# CVE-2025-40300: x86/vmscape: Add conditional IBPB mitigation

> A malicious guest VM can leak sensitive data from the host userspace hypervisor via branch predictor side-channels, requiring kernel patches to mitigate.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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-67238, AZL-74763, BELL-CVE-2025-40300, DEBIAN-CVE-2025-40300, GHSA-x8qw-46ww-c957, ROOT-OS-DEBIAN-11-CVE-2025-40300, ROOT-OS-DEBIAN-12-CVE-2025-40300, ROOT-OS-DEBIAN-13-CVE-2025-40300, ROOT-OS-UBUNTU-2204-CVE-2025-40300, ROOT-OS-UBUNTU-2404-CVE-2025-40300, UBUNTU-CVE-2025-40300, USN-7860-1, USN-7860-2, USN-7860-3, USN-7860-4, USN-7860-5, USN-7862-1, USN-7862-2, USN-7862-3, openSUSE-SU-2025:15553-1

## What is CVE-2025-40300?

CVE-2025-40300 is a medium-severity 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:

x86/vmscape: Add conditional IBPB mitigation

VMSCAPE is a vulnerability that exploits insufficient branch predictor
isolation between a guest and a userspace hypervisor (like QEMU). Existing
mitigations already protect kernel/KVM from a malicious guest. Userspace
can additionally be protected by flushing the branch predictors after a
VMexit.

Since it is the userspace that consumes the poisoned branch predictors,
conditionally issue an IBPB after a VMexit and before returning to
userspace. Workloads that frequently switch between hypervisor and
userspace will incur the most overhead from the new IBPB.

This new IBPB is not integrated with the existing IBPB sites. For
instance, a task can use the existing speculation control prctl() to
get an IBPB at context switch time. With this implementation, the
IBPB is doubled up: one at context switch and another before running
userspace.

The intent is to integrate and optimize these cases post-embargo.

[ dhansen: elaborate on suboptimal IBPB solution ]

## Which versions are affected by CVE-2025-40300?

It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `kernel-source` | openSUSE:Tumbleweed | All versions | 6.16.7-1.1 |

Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-40300?

Apply the vendor's patch.

## Is CVE-2025-40300 being exploited?

There is no known exploitation of CVE-2025-40300 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-2025-40300 and fix it if it is affected.

What it is: CVE-2025-40300 is a medium-severity 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-2025-40300 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-2025-40300/ (Markdown: https://insights.sbomify.com/CVE-2025-40300/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40300
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40300
- OSV: https://osv.dev/vulnerability/CVE-2025-40300

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40300/
Summaries are generated and may contain mistakes; the official description above is the record.