer.com/files/OpenBSD_DNS_Cache_Poisoning
_and_Multiple_OS_Predictable_IP_ID_Vulnerability.pdf>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
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[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC7528] Higgs, P. and J. Piesing, "A Uniform Resource Name (URN)
Namespace for the Hybrid Broadcast Broadband TV (HbbTV)
Association", RFC 7528, DOI 10.17487/RFC7528, April 2015,
<https://www.rfc-editor.org/info/rfc7528>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[Sanfilippo1998a]
Sanfilippo, S., "about the ip header id", Post to Bugtraq
mailing-list, Mon Dec 14 1998,
<http://seclists.org/bugtraq/1998/Dec/48>.
[Sanfilippo1998b]
Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list,
1998, <http://www.kyuzz.org/antirez/papers/dumbscan.html>.
Authors' Addresses
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
Ivan Arce
Quarkslab
Email: iarce@quarkslab.com
URI: https://www.quarkslab.com
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