# CVE-2026-31402: nfsd: fix heap overflow in NFSv4.0 LOCK replay cache

> Remote attackers can trigger a heap overflow in the Linux kernel NFS server via crafted NFSv4.0 LOCK requests, potentially leading to code execution or system crashes.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Apr 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-31402, DEBIAN-CVE-2026-31402, GHSA-7xf5-3qmr-j4c6, ROOT-OS-DEBIAN-11-CVE-2026-31402, ROOT-OS-DEBIAN-12-CVE-2026-31402, ROOT-OS-DEBIAN-13-CVE-2026-31402, ROOT-OS-UBUNTU-2204-CVE-2026-31402, ROOT-OS-UBUNTU-2404-CVE-2026-31402, UBUNTU-CVE-2026-31402

## What is CVE-2026-31402?

CVE-2026-31402 is a critical memory corruption vulnerability in Linux kernel, published on 3 Apr 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 heap overflow in NFSv4.0 LOCK replay cache

The NFSv4.0 replay cache uses a fixed 112-byte inline buffer
(rp_ibuf[NFSD4_REPLAY_ISIZE]) to store encoded operation responses.
This size was calculated based on OPEN responses and does not account
for LOCK denied responses, which include the conflicting lock owner as
a variable-length field up to 1024 bytes (NFS4_OPAQUE_LIMIT).

When a LOCK operation is denied due to a conflict with an existing lock
that has a large owner, nfsd4_encode_operation() copies the full encoded
response into the undersized replay buffer via read_bytes_from_xdr_buf()
with no bounds check. This results in a slab-out-of-bounds write of up
to 944 bytes past the end of the buffer, corrupting adjacent heap memory.

This can be triggered remotely by an unauthenticated attacker with two
cooperating NFSv4.0 clients: one sets a lock with a large owner string,
then the other requests a conflicting lock to provoke the denial.

We could fix this by increasing NFSD4_REPLAY_ISIZE to allow for a full
opaque, but that would increase the size of every stateowner, when most
lockowners are not that large.

Instead, fix this by checking the encoded response length against
NFSD4_REPLAY_ISIZE before copying into the replay buffer. If the
response is too large, set rp_buflen to 0 to skip caching the replay
payload. The status is still cached, and the client already received the
correct response on the original request.

Weakness: CWE-787 (Out-of-bounds Write), CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-31402?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31402?

Apply the vendor's patch.

## Is CVE-2026-31402 being exploited?

There is no known exploitation of CVE-2026-31402 in the wild and no public exploit code.

Public exploit code:

- CVE-2026-31402 (github_poc): https://github.com/0xredash/CVE-2026-31402

## 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-2026-31402 and fix it if it is affected.

What it is: CVE-2026-31402 is a critical memory corruption vulnerability in Linux kernel, published on 3 Apr 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-2026-31402 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-2026-31402/ (Markdown: https://insights.sbomify.com/CVE-2026-31402/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31402
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31402
- OSV: https://osv.dev/vulnerability/CVE-2026-31402

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31402/
Summaries are generated and may contain mistakes; the official description above is the record.