# CVE-2021-20321: Race condition in kernel

> A local attacker can crash the system by exploiting a race condition in the Linux kernel OverlayFS subsystem during specific rename operations.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:L/AC:H/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:** 18 Feb 2022
- **Updated:** 3 Aug 2024
- **Also known as:** A-222644279, ALSA-2021:5227, DEBIAN-CVE-2021-20321, PUB-A-222644279, RHSA-2021:5227, RHSA-2021:5241, RLSA-2021:5227, RLSA-2021:5241, UBUNTU-CVE-2021-20321

## What is CVE-2021-20321?

CVE-2021-20321 is a medium-severity vulnerability in Debian Debian Linux, published on 18 Feb 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A race condition accessing file object in the Linux kernel OverlayFS subsystem was found in the way users do rename in specific way with OverlayFS. A local user could use this flaw to crash the system.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2021-20321?

It affects 3 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `kernel-tools-libs-devel` | AlmaLinux:8 | All versions | 4.18.0-348.7.1.el8_5 |
| `bpftool` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `bpftool-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-abi-stablelists` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-core` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-cross-headers` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-core` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-devel` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-modules` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-modules-extra` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-aarch64` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-ppc64le` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-s390x` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-x86_64` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-devel` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-doc` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-modules` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-modules-extra` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-tools` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-tools-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-tools-libs` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump-core` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump-devel` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump-modules` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-zfcpdump-modules-extra` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `perf` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `perf-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `python3-perf` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `python3-perf-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:4.18.0-348.7.1.el8_5 |
| `bpftool-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debug-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-aarch64` | Red Hat:enterprise_linux:8::crb | All versions | 0:4.18.0-348.7.1.el8_5 |
| `kernel-debuginfo-common-ppc64le` | Red Hat:enterprise_linux:8::crb | All versions | 0:4.18.0-348.7.1.el8_5 |

Products: Debian Debian Linux, Linux kernel, Redhat Enterprise Linux.

## How do I fix CVE-2021-20321?

Apply the vendor's patch.

## Is CVE-2021-20321 being exploited?

There is no known exploitation of CVE-2021-20321 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

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- [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-2021-20321 and fix it if it is affected.

What it is: CVE-2021-20321 is a medium-severity vulnerability in Debian Debian Linux, published on 18 Feb 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-2021-20321 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-2021-20321/ (Markdown: https://insights.sbomify.com/CVE-2021-20321/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20321
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20321
- OSV: https://osv.dev/vulnerability/CVE-2021-20321

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-20321/
Summaries are generated and may contain mistakes; the official description above is the record.