CVE-2026-98255 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
tcp: exclude old ACKs from tcp fast path
Exclude old ACKs before SND.UNA from the tcp fast path as well as ACKs after SND.NXT.
Such ACKs will fall through to the slow path, where tcp_ack() performs the appropriate validation and challenge ACK handling according to RFC5961 and Commit 3d501dd326fb1c7 ("tcp: do not accept ACK of bytes we never sent").
This prevents old ACKs from being accepted or modifying connection state as part of the fast path before appropriate ACK validation is applied. In particular, this prevents payload carried by a segment with an excessively old ACK from advancing RCV.NXT before the ACK is rejected.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel TCP stack implementation contains a logic flaw in its fast-path acknowledgment processing mechanism that allows for the acceptance of invalid or stale acknowledgments under specific conditions. This vulnerability stems from insufficient validation checks within the high-performance code path designed to handle incoming network segments efficiently. Specifically, the system fails to adequately filter out old ACKs that fall outside the valid sequence number window defined by SND.UNA and SND.NXT before these packets are processed for state updates. By allowing such acknowledgments to bypass rigorous scrutiny in the fast path, the kernel inadvertently permits malformed or maliciously crafted TCP segments to influence critical connection variables without undergoing the necessary security checks reserved for slower processing paths.
The technical core of this issue lies in how the Transmission Control Protocol manages sequence numbers and acknowledgment windows during data transmission. In a healthy TCP session, acknowledgments must correspond to bytes that have been successfully sent but not yet acknowledged by the receiver. The vulnerability allows ACKs older than SND.UNA or newer than SND.NXT to be processed prematurely. These out-of-window acknowledgments are typically handled in the slow path where tcp_ack() performs comprehensive validation against RFC 5961 and specific kernel commits aimed at preventing acceptance of ACKs for bytes never sent. However, when these invalid ACKs slip through the fast path, they can trigger unintended side effects before the slower, more secure validation routines have a chance to reject them. This bypass undermines the integrity checks designed to protect against sequence number prediction attacks and other TCP-based exploits.
The operational impact of this vulnerability is significant for network stability and security. The primary consequence is that payload carried by segments with excessively old ACKs can incorrectly advance RCV.NXT, which represents the next expected byte in a receive stream. This premature advancement disrupts the normal flow control mechanisms of TCP, potentially leading to data corruption or loss if subsequent packets are processed based on an incorrect state assumption. Furthermore, this behavior creates a potential vector for denial-of-service attacks where an adversary could manipulate connection states by injecting carefully crafted ACK segments that appear valid in the fast path but are technically invalid according to protocol standards. This can lead to resource exhaustion as the system attempts to manage connections with corrupted sequence number tracking or cause unexpected connection resets due to state inconsistencies.
To mitigate this risk, it is essential to apply the latest kernel patches that enforce strict validation of acknowledgment numbers before they interact with fast-path logic. System administrators should ensure their Linux environments are updated to versions where tcp_ack() and related functions properly filter out ACKs outside the SND.UNA to SND.NXT range prior to any state modification. Additionally, network security strategies should include monitoring for anomalous TCP traffic patterns that exhibit signs of sequence number manipulation or unusual acknowledgment frequencies. Implementing ingress filtering rules at network boundaries can also help reduce exposure by blocking packets with suspicious header values before they reach vulnerable kernel components. Adherence to industry standards such as CWE-20 Improper Input Validation and ATT&CK techniques related to protocol abuse is recommended for comprehensive defense-in-depth strategies against this class of vulnerabilities.