---
id: "CVE-2026-99126"
kind: "vulnerability"
aliases: ["CVE-2026-99126"]
title: "Linux kernel s390 CTCM driver crashes on NULL XID pointer"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["ibm-z", "infrastructure", "linux-kernel", "mainframe", "s390"]
not_affected_if: ["ctcm-not-loaded", "non-s390-architecture"]
published_at: "2026-10-28T11:17:32+00:00"
first_seen_at: "2026-10-28T11:32:35+00:00"
updated_at: "2026-10-28T11:37:29+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99126", "first_seen_at": "2026-10-28T11:32:35+00:00"}]
---

# Linux kernel s390 CTCM driver crashes on NULL XID pointer

The s390 CTCM channel-to-channel network driver in the Linux kernel can dereference a NULL pointer during XID validation when the peer XID is missing or read/write channel pairing mismatches. The sources describe a kernel null dereference, which typically leads to a crash or denial of service rather than code execution.

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

## How to check

- `uname -r` to confirm the running kernel version, then compare against the fixed kernel release in the vendor advisory
- Check whether the ctcm module is loaded with `lsmod | grep ctcm` on IBM Z (s390) systems
- unknown for any other specific check, as the sources do not say

## Fix

- Update the Linux kernel to a release containing the s390/ctcm XID null dereference fix; the exact version is unknown from the sources
- unknown for any other action, as the sources do not say

## Timeline

- 2026-10-28T11:32:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99126

## From the sources

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

s390/ctcm: Prevent XID null dereference

The mpc_validate_xid() function sets grp->saved_xid2->xid2_flag2 to 0x40
to signal XID validation error. If peer XID is NULL or r/w channel
pairing mismatch happens, grp->saved_xid2 is never initialized. An
attempt to set the flag in such case leads to NULL dereference.

Fix this by using the always available priv->xid->xid2_flag2 instead of
grp->saved_xid2->xid2_flag2 for validation errors.

---

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