VulnerabilityPublished 18 Oct 2026

CVE-2026-98621: Linux kernel · net: gue: reject invalid REMCSUM offsets

Affects Linux kernelSource CVE Database V5

Not known to be exploited · fixed in c20f755338ca234e523dfc943719f194dfc1f903 : patch Linux kernel soon.

Data as of 26 Oct 2026, 14:05 UTC

Severity

High

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 11% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in c20f755338ca234e523dfc943719f194dfc1f903, 2566866fc30965d915d0b52b5c3323b362619f0e, 97f907203321eee2cc9132d5c9eb47698fb41d35 +5

Exploitation evidence

Disclosed: evidence found.
CVE published 18 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 ≥ fe881ef11cf0220f118816181930494d484c4883, < c20f755338ca234e523dfc943719f194dfc1f903	Fixed in c20f755338ca234e523dfc943719f194dfc1f903
LinuxLinux	Affected ≥ fe881ef11cf0220f118816181930494d484c4883, < 2566866fc30965d915d0b52b5c3323b362619f0e	Fixed in 2566866fc30965d915d0b52b5c3323b362619f0e
LinuxLinux	Affected ≥ fe881ef11cf0220f118816181930494d484c4883, < 97f907203321eee2cc9132d5c9eb47698fb41d35	Fixed in 97f907203321eee2cc9132d5c9eb47698fb41d35
LinuxLinux	Affected ≥ fe881ef11cf0220f118816181930494d484c4883, < 43efb1f997e976f386aa89716f55870cd2cff9ad	Fixed in 43efb1f997e976f386aa89716f55870cd2cff9ad
LinuxLinux	Affected ≥ fe881ef11cf0220f118816181930494d484c4883, < 40aeafbcbc7bc4018e8f74bbbb4d29ed8846f57f	Fixed in 40aeafbcbc7bc4018e8f74bbbb4d29ed8846f57f
LinuxLinux	Affected ≥ fe881ef11cf0220f118816181930494d484c4883, < 4b9bc21b2396c396d88dd909caaf73d5be6ff593	Fixed in 4b9bc21b2396c396d88dd909caaf73d5be6ff593

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available and has been applied in multiple Linux kernel commits. Users and administrators should update to a fixed kernel version incorporating these patches to remediate the vulnerability.

Description

CVE-2026-98621 is a vulnerability in the Linux kernel's Generic UDP Encapsulation (GUE) module where invalid REMCSUM offsets can cause an underflow. This flaw allows a forwarded packet with malformed checksum metadata to trigger a buffer overflow by writing beyond the destination buffer. The issue is fixed by rejecting reversed checksum offset tuples during packet validation.

Impact

Exploitation of this vulnerability can cause a buffer overflow by writing beyond the destination buffer, potentially leading to memory corruption. This could affect kernel stability or security depending on the context in which the malformed packet is processed. No known exploits in the wild have been reported.

Summary

The vulnerability occurs in the Linux kernel's GUE module where the REMCSUM option carries absolute checksum start and offset fields. If the offset is less than the start, an underflow happens when calculating skb->csum_offset. This can lead to a forwarded packet retaining CHECKSUM_PARTIAL status and reaching a network driver that trusts this metadata, causing skb_copy_and_csum_dev() to write two bytes approximately 64 KiB beyond the intended buffer boundary. The fix involves rejecting reversed tuples in validate_gue_flags() to enforce correct ordering of checksum offsets.

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Oct 2026, 10:08 UTC

Technical details

References

Community reviews

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