CVE-2026-98255: Linux kernel · tcp: exclude old ACKs from tcp fast path
Affects Linux kernelSource GCVE Database
Not known to be exploited.
Data as of 28 Oct 2026, 01:27 UTC
Rated by Threat Radar · CVSS not yet scored by NVD
Exploitation evidence
Disclosed: evidence found.
CVE published 6 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.
What to do
Affected software
| Product | Affected | Fixed in |
|---|---|---|
Ubuntu 26.04 LTSlinuxpkg:deb/ubuntu/linux?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSlinux-awspkg:deb/ubuntu/linux-aws?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSlinux-azurepkg:deb/ubuntu/linux-azure?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSlinux-azure-fdepkg:deb/ubuntu/linux-azure-fde?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSlinux-gcppkg:deb/ubuntu/linux-gcp?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSlinux-gkepkg:deb/ubuntu/linux-gke?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
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.
References
- ubuntu.com/security/CVE-2026-98255 (opens in a new tab)
- cve.org/CVERecord?id=CVE-2026-98255 (opens in a new tab)
- git.kernel.org/linus/f81e6c3fb06327bc49cdd6e559845293ba06a704 (opens in a new tab)
- git.kernel.org/stable/c/07cacab90290f795bad2e566790f4ba8e84cdf5f (opens in a new tab)
- git.kernel.org/stable/c/1c8448db5cf35a13d82b0c9004ffdb49358883e7 (opens in a new tab)
- git.kernel.org/stable/c/6eb9b697f50b73b9684e466c1e97f4ca8e947a59 (opens in a new tab)
- git.kernel.org/stable/c/8205ac5ddb42a1843fa8d6a8c295b5c7f4f8c6ed (opens in a new tab)
- git.kernel.org/stable/c/8309dc2cc360056aa9cd2acfbfc25930798c3b66 (opens in a new tab)
- git.kernel.org/stable/c/b86e1e5a4bbcbbccde8b7a3d3486604b581f6628 (opens in a new tab)
- git.kernel.org/stable/c/f81e6c3fb06327bc49cdd6e559845293ba06a704 (opens in a new tab)
- git.kernel.org/stable/c/ff037920c688ae6ca1153fec01e90e6782712455 (opens in a new tab)