# CVE-2026-89678: nfsd: fix partial-write detection in nfsd_direct_write

> A logic error in the Linux kernel NFS server allows partial writes to corrupt file offsets, leading to data modification.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-89678, GHSA-qpfv-2qf2-xr5w, UBUNTU-CVE-2026-89678

## What is CVE-2026-89678?

CVE-2026-89678 is a high-severity vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

nfsd: fix partial-write detection in nfsd_direct_write

nfsd_direct_write() walks a list of write segments and, after each
vfs_iocb_iter_write(), tries to detect a short write so the loop can
stop before placing the next segment at a wrong file offset:

    host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter);
    if (host_err < 0)
            return host_err;
    *cnt += host_err;
    if (host_err < segments[i].iter.count)
            break;	/* partial write */

vfs_iocb_iter_write() runs the iter through ->write_iter(), which
advances the iter by the number of bytes written. By the time the
check runs, segments[i].iter.count is the residual, not the original
request length:

    before write_iter: iter.count == original_len
    after  write_iter: iter.count == original_len - host_err

The condition then reduces to host_err < original_len - host_err, so
the break fires only when less than half of the segment was written.
Any short write completing between 50% and 99% of the segment slips
through; the loop advances to the next segment with kiocb->ki_pos
only bumped by the short amount, writing the next segment's payload
at the wrong offset and over-reporting *cnt to the NFS client.

Snapshot the segment's byte count before the write and compare
host_err against that snapshot so any short write breaks the loop.

## Which versions are affected by CVE-2026-89678?


## How do I fix CVE-2026-89678?

Apply the vendor's patch.

## Is CVE-2026-89678 being exploited?

There is no known exploitation of CVE-2026-89678 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89678 and fix it if it is affected.

What it is: CVE-2026-89678 is a high-severity vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89678 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-2026-89678/ (Markdown: https://insights.sbomify.com/CVE-2026-89678/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89678
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89678
- OSV: https://osv.dev/vulnerability/CVE-2026-89678

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89678/
Summaries are generated and may contain mistakes; the official description above is the record.