# CVE-2021-3656: Privilege escalation in KVM

> A malicious L1 guest can bypass intercepts to read/write host physical memory, causing crashes, data leaks, or guest-to-host escape.

- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/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:** 4 Mar 2022
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-3656, UBUNTU-CVE-2021-3656

## What is CVE-2021-3656?

CVE-2021-3656 is a high-severity vulnerability in Fedoraproject Fedora, published on 4 Mar 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "virt_ext" field, this issue could allow a malicious L1 to disable both VMLOAD/VMSAVE intercepts and VLS (Virtual VMLOAD/VMSAVE) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape.

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2021-3656?

It affects 26 products.


Products: Fedoraproject Fedora, Linux kernel, Redhat 3scale Api Management, Redhat Codeready Linux Builder, Redhat Enterprise Linux, Redhat Enterprise Linux Desktop, Redhat Enterprise Linux Eus, Redhat Enterprise Linux for Ibm Z Systems, Redhat Enterprise Linux for Ibm Z Systems Eus, Redhat Enterprise Linux for Power Big Endian, Redhat Enterprise Linux for Power Little Endian, Redhat Enterprise Linux for Power Little Endian Eus, Redhat Enterprise Linux for Real Time, Redhat Enterprise Linux for Real Time for Nfv, Redhat Enterprise Linux for Real Time for Nfv Tus, Redhat Enterprise Linux for Real Time Tus, Redhat Enterprise Linux for Scientific Computing, Redhat Enterprise Linux Server, Redhat Enterprise Linux Server Aus, Redhat Enterprise Linux Server for Power Little Endian Update Services for Sap Solutions and 6 more.

## How do I fix CVE-2021-3656?

Apply the vendor's patch.

## Is CVE-2021-3656 being exploited?

There is no known exploitation of CVE-2021-3656 in the wild and no public exploit code.

Public exploit code:

- https://github.com/rami08448/CVE-2021-3656-Demo (github_poc): https://github.com/rami08448/CVE-2021-3656-Demo

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3656 and fix it if it is affected.

What it is: CVE-2021-3656 is a high-severity vulnerability in Fedoraproject Fedora, published on 4 Mar 2022. 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-2021-3656 in the wild and no public exploit code.

Affected product: Fedoraproject Fedora. 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-2021-3656/ (Markdown: https://insights.sbomify.com/CVE-2021-3656/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3656
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3656
- OSV: https://osv.dev/vulnerability/CVE-2021-3656

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3656/
Summaries are generated and may contain mistakes; the official description above is the record.