# CVE-2026-23249: xfs: check for deleted cursors when revalidating two btrees

> A null pointer dereference in the Linux kernel XFS filesystem can cause a system crash during btree revalidation.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Mar 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-23249, DEBIAN-CVE-2026-23249, GHSA-w9wm-69p8-g28q, ROOT-OS-DEBIAN-13-CVE-2026-23249, ROOT-OS-UBUNTU-2404-CVE-2026-23249, UBUNTU-CVE-2026-23249

## What is CVE-2026-23249?

CVE-2026-23249 is a medium-severity vulnerability in Linux kernel, published on 18 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

xfs: check for deleted cursors when revalidating two btrees

The free space and inode btree repair functions will rebuild both btrees
at the same time, after which it needs to evaluate both btrees to
confirm that the corruptions are gone.

However, Jiaming Zhang ran syzbot and produced a crash in the second
xchk_allocbt call.  His root-cause analysis is as follows (with minor
corrections):

 In xrep_revalidate_allocbt(), xchk_allocbt() is called twice (first
 for BNOBT, second for CNTBT). The cause of this issue is that the
 first call nullified the cursor required by the second call.

 Let's first enter xrep_revalidate_allocbt() via following call chain:

 xfs_file_ioctl() ->
 xfs_ioc_scrubv_metadata() ->
 xfs_scrub_metadata() ->
 `sc->ops->repair_eval(sc)` ->
 xrep_revalidate_allocbt()

 xchk_allocbt() is called twice in this function. In the first call:

 /* Note that sc->sm->sm_type is XFS_SCRUB_TYPE_BNOPT now */
 xchk_allocbt() ->
 xchk_btree() ->
 `bs->scrub_rec(bs, recp)` ->
 xchk_allocbt_rec() ->
 xchk_allocbt_xref() ->
 xchk_allocbt_xref_other()

 since sm_type is XFS_SCRUB_TYPE_BNOBT, pur is set to &sc->sa.cnt_cur.
 Kernel called xfs_alloc_get_rec() and returned -EFSCORRUPTED. Call
 chain:

 xfs_alloc_get_rec() ->
 xfs_btree_get_rec() ->
 xfs_btree_check_block() ->
 (XFS_IS_CORRUPT || XFS_TEST_ERROR), the former is false and the latter
 is true, return -EFSCORRUPTED. This should be caused by
 ioctl$XFS_IOC_ERROR_INJECTION I guess.

 Back to xchk_allocbt_xref_other(), after receiving -EFSCORRUPTED from
 xfs_alloc_get_rec(), kernel called xchk_should_check_xref(). In this
 function, *curpp (points to sc->sa.cnt_cur) is nullified.

 Back to xrep_revalidate_allocbt(), since sc->sa.cnt_cur has been
 nullified, it then triggered null-ptr-deref via xchk_allocbt() (second
 call) -> xchk_btree().

So.  The bnobt revalidation failed on a cross-reference attempt, so we
deleted the cntbt cursor, and then crashed when we tried to revalidate
the cntbt.  Therefore, check for a null cntbt cursor before that
revalidation, and mark the repair incomplete.  Also we can ignore the
second tree entirely if the first tree was rebuilt but is already
corrupt.

Apply the same fix to xrep_revalidate_iallocbt because it has the same
problem.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2026-23249?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23249?

Apply the vendor's patch.

## Is CVE-2026-23249 being exploited?

There is no known exploitation of CVE-2026-23249 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23249 and fix it if it is affected.

What it is: CVE-2026-23249 is a medium-severity vulnerability in Linux kernel, published on 18 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-23249 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-23249/ (Markdown: https://insights.sbomify.com/CVE-2026-23249/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23249
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23249
- OSV: https://osv.dev/vulnerability/CVE-2026-23249

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23249/
Summaries are generated and may contain mistakes; the official description above is the record.