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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 30, 2021                                  July 29, 2020


 Security Considerations for Transient Numeric Identifiers Employed in
                           Network Protocols
              draft-gont-numeric-ids-sec-considerations-05

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) to data
   injection and information leakage that can be exploited by pervasive
   monitoring.  To prevent such flaws in future protocols and
   implementations, this document updates RFC 3552, requiring future
   RFCs to contain analysis of the security and privacy properties of
   any transient numeric identifiers specified by the protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 30, 2021.

Copyright Notice

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   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Issues with the Specification of Transient 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 an associated failure
   severity when such properties not met.

   The TCP/IP protocol suite alone has been subject to variety of
   attacks on its numerical identifiers over the past 30 years or more,
   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.irtf-pearg-numeric-ids-history] that doing so
   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:




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   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.

   Section 3 provides an overview of common flaws in the specification
   of transient numeric identifiers.  Section 4 provides an overview of
   the implications of predictable transient numeric identifiers.
   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 (see e.g.  [IANA-PROT]).  We note that different
      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 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".

   Hard Failure:



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      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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Issues with the Specification of Transient Identifiers

   A recent survey of transient numerical identifier usage in protocol
   specifications and implementations
   [I-D.irtf-pearg-numeric-ids-history] revealed 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.
   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



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   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.  That is, appropriate
   guidance is provided by the protocol specification (whether the core
   specification or or an update to it), but an implementation simply
   fails to follow such guidance.  For example, some popular operating
   systems (notably Microsoft Windows) still fails to implement
   transport-protocol port randomization, as specified in [RFC6056].

   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 security and privacy analysis of the
   transient numeric identifiers they specify, to prevent the
   corresponding considerations from being overlooked.

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 pseudo-random 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 made possible.  For




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   example, 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, and the
   privacy and security analysis 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
   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 counted values to be a
   predictable.

   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.irtf-pearg-numeric-ids-generation]).
   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.
   See e.g.[Klein2007].

5.  Security and Privacy Requirements for Identifiers

   When a protocol specifies transient numerical identifiers, it is
   critical for the protocol specification to:

   1.  Clearly specify the interoperability requirements for the
       aforementioned identifiers (e.g., required properties such as
       uniqueness, along with the failure severity if such properties
       are not met).

   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.irtf-pearg-numeric-ids-generation].




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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 and privacy implications of transient numeric
   identifiers are considered when writing the "Security Considerations"
   section of future RFCs.

8.  Acknowledgements

   The authors would like to thank Benjamin Kaduk for providing valuable
   comments and suggestions that have been incorporated into this
   document.

   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

   [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>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.





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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.

   [I-D.irtf-pearg-numeric-ids-generation]
              Gont, F. and I. Arce, "On the Generation of Transient
              Numeric Identifiers", draft-irtf-pearg-numeric-ids-
              generation-02 (work in progress), May 2020.

   [I-D.irtf-pearg-numeric-ids-history]
              Gont, F. and I. Arce, "Unfortunate History of Transient
              Numeric Identifiers", draft-irtf-pearg-numeric-ids-
              history-02 (work in progress), April 2020.

   [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", 2007,
              <http://www.trusteer.com/files/OpenBSD_DNS_Cache_Poisoning
              _and_Multiple_OS_Predictable_IP_ID_Vulnerability.pdf>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [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>.

   [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>.

   [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>.

   [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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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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