# CVE-2021-4477: Hirschmann HiLCOS OpenBAT BAT450 IPv6 IPsec Firewall Bypass

> Attackers can bypass firewall rules on Hirschmann HiLCOS devices by exploiting IPv6 IPsec configurations, allowing unauthorized traffic access.

- **Severity:** Critical 9.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 3 Apr 2026
- **Updated:** 6 Apr 2026
- **Also known as:** GHSA-pp73-vg9m-6fvw

## What is CVE-2021-4477?

CVE-2021-4477 is a critical vulnerability in Belden Hirschmann HiLCOS OpenBAT, published on 3 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Hirschmann HiLCOS OpenBAT and BAT450 products contain a firewall bypass vulnerability in IPv6 IPsec deployments that allows traffic from VPN connections to bypass configured firewall rules. Attackers can exploit this vulnerability by establishing IPv6 IPsec connections (IKEv1 or IKEv2) while simultaneously using an IPv6 Internet connection to circumvent firewall policy enforcement.

Weakness: CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2021-4477?


## How do I fix CVE-2021-4477?

No fix has been published for CVE-2021-4477 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-4477 being exploited?

There is no known exploitation of CVE-2021-4477 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-4477 and fix it if it is affected.

What it is: CVE-2021-4477 is a critical vulnerability in Belden Hirschmann HiLCOS OpenBAT, published on 3 Apr 2026. 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-2021-4477 in the wild and no public exploit code.

Affected product: Belden Hirschmann HiLCOS OpenBAT. 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-2021-4477/ (Markdown: https://insights.sbomify.com/CVE-2021-4477/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4477
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4477
- OSV: https://osv.dev/vulnerability/CVE-2021-4477

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4477/
Summaries are generated and may contain mistakes; the official description above is the record.