# CVE-2025-39738: btrfs: do not allow relocation of partially dropped subvolumes

> A flaw in the Linux kernel's btrfs filesystem allows a local attacker to trigger a transaction abort and system crash by relocating partially dropped subvolumes.

- **Severity:** High 7.3 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/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:** 11 Sep 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-67244, AZL-73938, BELL-CVE-2025-39738, DEBIAN-CVE-2025-39738, GHSA-8x83-vv5w-vmh3, ROOT-OS-DEBIAN-11-CVE-2025-39738, ROOT-OS-UBUNTU-2204-CVE-2025-39738, ROOT-OS-UBUNTU-2404-CVE-2025-39738, UBUNTU-CVE-2025-39738

## What is CVE-2025-39738?

CVE-2025-39738 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 Sep 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: do not allow relocation of partially dropped subvolumes

[BUG]
There is an internal report that balance triggered transaction abort,
with the following call trace:

  item 85 key (594509824 169 0) itemoff 12599 itemsize 33
          extent refs 1 gen 197740 flags 2
          ref#0: tree block backref root 7
  item 86 key (594558976 169 0) itemoff 12566 itemsize 33
          extent refs 1 gen 197522 flags 2
          ref#0: tree block backref root 7
 ...
 BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0
 BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117
 ------------[ cut here ]------------
 BTRFS: Transaction aborted (error -117)
 WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs]

And btrfs check doesn't report anything wrong related to the extent
tree.

[CAUSE]
The cause is a little complex, firstly the extent tree indeed doesn't
have the backref for 594526208.

The extent tree only have the following two backrefs around that bytenr
on-disk:

        item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33
                refs 1 gen 197740 flags TREE_BLOCK
                tree block skinny level 0
                (176 0x7) tree block backref root CSUM_TREE
        item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33
                refs 1 gen 197522 flags TREE_BLOCK
                tree block skinny level 0
                (176 0x7) tree block backref root CSUM_TREE

But the such missing backref item is not an corruption on disk, as the
offending delayed ref belongs to subvolume 934, and that subvolume is
being dropped:

        item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439
                generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328
                last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0
                drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2
                level 2 generation_v2 198229

And that offending tree block 594526208 is inside the dropped range of
that subvolume.  That explains why there is no backref item for that
bytenr and why btrfs check is not reporting anything wrong.

But this also shows another problem, as btrfs will do all the orphan
subvolume cleanup at a read-write mount.

So half-dropped subvolume should not exist after an RW mount, and
balance itself is also exclusive to subvolume cleanup, meaning we
shouldn't hit a subvolume half-dropped during relocation.

The root cause is, there is no orphan item for this subvolume.
In fact there are 5 subvolumes from around 2021 that have the same
problem.

It looks like the original report has some older kernels running, and
caused those zombie subvolumes.

Thankfully upstream commit 8d488a8c7ba2 ("btrfs: fix subvolume/snapshot
deletion not triggered on mount") has long fixed the bug.

[ENHANCEMENT]
For repairing such old fs, btrfs-progs will be enhanced.

Considering how delayed the problem will show up (at run delayed ref
time) and at that time we have to abort transaction already, it is too
late.

Instead here we reject any half-dropped subvolume for reloc tree at the
earliest time, preventing confusion and extra time wasted on debugging
similar bugs.

## Which versions are affected by CVE-2025-39738?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39738?

Apply the vendor's patch.

## Is CVE-2025-39738 being exploited?

There is no known exploitation of CVE-2025-39738 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39738 and fix it if it is affected.

What it is: CVE-2025-39738 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 Sep 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-2025-39738 in the wild and no public exploit code.

Affected product: Debian Debian 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-2025-39738/ (Markdown: https://insights.sbomify.com/CVE-2025-39738/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39738
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39738
- OSV: https://osv.dev/vulnerability/CVE-2025-39738

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39738/
Summaries are generated and may contain mistakes; the official description above is the record.