---
id: "CVE-2026-98621"
kind: "vulnerability"
aliases: ["CVE-2026-98621"]
title: "Linux kernel: GUE REMCSUM offsets can write past a buffer"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["gue-tunnels", "kernel", "linux", "networking", "package"]
not_affected_if: ["default-config", "no-gue-tunnels"]
published_at: "2026-10-18T16:17:14+00:00"
first_seen_at: "2026-10-23T21:57:16+00:00"
updated_at: "2026-10-25T05:21:15+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98621", "first_seen_at": "2026-10-23T21:57:16+00:00"}]
---

# Linux kernel: GUE REMCSUM offsets can write past a buffer

The Linux kernel's GUE tunnel parser accepted REMCSUM option offsets in reversed order, which underflows a 16-bit checksum offset. A forwarded packet can then make a hardware checksum driver write two bytes about 64 KiB past its destination buffer.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and compare with the first fixed kernel in your distribution's advisory
- Check whether GUE tunnels (Foo-over-UDP with the REMCSUM option) are configured with `ip link show type gue`

## Fix

- Update the kernel to a release that includes the upstream fix ("reject invalid REMCSUM offsets"), using the version your distribution publishes
- Reboot into the patched kernel

## Timeline

- 2026-10-23T21:57:16+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98621

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

net: gue: reject invalid REMCSUM offsets

The REMCSUM option carries an absolute checksum start and checksum field
offset. gue_remcsum() passes them to skb_remcsum_process(), whose
partial path stores offset - start in the u16 skb->csum_offset variable.
If offset is less than start, this underflows.

A forwarded packet can retain CHECKSUM_PARTIAL and reach a NETIF_F_HW_CSUM
driver which trusts the metadata, leading skb_copy_and_csum_dev() to write
two bytes about 64 KiB beyond the destination buffer.

Reject reversed tuples in validate_gue_flags(), after the existing length
validation, so all GUE parsers enforce the ordering in one place.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_