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PatchSiren cyber security CVE debrief

CVE-2021-4477 Belden CVE debrief

CVE-2021-4477 is a critical vulnerability in Hirschmann HiLCOS OpenBAT and BAT450 products that allows traffic from VPN connections to bypass configured firewall rules in IPv6 IPsec deployments. The vulnerability has a significant impact on organizations using these products, as it could allow attackers to bypass security controls and access sensitive information. To address this vulnerability, defenders should apply patches or updates provided by the vendor, verify and enforce secure configuration of IPv6 IPsec deployments, and monitor VPN connections and firewall rules for suspicious activity.

Vendor
Belden
Product
Hirschmann HiLCOS OpenBAT
CVSS
CRITICAL 9.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-03
Original CVE updated
2026-07-24
Advisory published
2026-04-03
Advisory updated
2026-07-24

Who should care

Organizations using Hirschmann HiLCOS OpenBAT and BAT450 products should prioritize patching this vulnerability to prevent potential exploitation. This vulnerability affects operators managing these products, as well as platform administrators, vulnerability management teams, and security teams responsible for ensuring the security and integrity of their networks.

Technical summary

The vulnerability is caused by a firewall bypass issue in IPv6 IPsec deployments in Hirschmann HiLCOS OpenBAT and BAT450 products. This allows attackers to establish IPv6 IPsec connections (IKEv1 or IKEv2) and use an IPv6 Internet connection to bypass configured firewall rules. The affected products are Hirschmann HiLCOS OpenBAT and BAT450. The vulnerability has a significant impact on organizations using these products, as it could allow attackers to bypass security controls and access sensitive information.

Defensive priority

High

Recommended defensive actions

  • Apply patches or updates provided by the vendor
  • Verify and enforce secure configuration of IPv6 IPsec deployments
  • Monitor VPN connections and firewall rules for suspicious activity
  • Consider implementing compensating controls such as network segmentation
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-04-03T23:17:01.043Z and was last modified on 2026-07-24T22:10:00.140Z. The NVD entry is currently Awaiting Analysis. Evidence is limited, and defenders should verify the affected scope and vendor guidance. The vulnerability affects Hirschmann HiLCOS OpenBAT and BAT450 products, and it allows traffic from VPN connections to bypass configured firewall rules in IPv6 IPsec deployments. Defenders should review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.

Sources and references

Verified primary and authoritative sources

  • CVE-2021-4477 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2021-4477

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2021-4477 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2021-4477

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.