CVE-2026-98255: tcp: exclude old ACKs from tcp fast path

CVE-2026-98255: tcp: exclude old ACKs from tcp fast path

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-98255 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 5.10.271 with commit 07cacab90290f795bad2e566790f4ba8e84cdf5f
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 5.15.222 with commit 6eb9b697f50b73b9684e466c1e97f4ca8e947a59
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 6.1.189 with commit b86e1e5a4bbcbbccde8b7a3d3486604b581f6628
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 6.6.158 with commit 1c8448db5cf35a13d82b0c9004ffdb49358883e7
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 6.12.112 with commit 8309dc2cc360056aa9cd2acfbfc25930798c3b66
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 6.18.54 with commit ff037920c688ae6ca1153fec01e90e6782712455
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 7.2.8 with commit 8205ac5ddb42a1843fa8d6a8c295b5c7f4f8c6ed
	Issue introduced in 4.14 with commit 31770e34e43d6c8dee129bfee77e56c34e61f0e5 and fixed in 7.3-rc4 with commit f81e6c3fb06327bc49cdd6e559845293ba06a704

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-98255
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/tcp_input.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/07cacab90290f795bad2e566790f4ba8e84cdf5f
	https://git.kernel.org/stable/c/6eb9b697f50b73b9684e466c1e97f4ca8e947a59
	https://git.kernel.org/stable/c/b86e1e5a4bbcbbccde8b7a3d3486604b581f6628
	https://git.kernel.org/stable/c/1c8448db5cf35a13d82b0c9004ffdb49358883e7
	https://git.kernel.org/stable/c/8309dc2cc360056aa9cd2acfbfc25930798c3b66
	https://git.kernel.org/stable/c/ff037920c688ae6ca1153fec01e90e6782712455
	https://git.kernel.org/stable/c/8205ac5ddb42a1843fa8d6a8c295b5c7f4f8c6ed
	https://git.kernel.org/stable/c/f81e6c3fb06327bc49cdd6e559845293ba06a704