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IETF RFC 9416
Last modified on Saturday, July 22nd, 2023
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Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 9416 SI6 Networks
BCP: 72 I. Arce
Updates: 3552 Quarkslab
Category: Best Current Practice July 2023
ISSN: 2070-1721
Security Considerations for Transient Numeric Identifiers Employed in
Network Protocols
 Abstract
Poor selection of transient numerical identifiers in protocols such
as the TCP/IP suite has historically led to a number of attacks on
implementations, ranging from Denial of Service (DoS) or data
injection to information leakages that can be exploited by pervasive
monitoring. Due diligence in the specification of transient numeric
identifiers is required even when cryptographic techniques are
employed, since these techniques might not mitigate all the
associated issues. This document formally updates RFC 3552,
incorporating requirements for transient numeric identifiers, to
prevent flaws in future protocols and implementations.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9416.
Copyright Notice
Copyright (c) 2023 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Issues with the Specification of Transient Numeric Identifiers
4. Common Flaws in the Generation of Transient Numeric Identifiers
5. Requirements for Transient Numeric Identifiers
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Networking protocols employ a variety of transient numeric
identifiers for different protocol objects, such as IPv4 and IPv6
Identification values [RFC 791] [RFC 8200], IPv6 Interface Identifiers
(IIDs) [RFC 4291], transport-protocol ephemeral port numbers
[RFC 6056], TCP Initial Sequence Numbers (ISNs) [RFC 9293], NTP
Reference IDs (REFIDs) [RFC 5905], and DNS IDs [RFC 1035]. These
identifiers typically have specific requirements (e.g., uniqueness
during a specified period of time) that must be satisfied such that
they do not result in negative interoperability implications, and an
associated failure severity when such requirements are not met
[RFC 9415].
| NOTE: Some documents refer to the DNS ID as the DNS "Query ID"
| or "TxID".
For more than 30 years, a large number of implementations of IETF
protocols have been subject to a variety of attacks, with effects
ranging from Denial of Service (DoS) or data injection to information
leakages that could be exploited for pervasive monitoring [RFC 7258].
The root cause 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. While it
is generally trivial to identify an algorithm that can satisfy the
interoperability requirements of a given transient numeric
identifier, empirical evidence exists that doing so without
negatively affecting the security and/or privacy properties of the
aforementioned protocols is prone to error [RFC 9414].
For example, implementations have been subject to security and/or
privacy issues resulting from:
* predictable IPv4 or IPv6 Identification values (e.g., see
[Sanfilippo1998a], [RFC 6274], and [RFC 7739]),
* predictable IPv6 IIDs (e.g., see [RFC 7217], [RFC 7707], and
[RFC 7721]),
* predictable transport-protocol ephemeral port numbers (e.g., see
[RFC 6056] and [Silbersack2005]),
* predictable TCP Initial Sequence Numbers (ISNs) (e.g., see
[Morris1985], [Bellovin1989], and [RFC 6528]),
* predictable initial timestamps in TCP timestamps options (e.g.,
see [TCPT-uptime] and [RFC 7323]), and
* predictable DNS IDs (see, e.g., [Schuba1993] and [Klein2007]).
Recent history indicates that, when new protocols are standardized or
new protocol implementations are produced, the security and privacy
properties of the associated transient numeric identifiers tend to be
overlooked, and inappropriate algorithms to generate such identifiers
are either suggested in the specifications or selected by
implementers. As a result, advice in this area is warranted.
We note that the use of cryptographic techniques for confidentiality
and authentication might not eliminate all the issues associated with
predictable transient numeric identifiers. Therefore, due diligence
in the specification of transient numeric identifiers is required
even when cryptographic techniques are employed.
| NOTE: For example, cryptographic authentication can readily
| mitigate data injection attacks even in the presence of
| predictable transient numeric identifiers (such as "sequence
| numbers"). However, use of flawed algorithms (such as global
| counters) for generating transient numeric identifiers could
| still result in information leakages even when cryptographic
| techniques are employed. These information leakages could in
| turn be leveraged to perform other devastating attacks (please
| see [RFC 9415] for further details).
Section 3 provides an overview of common flaws in the specification
of transient numeric identifiers. Section 4 provides an overview of
common flaws in the generation of transient numeric identifiers and
their associated security and privacy implications. Finally,
Section 5 provides key guidelines for protocol designers.
2. Terminology
Transient Numeric 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. Transient
numeric identifiers are usually defined as a series of bits and
represented using integer values. These identifiers are typically
dynamically selected, as opposed to statically assigned numeric
identifiers (e.g., see [IANA-PROT]). We note that different
transient numeric identifiers may have additional requirements or
properties depending on their specific use in a protocol. We use
the term "transient numeric identifier" (or simply "numeric
identifier" or "identifier" as short forms) as a generic term to
refer to any data object in a protocol specification that
satisfies the identification property stated above.
Failure Severity:
The interoperability 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".
Soft Failure:
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.
