# CVE-2025-40205: btrfs: avoid potential out-of-bounds in btrfs_encode_fh()

> A potential out-of-bounds write in the Linux kernel's btrfs filesystem allows attackers to corrupt memory, potentially leading to code execution or privilege escalation.

- **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:** 12 Nov 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-70097, BELL-CVE-2025-40205, DEBIAN-CVE-2025-40205, GHSA-7v4m-qg52-mxg3, ROOT-OS-UBUNTU-2404-CVE-2025-40205, UBUNTU-CVE-2025-40205

## What is CVE-2025-40205?

CVE-2025-40205 is a high-severity memory corruption vulnerability in Linux, published on 12 Nov 2025. 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: avoid potential out-of-bounds in btrfs_encode_fh()

The function btrfs_encode_fh() does not properly account for the three
cases it handles.

Before writing to the file handle (fh), the function only returns to the
user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or
BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).

However, when a parent exists and the root ID of the parent and the
inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT
(10 dwords, 40 bytes).

If *max_len is not large enough, this write goes out of bounds because
BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than
BTRFS_FID_SIZE_CONNECTABLE originally returned.

This results in an 8-byte out-of-bounds write at
fid->parent_root_objectid = parent_root_id.

A previous attempt to fix this issue was made but was lost.

https://lore.kernel.org/all/4CADAEEC020000780001B32C@vpn.id2.novell.com/

Although this issue does not seem to be easily triggerable, it is a
potential memory corruption bug that should be fixed. This patch
resolves the issue by ensuring the function returns the appropriate size
for all three cases and validates that *max_len is large enough before
writing any data.

## Which versions are affected by CVE-2025-40205?


## How do I fix CVE-2025-40205?

Apply the vendor's patch.

## Is CVE-2025-40205 being exploited?

There is no known exploitation of CVE-2025-40205 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40205 and fix it if it is affected.

What it is: CVE-2025-40205 is a high-severity memory corruption vulnerability in Linux, published on 12 Nov 2025. 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-2025-40205 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-2025-40205/ (Markdown: https://insights.sbomify.com/CVE-2025-40205/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40205
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40205
- OSV: https://osv.dev/vulnerability/CVE-2025-40205

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40205/
Summaries are generated and may contain mistakes; the official description above is the record.