Improve TCP Handling of Out-of-Window Packets to Mitigate Ghost ACKs
draft-ietf-tcpm-tcp-ghost-acks-09

TCPM                                                              Y. Pan
Internet-Draft                                                 C. Rossow
Intended status: Standards Track                                   CISPA
Expires: 1 February 2027                                    31 July 2026

  Improve TCP Handling of Out-of-Window Packets to Mitigate Ghost ACKs
                   draft-ietf-tcpm-tcp-ghost-acks-09

Abstract

   Historically, TCP as specified in RFC 793 was threatened by the blind
   data injection attack because of the loose SEG.ACK value validation,
   where the SEG.ACK value of a TCP segment is considered valid as long
   as it does not acknowledge data ahead of what has been sent.  RFC
   5961 improved the input validation by shrinking the range of
   acceptable SEG.ACK values in a TCP segment.  Later, RFC 9293
   incorporated the updates proposed by RFC 5961 as a TCP stack
   implementation option.
   However, an endpoint that follows the RFC 9293 specifications can
   still accept a TCP segment containing an SEG.ACK value acknowledging
   data that the endpoint has never sent.  This document specifies small
   modifications to the way TCP verifies incoming TCP segments' SEG.ACK
   value to prevent TCP from accepting such invalid SEG.ACK values.

Status of This Memo

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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Ghost ACKs  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Mitigation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Pseudo Code Example 1  . . . . . . . . . . . . . . .   5
   Appendix B.  Pseudo Code Example 2  . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   TCP (as originally specified by [RFC0793]) is widely deployed in
   today's Internet.  Against the threat of the blind data injection
   attack, Section 5 of [RFC5961] proposed to improve the validation of
   the SEG.ACK field of incoming TCP segments.  Currently, [RFC9293] is
   the latest main document for TCP, which obsoletes [RFC0793] and
   incorporates the SEG.ACK validation proposed by [RFC5961] as an
   optional implementation choice.  The SEG.ACK validation introduced in
   [RFC9293] (with or without the [RFC5961] implementation choice)
   accepts a certain range of SEG.ACK values before SND.UNA as
   duplicate/old ACK values.  This also applies to connections without
   data (or with little data) transferred previously.  Consequently,
   current SEG.ACK validation accepts segments with invalid SEG.ACK
   values that acknowledge data that an endpoint has never sent as
   "duplicate/old" SEG.ACK values (ghost ACKs).
   This document improves the SEG.ACK value validation in [RFC9293],
   such that TCP would only accept duplicate/old SEG.ACK values
   acknowledging data already sent by the endpoint, eliminating the
   security risks imposed by ghost ACKs.

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1.1.  Terminology

   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.

   TCP terminology should be interpreted as described in [RFC9293].

2.  Ghost ACKs

   As described in [RFC9293], when receiving a segment, the endpoint
   performs checks on the SEG.ACK field of the incoming segment.
   Suppose the TCP stack has implemented the mitigation for blind data
   injection attack proposed by Section 5 of [RFC5961], an incoming
   segment whose SEG.ACK value satisfies the condition SND.UNA -
   MAX.SND.WND =< SEG.ACK =< SND.NXT is considered acceptable, and the
   segment is further processed.  When the [RFC5961] mitigation is not
   implemented, an incoming segment with SEG.ACK =< SND.NXT is accepted
   and further processed.

   However, there are cases where the number of bytes sent by the
   endpoint is less than MAX.SND.WND or 2^31 - 1, and this can result in
   accepting a segment with an SEG.ACK value acknowledging bytes the
   endpoint has never sent.  As a concrete example, consider a newly
   established TCP connection without data transferred during the
   handshake.  There is SND.UNA == SND.NXT == ISS + 1.  In this case,
   any segments with SEG.ACK < SND.UNA acknowledges bytes that the
   endpoint has never sent, but they are still considered acceptable
   since they satisfy the above SEG.ACK validation condition.

   Ghost ACKs therefore reduce the effectiveness of ACK validation
   against blind data injection attacks, particularly for newly
   established TCP connections.

3.  Mitigation

   TCP stacks that implement this specification MUST add an input check
   for acknowledgements that SEG.ACK is the next sequence number of a
   sequence number occupied by one of the segments, which have been sent
   before.  All incoming segments whose SEG.ACK value doesn't satisfy
   this condition MUST be discarded and a challenge ACK sent back.