Hard Failure:
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. Issues with the Specification of Transient Numeric Identifiers
Recent work on transient numeric identifier usage in protocol
specifications and implementations [RFC 9414] [RFC 9415] revealed that
most of the issues discussed in this document arise as a result of
one of the following conditions:
* protocol specifications that under specify their transient numeric
identifiers
* protocol specifications that over specify their transient numeric
identifiers
* protocol implementations that simply fail to comply with the
specified requirements
Both under specifying and over specifying transient numeric
identifiers is hazardous. TCP local ports [RFC 793], as well as DNS
IDs [RFC 1035], were originally under specified, leading to
implementations that resulted in predictable values and thus were
vulnerable to numerous off-path attacks. Over specification, as for
IPv6 Interface Identifiers (IIDs) [RFC 4291] and IPv6 Identification
values [RFC 2460], left implementations unable to respond to security
and privacy issues stemming from the mandated or recommended
algorithms -- IPv6 IIDs need not expose privacy-sensitive link-layer
addresses, and predictable IPv6 Fragment Header Identification values
invite the same off-path attacks that plague TCP.
Finally, there are protocol implementations that simply fail to
comply with existing protocol specifications. That is, appropriate
guidance is provided by the protocol specification (whether it be the
core specification or an update to it), but an implementation simply
fails to follow such guidance. For example, some popular operating
systems still fail to implement transport-protocol port
randomization, as specified in [RFC 6056].
Clear specification of the interoperability requirements for the
transient numeric identifiers will help identify possible algorithms
that could be employed to generate them and also make evident if such
identifiers are being over specified. A protocol specification will
usually also benefit from a vulnerability assessment of the transient
numeric identifiers they specify to prevent the corresponding
considerations from being overlooked.
4. Common Flaws in the Generation of Transient Numeric Identifiers
This section briefly notes common flaws associated with the
generation of transient numeric identifiers. Such common flaws
include, but are not limited to:
* employing trivial algorithms (e.g., global counters) that result
in predictable identifiers,
* employing the same identifier across contexts in which constancy
is not required,
* reusing identifiers across different protocols or layers of the
protocol stack,
* initializing counters or timers to constant values when such
initialization is not required,
* employing the same increment space across different contexts, and
* use of flawed Pseudorandom Number Generators (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.
When one identifier is employed across contexts where such constancy
is not needed, activity correlation is made possible. For example,
employing an identifier that is constant across networks allows for
node tracking across networks.
Reusing identifiers across different layers or protocols ties the
security and privacy properties of the protocol reusing the
identifier to the security and privacy properties of the original
identifier (over which the protocol reusing the identifier may have
no control regarding its generation). Besides, when reusing an
identifier across protocols from different layers, the goal of
isolating the properties of a layer from those of another layer is
broken, and the vulnerability assessment may be harder to perform
since the combined system, rather than each protocol in isolation,
will have to be assessed.
At times, a protocol needs to convey order information (whether it be
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. Similarly, there may not be a need
for the difference between successive counter values to be
predictable.
A node that implements a per-context linear function may share the
increment space among different contexts (please see the "Simple PRF-
Based Algorithm" section in [RFC 9415]). 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.
Finally, some implementations have been found to employ flawed PRNGs
(e.g., see [Klein2007]).
5. Requirements for Transient Numeric Identifiers
Protocol specifications that employ transient numeric identifiers
MUST explicitly specify the interoperability requirements for the
aforementioned transient numeric identifiers (e.g., required
properties such as uniqueness, along with the failure severity if
such requirements are not met).
A vulnerability assessment of the aforementioned transient numeric
identifiers MUST be performed as part of the specification process.
Such vulnerability assessment should cover, at least, spoofing,
tampering, repudiation, information disclosure, DoS, and elevation of
privilege.
| NOTE: Sections 8 and 9 of [RFC 9415] provide a general
| vulnerability assessment of transient numeric identifiers,
| along with a vulnerability assessment of common algorithms for
| generating transient numeric identifiers. Please see
| [Shostack2014] for further guidance on threat modeling.
Protocol specifications SHOULD NOT employ predictable transient
numeric identifiers, except when such predictability is the result of
their interoperability requirements.
Protocol specifications that employ transient numeric identifiers
SHOULD recommend an algorithm for generating the aforementioned
transient numeric identifiers that mitigates the vulnerabilities
identified in the previous step, such as those discussed in
[RFC 9415].
As discussed in Section 1, use of cryptographic techniques for
confidentiality and authentication might not eliminate all the issues
associated with predictable transient numeric identifiers.
Therefore, the advice from this section MUST still be applied for
cases where cryptographic techniques for confidentiality or
authentication are employed.
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
This entire document is about the security and privacy implications
of transient numeric identifiers and formally updates [RFC 3552] such
that the security and privacy implications of transient numeric
identifiers are addressed when writing the "Security Considerations"
section of future RFCs.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC 3552, July 2003,
<https://www.rfc-editor.org/info/RFC 3552>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
8.2. Informative References
[Bellovin1989]
Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", Computer Communications Review, vol. 19, no. 2,
pp. 32-48, April 1989,
<https://www.cs.columbia.edu/~smb/papers/ipext.pdf>.
[IANA-PROT]
IANA, "Protocol Registries",
<https://www.iana.org/protocols>.
