# CVE-2021-28691: Use after free in Linux

> A malicious network frontend can trigger a use-after-free in the Linux kernel netback driver, potentially causing denial of service or code execution.

- **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:** 29 Jun 2021
- **Updated:** 3 Aug 2024
- **Also known as:** AZL-6537, DEBIAN-CVE-2021-28691, GHSA-5gvc-4qph-j93r, UBUNTU-CVE-2021-28691

## What is CVE-2021-28691?

CVE-2021-28691 is a high-severity use after free vulnerability in Linux kernel, published on 29 Jun 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Guest triggered use-after-free in Linux xen-netback A malicious or buggy network PV frontend can force Linux netback to disable the interface and terminate the receive kernel thread associated with queue 0 in response to the frontend sending a malformed packet. Such kernel thread termination will lead to a use-after-free in Linux netback when the backend is destroyed, as the kernel thread associated with queue 0 will have already exited and thus the call to kthread_stop will be performed against a stale pointer.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-28691?

It affects 11 products.


Products: Linux kernel, Netapp Cloud Backup, Netapp H300e Firmware, Netapp H300s Firmware, Netapp H410c Firmware, Netapp H410s Firmware, Netapp H500e Firmware, Netapp H500s Firmware, Netapp H700e Firmware, Netapp H700s Firmware, Netapp Solidfire Baseboard Management Controller.

## How do I fix CVE-2021-28691?

Apply the vendor's patch.

## Is CVE-2021-28691 being exploited?

There is no known exploitation of CVE-2021-28691 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-2021-28691 and fix it if it is affected.

What it is: CVE-2021-28691 is a high-severity use after free vulnerability in Linux kernel, published on 29 Jun 2021. 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-2021-28691 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-2021-28691/ (Markdown: https://insights.sbomify.com/CVE-2021-28691/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28691
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28691
- OSV: https://osv.dev/vulnerability/CVE-2021-28691

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-28691/
Summaries are generated and may contain mistakes; the official description above is the record.