# CVE-2026-46107: dm-thin: fix metadata refcount underflow

> A refcount underflow in the Linux kernel's dm-thin module can cause errors and potential denial of service when shared btree nodes are rebalanced.

- **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:** 28 May 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-89061, BELL-CVE-2026-46107, DEBIAN-CVE-2026-46107, GHSA-c6hr-v224-3w2w, ROOT-OS-DEBIAN-11-CVE-2026-46107, ROOT-OS-DEBIAN-12-CVE-2026-46107, ROOT-OS-UBUNTU-2204-CVE-2026-46107, ROOT-OS-UBUNTU-2404-CVE-2026-46107, UBUNTU-CVE-2026-46107

## What is CVE-2026-46107?

CVE-2026-46107 is a high-severity vulnerability in Linux kernel, published on 28 May 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:

dm-thin: fix metadata refcount underflow

There's a bug in dm-thin in the function rebalance_children. If the
internal btree node has one entry, the code tries to copy all btree
entries from the node's child to the node itself and then decrement the
child's reference count.

If the child node is shared (it has reference count > 1), we won't free
it, so there would be two pointers to each of the grandchildren nodes.
But the reference counts of the grandchildren is not increased, thus the
reference count doesn't match the number of pointers that point to the
grandchildren. This results in "device mapper: space map common: unable
to decrement block" errors.

Fix this bug by incrementing reference counts on the grandchildren if the
btree node is shared.

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound)), CWE-667 (Improper Locking)

## Which versions are affected by CVE-2026-46107?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-46107?

Apply the vendor's patch.

## Is CVE-2026-46107 being exploited?

There is no known exploitation of CVE-2026-46107 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-2026-46107 and fix it if it is affected.

What it is: CVE-2026-46107 is a high-severity vulnerability in Linux kernel, published on 28 May 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-2026-46107 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-2026-46107/ (Markdown: https://insights.sbomify.com/CVE-2026-46107/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-46107
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-46107
- OSV: https://osv.dev/vulnerability/CVE-2026-46107

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-46107/
Summaries are generated and may contain mistakes; the official description above is the record.