[Klein2007]
Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
Predictable IP ID Vulnerability", October 2007,
<https://dl.packetstormsecurity.net/papers/attack/OpenBSD_
DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP_ID_Vuln
erability.pdf>.
[Morris1985]
Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP
Software", CSTR 117, AT&T Bell Laboratories, Murray Hill,
NJ, February 1985,
<https://pdos.csail.mit.edu/~rtm/papers/117.pdf>.
[PREDICTABLE-NUMERIC-IDS]
Gont, F. and I. Arce, "Security and Privacy Implications
of Numeric Identifiers Employed in Network Protocols",
Work in Progress, Internet-Draft, draft-gont-predictable-
numeric-ids-03, 11 March 2019,
<https://datatracker.ietf.org/doc/html/draft-gont-
predictable-numeric-ids-03>.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC 791, September 1981,
<https://www.rfc-editor.org/info/RFC 791>.
[RFC 793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC 793, September 1981,
<https://www.rfc-editor.org/info/RFC 793>.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <https://www.rfc-editor.org/info/RFC 1035>.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC 2460,
December 1998, <https://www.rfc-editor.org/info/RFC 2460>.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC 4291, February
2006, <https://www.rfc-editor.org/info/RFC 4291>.
[RFC 5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC 5905, June 2010,
<https://www.rfc-editor.org/info/RFC 5905>.
[RFC 6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC 6056, January 2011,
<https://www.rfc-editor.org/info/RFC 6056>.
[RFC 6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, DOI 10.17487/RFC 6274, July 2011,
<https://www.rfc-editor.org/info/RFC 6274>.
[RFC 6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC 6528, February
2012, <https://www.rfc-editor.org/info/RFC 6528>.
[RFC 7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC 7217, April 2014,
<https://www.rfc-editor.org/info/RFC 7217>.
[RFC 7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC 7258, May
2014, <https://www.rfc-editor.org/info/RFC 7258>.
[RFC 7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC 7323, September 2014,
<https://www.rfc-editor.org/info/RFC 7323>.
[RFC 7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC 7707, March 2016,
<https://www.rfc-editor.org/info/RFC 7707>.
[RFC 7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC 7721, March 2016,
<https://www.rfc-editor.org/info/RFC 7721>.
[RFC 7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC 7739,
February 2016, <https://www.rfc-editor.org/info/RFC 7739>.
[RFC 8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC 8200, July 2017,
<https://www.rfc-editor.org/info/RFC 8200>.
[RFC 9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC 9293, August 2022,
<https://www.rfc-editor.org/info/RFC 9293>.
[RFC 9414] Gont, F. and I. Arce, "Unfortunate History of Transient
Numeric Identifiers", RFC 9414, DOI 10.17487/RFC 9414, July
2023, <https://www.rfc-editor.org/info/RFC 9414>.
[RFC 9415] Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", RFC 9415, DOI 10.17487/RFC 9415, July
2023, <https://www.rfc-editor.org/info/RFC 9415>.
[Sanfilippo1998a]
Sanfilippo, S., "about the ip header id", message to the
Bugtraq mailing list, December 1998,
<https://seclists.org/bugtraq/1998/Dec/48>.
[Schuba1993]
Schuba, C., "Addressing Weakness in the Domain Name System
Protocol", August 1993,
<http://ftp.cerias.purdue.edu/pub/papers/christoph-schuba/
schuba-DNS-msthesis.pdf>.
[Shostack2014]
Shostack, A., "Threat Modeling: Designing for Security",
Wiley, 1st edition, February 2014.
[Silbersack2005]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability",
EuroBSDCon 2005 Conference, January 2005,
<http://www.silby.com/eurobsdcon05/
eurobsdcon_silbersack.pdf>.
[TCPT-uptime]
McDanel, B., "TCP Timestamping - Obtaining System Uptime
Remotely", message to the Bugtraq mailing list, March
2001, <https://seclists.org/bugtraq/2001/Mar/182>.
Acknowledgements
The authors would like to thank (in alphabetical order) Bernard
Aboba, Brian Carpenter, Roman Danyliw, Theo de Raadt, Lars Eggert,
Russ Housley, Benjamin Kaduk, Charlie Kaufman, Erik Kline, Alvaro
Retana, Joe Touch, Michael Tüxen, Robert Wilton, and Paul Wouters for
providing valuable comments on draft versions of this document.
The authors would like to thank (in alphabetical order) Steven
Bellovin, Joseph Lorenzo Hall, and Gre Norcie for providing valuable
comments on [PREDICTABLE-NUMERIC-IDS], on which the present document
is based.
The authors would like to thank Diego Armando Maradona for his magic
and inspiration.
Authors' Addresses
Fernando Gont
SI6 Networks
Segurola y Habana 4310 7mo piso
Ciudad Autonoma de Buenos Aires
Argentina
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Ivan Arce
Quarkslab
Segurola y Habana 4310 7mo piso
Ciudad Autonoma de Buenos Aires
Argentina
Email: iarce@quarkslab.com
URI: https://www.quarkslab.com
RFC TOTAL SIZE: 25004 bytes
PUBLICATION DATE: Saturday, July 22nd, 2023
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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