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Network Working Group F. Gont
Internet-Draft SI6 Networks
Updates: 3552 (if approved) I. Arce
Intended status: Best Current Practice Quarkslab
Expires: January 9, 2020 July 8, 2019
Security Considerations for Transient Numeric Identifiers Employed in
Network Protocols
draft-gont-numeric-ids-sec-considerations-04
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 numeric identifiers in such protocols, usually as a
result of insufficient or misleading specifications. This document
formally updates RFC3552, such that RFCs are required to include a
security and privacy analysis of the transient numeric identifiers
they specify.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 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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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
9.3. Informative References . . . . . . . . . . . . . . . . . 9
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 an associated failure
severity when such properties 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
[RFC7258]. The root of these issues has been, in many cases, the
poor selection of identifiers in such protocols, usually as a result
of insufficient or misleading specifications. While it is generally
trivial to identify an algorithm that can satisfy the
interoperability requirements for a given identifier, there exists
practical evidence [I-D.gont-numeric-ids-history] that doing so
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without negatively affecting the 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 such identifiers are either
suggested in the specification or selected by implementers. As a
result, advice in this area is warranted.
2. Terminology
Identifier:
A data object in a protocol specification that can be used to
definitely 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 these 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 numeric 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
[I-D.gont-numeric-ids-history], we found that most of the issues
discussed in this document arise as a result of one of the following
conditions:
o Protocol specifications that 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
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[RFC7721]. Similarly, [RFC2460] suggested the use of a global
counter 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 fails to
implement transport-protocol port randomization, 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 such identifiers can easily
predict future identifiers employed by the victim node. For example,
the algorithm for Fragment Identification selection in [RFC2460] and
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the algorithm for TCP ISN selection in [RFC0793] suffer from that
problem.
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 the original 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 layers, the goal of of isolating the
properties of a layer from that of another layer is broken. Re-using
link-layer addresses in IPv6 addresses as 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 [RFC8200] that gets
initialized to zero upon system bootstrapping will leak the number of
fragmented packets that this node has transmitted. Similarly, a node
that updates a timer to zero when bootstrapping will reveal the
"uptime" of the node.
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 number
of fragmented packets that has been transmitted by a target node.
Finally, some implementations have been found to employ flawed PRNGs.
See e.g.[Klein2007].
5. Security and Privacy Requirements for Identifiers
Protocol specifications that specify transient numeric identifiers
MUST:
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1. Clearly specify the interoperability requirements for the
aforementioned identifiers.
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-numeric-ids-generation].
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
This 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 transient 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.
The authors would like to thank Diego Armando Maradona for his magic
and inspiration.
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>.
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[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>.
[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>.
[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
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-03 (work in progress),
March 2019.
[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>.
[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>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
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[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>.
9.3. Informative References
[I-D.gont-numeric-ids-generation]
Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", draft-gont-numeric-ids-generation-03
(work in progress), March 2019.
[I-D.gont-numeric-ids-history]
Gont, F. and I. Arce, "Unfortunate History of Transient
Numeric Identifiers", draft-gont-numeric-ids-history-04
(work in progress), March 2019.
Authors' Addresses
Fernando Gont
SI6 Networks
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Ivan Arce
Quarkslab
Email: iarce@quarkslab.com
URI: https://www.quarkslab.com
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