# CVE-2021-3929: Use after free in QEMU

> A malicious guest can trigger a use-after-free in QEMU's NVMe emulation to crash the host or execute arbitrary code.

- **Severity:** High 8.2 (`CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Aug 2022
- **Updated:** 28 Feb 2025
- **Also known as:** AZL-10721, AZL-35152, DEBIAN-CVE-2021-3929, GHSA-h66w-323g-4q62, OESA-2022-1733, UBUNTU-CVE-2021-3929, openSUSE-SU-2024:11969-1

## What is CVE-2021-3929?

CVE-2021-3929 is a high-severity use after free vulnerability in Fedoraproject Fedora, published on 25 Aug 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A DMA reentrancy issue was found in the NVM Express Controller (NVME) emulation in QEMU. This CVE is similar to CVE-2021-3750 and, just like it, when the reentrancy write triggers the reset function nvme_ctrl_reset(), data structs will be freed leading to a use-after-free issue. A malicious guest could use this flaw to crash the QEMU process on the host, resulting in a denial of service condition or, potentially, executing arbitrary code within the context of the QEMU process on the host.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-3929?

It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `qemu` | openSUSE:Tumbleweed | All versions | 6.2.0-44.1 |

Products: Fedoraproject Fedora, Qemu Qemu.

## How do I fix CVE-2021-3929?

Apply the vendor's patch.

## Is CVE-2021-3929 being exploited?

There is no known exploitation of CVE-2021-3929 in the wild and no public exploit code.

Public exploit code:

- Recursive MMIO VM Escape PoC (github_poc): https://github.com/QiuhaoLi/CVE-2021-3929-3947
- QEMU 11.1.2 - High - Local - UAF - NVMe SGL span into BAR0 incomplete CVE-2021-3929 (github_poc): https://github.com/abraxas/qemu-nvme-sgl-bar

## 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-2021-3929 and fix it if it is affected.

What it is: CVE-2021-3929 is a high-severity use after free vulnerability in Fedoraproject Fedora, published on 25 Aug 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-2021-3929 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-2021-3929/ (Markdown: https://insights.sbomify.com/CVE-2021-3929/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3929
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3929
- OSV: https://osv.dev/vulnerability/CVE-2021-3929

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3929/
Summaries are generated and may contain mistakes; the official description above is the record.