# CVE-2026-64298: NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

> An attacker with read-only access can truncate files via NFSv4 due to missing write permission checks during O_TRUNC operations.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/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:** 25 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-93668, BELL-CVE-2026-64298, DEBIAN-CVE-2026-64298, GHSA-mj8v-3x8m-9rx7, ROOT-OS-DEBIAN-12-CVE-2026-64298, ROOT-OS-UBUNTU-2204-CVE-2026-64298, UBUNTU-CVE-2026-64298

## What is CVE-2026-64298?

CVE-2026-64298 is a high-severity missing authorization vulnerability in Linux kernel, published on 25 Jul 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:

NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

POSIX requires write permission to truncate a file, so an open() that
specifies O_TRUNC must be authorized for write access regardless of the
O_ACCMODE access mode.

nfs_open_permission_mask() builds the access mask passed to
nfs_may_open(), which is the local authorization gate for OPENs the
client serves itself from a cached write delegation via the
can_open_delegated() path in nfs4_try_open_cached().  The mask is
derived from O_ACCMODE alone, so an open(O_RDONLY | O_TRUNC) against a
file the caller cannot write requests only MAY_READ and passes the
local check.  The OPEN is then satisfied locally and the truncation is
issued to the server as a SETATTR(size=0) over the delegation stateid,
which the server accepts under standard write-delegation semantics.
POSIX requires that this open fail with EACCES.

Include MAY_WRITE in the mask whenever O_TRUNC is set so the local
check matches the access the server would have enforced.

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-64298?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64298?

Apply the vendor's patch.

## Is CVE-2026-64298 being exploited?

There is no known exploitation of CVE-2026-64298 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-2026-64298 and fix it if it is affected.

What it is: CVE-2026-64298 is a high-severity missing authorization vulnerability in Linux kernel, published on 25 Jul 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-2026-64298 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-2026-64298/ (Markdown: https://insights.sbomify.com/CVE-2026-64298/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64298
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64298
- OSV: https://osv.dev/vulnerability/CVE-2026-64298

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64298/
Summaries are generated and may contain mistakes; the official description above is the record.