# CVE-2021-20257: Denial of service in QEMU

> A guest can trigger an infinite loop in the QEMU e1000 NIC emulator, consuming host CPU cycles and causing a denial of service.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/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:** 16 Mar 2022
- **Updated:** 3 Aug 2024
- **Also known as:** AZL-9068, DEBIAN-CVE-2021-20257, GHSA-j586-x8f6-9xf2, UBUNTU-CVE-2021-20257

## What is CVE-2021-20257?

CVE-2021-20257 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 16 Mar 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

An infinite loop flaw was found in the e1000 NIC emulator of the QEMU. This issue occurs while processing transmits (tx) descriptors in process_tx_desc if various descriptor fields are initialized with invalid values. This flaw allows a guest to consume CPU cycles on the host, resulting in a denial of service. The highest threat from this vulnerability is to system availability.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2021-20257?

It affects 8 products.


Products: Debian Debian Linux, Fedoraproject Fedora, Qemu Qemu, Redhat Codeready Linux Builder, Redhat Enterprise Linux, Redhat Enterprise Linux for Ibm Z Systems, Redhat Enterprise Linux for Power Little Endian, Redhat Openstack Platform.

## How do I fix CVE-2021-20257?

Apply the vendor's patch.

## Is CVE-2021-20257 being exploited?

There is no known exploitation of CVE-2021-20257 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-2021-20257 and fix it if it is affected.

What it is: CVE-2021-20257 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 16 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-2021-20257 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-2021-20257/ (Markdown: https://insights.sbomify.com/CVE-2021-20257/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20257
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20257
- OSV: https://osv.dev/vulnerability/CVE-2021-20257

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20257/
Summaries are generated and may contain mistakes; the official description above is the record.