# CVE-2026-89723: nilfs2: fix slab-out-of-bounds in nilfs_direct_propagate after truncation

> A slab-out-of-bounds error in the nilfs2 filesystem allows attackers to cause denial of service or potentially execute code via crafted file truncation operations.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100109, BELL-CVE-2026-89723, DEBIAN-CVE-2026-89723, GHSA-ffm2-4xxc-m4vx, UBUNTU-CVE-2026-89723

## What is CVE-2026-89723?

CVE-2026-89723 is a high-severity memory corruption vulnerability in Linux, published on 11 Sep 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:

nilfs2: fix slab-out-of-bounds in nilfs_direct_propagate after truncation

Shuangpeng Bai reported that KASAN detected a slab-out-of-bounds error
in nilfs_direct_propagate() during testing.

Analysis revealed that after truncating a file, a node block immediately
below the B-tree root was not deleted.  Instead, it remained in the B-tree
node cache in a dirty state.  The log writer subsequently detected this
block and incorrectly invoked nilfs_direct_propagate() on it, which is
designed to handle only data blocks in direct mapping.

B-tree nodes in the cache are managed by virtual block numbers, and their
logical keys typically exceed the range expected by direct mapping.
Consequently, processing such a node as a direct mapping entry triggers
a slab-out-of-bounds access.

The root cause is that when a B-tree mapping collapses into a direct
mapping during truncation, an intermediate node block pointed to by the
root node is left behind as garbage instead of being explicitly deleted.

This resolves the issue by adding a nilfs_btree_discard() operation
to delete the remaining intermediate node block during the conversion.
A 'deform' flag is added to the bop_delete interface to explicitly signal
that the deletion is part of a mapping transformation.  This allows the
B-tree mapping implementation to perform the necessary cleanup and
discarding of the residual node structure that would be otherwise be left
orphaned after the transition.

## Which versions are affected by CVE-2026-89723?


## How do I fix CVE-2026-89723?

Apply the vendor's patch.

## Is CVE-2026-89723 being exploited?

There is no known exploitation of CVE-2026-89723 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89723 and fix it if it is affected.

What it is: CVE-2026-89723 is a high-severity memory corruption vulnerability in Linux, published on 11 Sep 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-2026-89723 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89723/ (Markdown: https://insights.sbomify.com/CVE-2026-89723/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89723
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89723
- OSV: https://osv.dev/vulnerability/CVE-2026-89723

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89723/
Summaries are generated and may contain mistakes; the official description above is the record.