   Appendix A and Appendix B provide pseudo code examples showing how to
   implement this mitigation under the assumption that the TCP stack
   supports [RFC5961].

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4.  Security Considerations

   This document defines ghost ACKs and provides two alternative
   mitigations against ghost ACK based attacks.  The security
   considerations in [RFC5961] and [RFC9293] still apply to
   implementations handling ghost ACKs as described in this document.

   TCP implementation supporting [RFC5961] might be subject to side
   channel attacks based on challenge ACKs.  When the ghost ACK
   mitigations are implemented, such attacks might be used to disclose
   the ISS as long as less than MAX.SND.WND bytes of data have been
   cumulatively acknowledged or the fact that more than MAX.SND.WND
   bytes have been cumulatively acknowledged.

5.  IANA Considerations

   This document does not require any actions from IANA.

6.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", RFC 793,
              DOI 10.17487/RFC793, 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>.

   [RFC4898]  Mathis, M., Heffner, J., and R. Raghunarayan, "TCP
              Extended Statistics MIB", RFC 4898, DOI 10.17487/RFC4898,
              May 2007, <https://www.rfc-editor.org/info/rfc4898>.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,
              <https://www.rfc-editor.org/info/rfc5961>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/info/rfc7323>.

   [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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   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

Appendix A.  Pseudo Code Example 1

   This pseudo code example requires that the TCP stack supports
   [RFC5961].  It adds an additional boolean state variable NO.ISS.CHECK
   for each established connection.  This variable is initialized to
   false.  At the beginning of the SEG.ACK validation, it is determined
   if the ISS-based check is still needed:

   if (!NO.ISS.CHECK && SND.UNA > ISS + (65535 << Snd.Wind.Shift)) {
       /* Checking SEG.ACK against ISS is definitely redundant. */
       NO.ISS.CHECK = true;
   }

   Snd.Wind.Shift is defined in [RFC7323].  For TCP stacks that do not
   implement [RFC7323], Snd.Wind.Shift is treated as 0.  Then a local
   variable ACK.MIN is computed, which is later used to validate the
   SEG.ACK.  It is used to perform the validation, which is stricter.

   if (NO.ISS.CHECK) {
       /* Check for too old ACKs (RFC 5961, Section 5.2). */
       ACK.MIN = SND.UNA - MAX.SND.WND;
   } else {
       if (ISS + 1 > SND.UNA - MAX.SND.WND) {
           /* Checking for ghost ACKs is stricter. */
           ACK.MIN = ISS + 1;
       } else {
           /*
            * Checking for too old ACKs (RFC 5961, Section 5.2)
            * is stricter.
            */
           ACK.MIN = SND.UNA - MAX.SND.WND;
       }
   }

   Finally the validation of SEG.ACK specified in Section 5.2 of
   [RFC5961] is changed to:

   if (SEG.ACK < ACK.MIN || SEG.ACK > SND.NXT) {
       send_challenge_ack();
       return;
   }

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Appendix B.  Pseudo Code Example 2

   This pseudo code example requires that the TCP stack supports
   [RFC4898] and [RFC5961].  It uses the tcpEStatsAppHCThruOctetsAcked
   counter (defined in [RFC4898]), which tracks the number of bytes that
   are already cumulatively acknowledged by the peer.

   A local variable ACK.MIN is computed, which is used to validate the
   SEG.ACK.

   if (tcpEStatsAppHCThruOctetsAcked < MAX.SND.WND) {
       ACK.MIN = SND.UNA - tcpEStatsAppHCThruOctetsAcked;
   } else {
       ACK.MIN = SND.UNA - MAX.SND.WND;
   }

   Finally the validation of SEG.ACK specified in Section 5.2 of
   [RFC5961] is changed to:

   if (SEG.ACK < ACK.MIN || SEG.ACK > SND.NXT) {
       send_challenge_ack();
       return;
   }

   Though unlikely to happen, the 64-bit tcpEStatsAppHCThruOctetsAcked
   counter can overflow.  It is preferred that an implementation handles
   tcpEStatsAppHCThruOctetsAcked overflow.

Acknowledgements

   We thank Eric Dumazet for proposing the second mitigation option
   using tcpEStatsAppHCThruOctetsAcked.

Authors' Addresses

   Yepeng Pan
   CISPA Helmholtz Center for Information Security
   Email: yepeng.pan@cispa.de

   Christian Rossow
   CISPA Helmholtz Center for Information Security
   Email: rossow@cispa.de

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