# CVE-2021-3773: Information disclosure in kernel

> A netfilter flaw allows attackers to infer OpenVPN endpoint information, potentially aiding further network attacks.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Feb 2022
- **Updated:** 28 Mar 2025
- **Also known as:** GHSA-3xjv-m925-6jcj, ROOT-OS-UBUNTU-2204-CVE-2021-3773, UBUNTU-CVE-2021-3773

## What is CVE-2021-3773?

CVE-2021-3773 is a critical vulnerability in Fedoraproject Fedora, published on 16 Feb 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A flaw in netfilter could allow a network-connected attacker to infer openvpn connection endpoint information for further use in traditional network attacks.

Weakness: CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2021-3773?

It affects 6 products.


Products: Fedoraproject Fedora, Linux kernel, Oracle Communications Cloud Native Core Binding Support Function, Oracle Communications Cloud Native Core Network Exposure Function, Oracle Communications Cloud Native Core Policy, Redhat Enterprise Linux.

## How do I fix CVE-2021-3773?

Apply the vendor's patch.

## Is CVE-2021-3773 being exploited?

There is no known exploitation of CVE-2021-3773 in the wild and no public exploit code.

Public exploit code:

- This script exploits the "Port Shadow" vulnerability (CVE-2021-3773) in OpenVPN servers using Netfilter for NAT. (github_poc): https://github.com/d0rb/CVE-2021-3773

## 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-2021-3773 and fix it if it is affected.

What it is: CVE-2021-3773 is a critical vulnerability in Fedoraproject Fedora, published on 16 Feb 2022. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-3773 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-2021-3773/ (Markdown: https://insights.sbomify.com/CVE-2021-3773/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3773
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3773
- OSV: https://osv.dev/vulnerability/CVE-2021-3773

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3773/
Summaries are generated and may contain mistakes; the official description above is the record.