VulnerabilityPublished 6 Oct 2026Updated 19 Oct 2026

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

Affects Linux kernelSource CVE Database V5

Not known to be exploited · fixed in ff037920c688ae6ca1153fec01e90e6782712455 : update Linux kernel when convenient.

Data as of 26 Oct 2026, 12:55 UTC

Severity

Medium

Rated Medium by Threat Radar's AI analysis · CVSS not yet scored by NVD

EPSS: 0.1% chance of exploitation in 30 days – higher than 7% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in ff037920c688ae6ca1153fec01e90e6782712455, f81e6c3fb06327bc49cdd6e559845293ba06a704, b86e1e5a4bbcbbccde8b7a3d3486604b581f6628 +5

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.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < ff037920c688ae6ca1153fec01e90e6782712455	Fixed in ff037920c688ae6ca1153fec01e90e6782712455
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < f81e6c3fb06327bc49cdd6e559845293ba06a704	Fixed in f81e6c3fb06327bc49cdd6e559845293ba06a704
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < b86e1e5a4bbcbbccde8b7a3d3486604b581f6628	Fixed in b86e1e5a4bbcbbccde8b7a3d3486604b581f6628
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < 8309dc2cc360056aa9cd2acfbfc25930798c3b66	Fixed in 8309dc2cc360056aa9cd2acfbfc25930798c3b66
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < 8205ac5ddb42a1843fa8d6a8c295b5c7f4f8c6ed	Fixed in 8205ac5ddb42a1843fa8d6a8c295b5c7f4f8c6ed
LinuxLinux	Affected ≥ 31770e34e43d6c8dee129bfee77e56c34e61f0e5, < 07cacab90290f795bad2e566790f4ba8e84cdf5f	Fixed in 07cacab90290f795bad2e566790f4ba8e84cdf5f

AI-suggested

A fix has been implemented in the Linux kernel, excluding old ACKs from the TCP fast path and ensuring proper validation in the slow path. Users should update to the fixed Linux kernel versions identified by the commit hashes provided. No additional mitigation is required beyond applying these updates.

Description

A vulnerability in the Linux kernel TCP implementation allowed old ACK packets, outside the expected sequence window, to be processed in the fast path without proper validation. This could lead to connection state being incorrectly modified by segments with excessively old ACKs. The issue has been resolved by excluding such old ACKs from the fast path, ensuring they are validated in the slow path according to RFC5961.

Impact

The vulnerability could allow an attacker to send TCP segments with old ACK numbers that bypass proper validation, potentially causing the receive sequence number to advance incorrectly. This could disrupt normal TCP connection state management, possibly leading to connection instability or denial of service. However, no known exploits in the wild have been reported.

Summary

CVE-2026-98255 addresses a flaw in the Linux kernel's TCP stack where old ACKs, specifically those before SND.UNA or after SND.NXT, were processed in the TCP fast path without appropriate validation. This could allow payload carried by segments with excessively old ACKs to advance the receive next sequence number (RCV.NXT) improperly. The fix excludes these old ACKs from the fast path, forcing them to be handled in the slow path where tcp_ack() performs validation and challenge ACK handling per RFC5961 and prior commits. This prevents acceptance of invalid ACKs and protects connection state integrity.

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Oct 2026, 12:08 UTC

Technical details

References

Community reviews

Related threats