Improve TCP Handling of Out-of-Window Packets to Mitigate Ghost ACKs

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.¶

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 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 aims to improve 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.¶

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 2³¹ - 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.¶

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. Both mitigation options assume [RFC5961] is already supported by the TCP stack.¶

          if (!NO_ISS_CHECK && SND.UNA >= ISS + (65535 << Snd.Wind.Shift)) {
              /* Checking SEG.ACK against ISS is definitely redundant. */
              NO_ISS_CHECK = true;
          }

          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;
              }
          }

          if (SEG.ACK < ACK.MIN) {
                send_challenge_ack;
                return;
          }

5. References