Review reviewCritical

CVE-2021-4477

Belden Hirschmann HiLCOS OpenBAT

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.

CVSS: 9.3
EPSS: 0.32%
CISA KEV: Not listed
Published: 2026.04.04

PRIORITY ASSESSMENT

Review review

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV

Exploit probability0.32%

Technical severityCVSS 9.3

Vulnerability overview

Affected product and versions

Product: Belden Hirschmann HiLCOS OpenBAT
Affected versions: >= 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
Fixed versions: No verified fixed-version field is available yet

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide

1
Identify
Confirm that Belden Hirschmann HiLCOS OpenBAT and an affected version are present.
2
Prioritize
Combine exploitation signals with asset exposure and business criticality.
3
Remediate
Follow the vendor advisory or supported update path and preserve rollback options.
4
Verify
Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CWE: CWE-284