# CVE-2026-90263: btrfs: check if root is readonly when setting posix acl

> Attackers can modify POSIX ACLs on read-only Btrfs filesystems, bypassing intended write restrictions and altering file permissions.

- **Severity:** Not scored
- **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:** 17 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-102465, BELL-CVE-2026-90263, DEBIAN-CVE-2026-90263, GHSA-wxw8-hpqh-3xg2, UBUNTU-CVE-2026-90263

## What is CVE-2026-90263?

CVE-2026-90263 is a vulnerability in Linux, published on 17 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:

btrfs: check if root is readonly when setting posix acl

For a filesystem which has btrfs read-only property set to true, all
write operations including acl and xattr should be denied. However, acl
can still be set even if btrfs ro property is true.

This happens because no function on the set_acl code path checks the root
is readonly or not. It was checked in btrfs_setxattr_trans() but got
removed in commit 353c2ea735e4 ("btrfs: remove redundant readonly root
check in btrfs_setxattr_trans")

That commit didn't check if all the callers properly check the root's
read-only flag. A previous fix is commit b51111271b03 ("btrfs: check if
root is readonly while setting security xattr").

Always check if the root is read-only before performing the set acl
operation.

## Which versions are affected by CVE-2026-90263?


## How do I fix CVE-2026-90263?

Apply the vendor's patch.

## Is CVE-2026-90263 being exploited?

There is no known exploitation of CVE-2026-90263 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90263 and fix it if it is affected.

What it is: CVE-2026-90263 is a vulnerability in Linux, published on 17 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-2026-90263 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-2026-90263/ (Markdown: https://insights.sbomify.com/CVE-2026-90263/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90263
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90263
- OSV: https://osv.dev/vulnerability/CVE-2026-90263

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90263/
Summaries are generated and may contain mistakes; the official description above is the record.