# GVD-2026-0003: Use-after-free in the Linux kernel during IPv4 TCP connection setup

| | |
|---|---|
| Advisory | GVD-2026-0003 |
| Kind | Third Party |
| Severity | Critical |
| Published | 2026-10-30 |
| Last updated | 2026-10-30 |
| Fixed in | None yet |
| CSAF | https://security.gravwell.io/.well-known/csaf/white/2026/gvd-2026-0003.json |
| Web page | https://security.gravwell.io/advisories/GVD-2026-0003/ |

## What you need to do

### Self-Hosted

Do these steps on each Linux host that runs Gravwell services or ingesters:

1. Install the kernel update for CVE-2026-98432 from the vendor of your operating system. On Debian 13, the fixed
   version is `linux` 6.12.113-1.
2. Reboot the host. The fixed kernel runs only after a reboot.
3. Make sure that `uname -r` shows the fixed kernel version.

If you run Gravwell in Docker, do these steps on the Docker host. Containers use the kernel of their host.

### Gravwell Cloud

No action required from users of Gravwell Cloud services.

If you run edge-deployed ingesters or federators on your own hosts, do the Self-Hosted steps on those hosts.

## Vulnerabilities

| Identifier | Severity | CVSS 4.0 | CWE | Status | Fixed in |
|---|---|---|---|---|---|
| CVE-2026-98432 | Critical | 9.3 | CWE-416 | Under investigation | - |

### CVE-2026-98432: Use-after-free in the Linux kernel during IPv4 TCP connection setup

- Component: The Linux kernel on hosts that run Gravwell services or ingesters
- Status: Under investigation
- Affected versions: All versions
- CWE: CWE-416: Use After Free
- CVSS 4.0 (Gravwell's assessment): 9.3 Critical, `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`
- CVSS 3.1 (Gravwell's assessment): 9.8 Critical, `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

CVE-2026-98432 is a use-after-free in the Linux kernel during IPv4 TCP connection setup. An attacker on the
network can trigger the bug without credentials. It affects kernels from 4.4 up to the stable releases that
contain the fix. Debian 13 fixes it in `linux` 6.12.113-1.

On 2026-10-29, the Go security team reported a demonstrated attack that ran code remotely through this bug. The
attack used Multipath TCP (MPTCP) connections. Programs built with Go 1.24 or later accept MPTCP connections by
default. The Go team states that the bug is not specific to MPTCP. A kernel update is necessary to fix it.

Upstream references:

- [CVE record: CVE-2026-98432](https://www.cve.org/CVERecord?id=CVE-2026-98432)
- [NVD: CVE-2026-98432](https://nvd.nist.gov/vuln/detail/CVE-2026-98432)
- [Debian security tracker: CVE-2026-98432](https://security-tracker.debian.org/tracker/CVE-2026-98432)
- [oss-security: Go announcement of the MPTCP exploit](https://www.openwall.com/lists/oss-security/2026/10/30/1)
- [Go Issue #82717](https://go.dev/issue/82717)

## Summary

CVE-2026-98432 is a use-after-free in the Linux kernel during IPv4 TCP connection setup. An attacker on the network can
trigger the bug without credentials. On 2026-10-29, the Go security team reported a demonstrated attack that ran code
remotely through this bug. The attack used Multipath TCP (MPTCP) connections, which programs built with Go 1.24 or
later accept by default. Gravwell builds current releases with Go 1.26.

Self-Hosted customers must install the kernel update from the vendor of their operating system, then reboot each host.
No action required from users of Gravwell Cloud services. Gravwell continues to investigate the impact on Gravwell
components. When the investigation ends, Gravwell will update this advisory.

## Mitigations

If you cannot reboot a host soon, run these commands on that host. They disable MPTCP and restart the Gravwell
services.

```
sudo sysctl -w net.mptcp.enabled=0
sudo systemctl restart 'gravwell_*'
```

The restart is necessary because a listener that is already open keeps MPTCP until its service restarts.

If a Gravwell container does not use `--network host`, start it with `--sysctl net.mptcp.enabled=0`. Such a
container has its own network stack. The host setting does not apply to it.

This setting blocks only the demonstrated attack. It does not fix the kernel bug.

## Changelog

- 2026-10-30: Initial publication while Gravwell investigates.

Questions this advisory doesn't answer? Email the Gravwell Security Team at security@gravwell.io.