TCPM Y. Pan Internet-Draft C. Rossow Intended status: Standards TrackCISPA Helmholtz Center for Information Security Expires: 9 January 2025 8 July 2024 Improve TCP Handling of Out-of-Window Packets to Mitigate Ghost ACKs draft-pan-tcpm-tcp-ghost-acks-00 Abstract Historically, TCP as specified in RFC 793 was threatened by the blind data injection attack because of the loose acceptable ACK value validation, where the ACK value of a data segment is considered valid as long as it does not acknowledge data ahead of the next segment to send. RFC 5961 improved the defect by shrinking the range of acceptable ACK values in a data segment. Later, RFC 9293 involved 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 data segment containing an ACK value acknowledging data that the endpoint has never sent. This document specifies small modifications to the way TCP verifies incoming data segments' ACK value to prevent TCP from accepting invalid ACK values. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 9 January 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. Pan & Rossow Expires 9 January 2025 [Page 1] Internet-Draft Ghost ACKs Mitigation July 2024 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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Ghost ACKs . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Security Implications of Ghost ACKs . . . . . . . . . . . . . 3 4. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . 3 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Normative References . . . . . . . . . . . . . . . . . . 4 5.2. Informative References . . . . . . . . . . . . . . . . . 5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction TCP as specified in [RFC0793] is widely deployed in today's Internet. Against the threat of the blind data injection attack, [RFC5961] section 5 proposed to improve the validation of data segments with duplicate/old ACK values. Currently, [RFC9293] is the latest main document for TCP, which obsoletes [RFC0793] and involves the ACK validation proposed by [RFC5961] as an optional implementation choice. ACK validation introduced in [RFC9293] (with or without the [RFC5961] implementation choice) accepts a certain range of 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 ACK validation accepts segments with invalid ACK values that acknowledge data that a server has never sent as "duplicate/old" ACK values (ghost ACKs). This document aims to improve the ACK value validation in [RFC9293], such that TCP would only accept duplicate/old ACK values acknowledging data already sent by the endpoint, eliminating the security risks imposed by ghost ACKs. Pan & Rossow Expires 9 January 2025 [Page 2] Internet-Draft Ghost ACKs Mitigation July 2024 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 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 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 SND.UNA - (2^31 - 1) =< 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 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 ACK validation condition. 3. Security Implications of Ghost ACKs Ghost ACKs allow an attacker to inject payloads into a newly established connection. This extends the threat model as described in [RFC5961], where an off-path attacker can perform injection attacks against an existing foreign connection. Ghost ACKs further allow attackers that spoof the TCP handshake to use the spoofed TCP connection and transmit payloads [SP2024Spoof]. 4. Mitigation TCP stacks MAY implement one of the following two mitigations. Pan & Rossow Expires 9 January 2025 [Page 3] Internet-Draft Ghost ACKs Mitigation July 2024 Mitigation Option 1 (generic): TCP stacks that implement this mitigation MUST add an additional state variable for each established connection, ISS_CHECK_NOT_REQUIRED. When validating the ACK value of any incoming segments, TCP stacks that implement this mitigation MUST add an additional input check ISS_CHECK_NOT_REQUIRED || SND.UNA =< SEG.ACK || ISS + 1 =< SEG.ACK. The first part of the condition checks if the ISS comparison is required, more details regarding the ISS_CHECK_NOT_REQUIRED state follow later. When ISS comparison is required, the second part of the condition verifies if the segment acknowledges unacknowledged data (no ghost ACKs by definition). If not, the last part of the condition verifies whether the segment's ACK value is after ISS, which guarantees no ghost ACKs are accepted. Segments with ACK values satisfying the above condition are further validated and processed as specified in [RFC9293]. Otherwise, the incoming segment MUST be discarded and a challenge ACK sent back. When a connection is initialized, ISS_CHECK_NOT_REQUIRED is set to false. Once the SND.UNA satisfies condition SND.UNA != ISS && ISS + 1 =< SND.UNA - (2^31-1), ISS_CHECK_NOT_REQUIRED is set to true. When the current send window meets the above condition, any acceptable old ACK values are after ISS, and thus no ghost ACKs by definition. In this case, the original [RFC9293] specification is enough to validate the ACK value. Mitigation Option 2 (assumes [RFC4898] support) TCP stacks that implemented the [RFC4898] tcpEStatsAppHCThruOctetsAcked statistics, which tracks the number of bytes that are already acknowledged by the peer, can adopt this option. To implement this mitigation, TCP stacks MUST add the SND.UNA - min(MAX.SND.WND, tcpEStatsAppHCThruOctetsAcked) =< SEG.ACK input checks for ACK values of any incoming segments. Segments with ACK values satisfying the above condition are further validated and processed as specified in [RFC9293]. Otherwise, the segment MUST be discarded and an ACK sent back. For TCP stacks that do not implement the [RFC5961] ACK validation, MAX.SND.WND should be replaced with 2^31 - 1. 5. References 5.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", RFC 793, DOI 10.17487/RFC0793, September 1981, . Pan & Rossow Expires 9 January 2025 [Page 4] Internet-Draft Ghost ACKs Mitigation July 2024 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4898] Mathis, M., Heffner, J., and R. Raghunarayan, "TCP Extended Statistics MIB", RFC 4898, DOI 10.17487/RFC4898, May 2007, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022, . 5.2. Informative References [SP2024Spoof] Pan, Y. and C. Rossow, "TCP Spoofing: Reliable Payload Transmission Past the Spoofed TCP Handshake", DOI 10.1109/SP54263.2024.00182, May 2024, . Acknowledgements This template uses extracts from templates written by Pekka Savola, Elwyn Davies and Henrik Levkowetz. 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 Pan & Rossow Expires 9 January 2025 [Page 5] Internet-Draft Ghost ACKs Mitigation July 2024 Email: rossow@cispa.de Pan & Rossow Expires 9 January 2025 [Page 6]