CVE-2021-4477
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedHirschmann 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceThe vulnerability allows IPv6 IPsec VPN traffic to bypass configured firewall rules on Hirschmann HiLCOS OpenBAT and BAT450 devices. Attackers can establish IPv6 IPsec connections (IKEv1 or IKEv2) while simultaneously using an IPv6 Internet connection, enabling them to circumvent firewall policy enforcement and access protected networks.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify device model and confirm it is Hirschmann HiLCOS OpenBAT or BAT450Log into the device web interface or CLI and check the hardware model information under System > Status or use the 'show system' commandAffected if The device is an Hirschmann OpenBAT or BAT450 model running HiLCOS firmware
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Check the installed HiLCOS firmware versionNavigate to System > Firmware or use the 'show version' command in CLI to retrieve the current HiLCOS version numberAffected if The installed HiLCOS version has not been patched to address CVE-2021-4477 (compare against vendor advisories for fixed versions)
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Verify if IPv6 is enabled on the deviceCheck the network configuration under IPv6 settings in the web interface or use 'show ipv6 configuration' command in CLIAffected if IPv6 is enabled and configured on the device
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Confirm IPsec VPN is configured for IPv6Review IPsec VPN settings in the configuration menu, specifically looking for IKEv1 or IKEv2 tunnels with IPv6 addresses or networks, or use 'show ipsec sa' commandAffected if An IPv6 IPsec VPN tunnel (IKEv1 or IKEv2) is active or configured on the device
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Check firewall rules applied to IPv6 IPsec trafficReview firewall rule sets in the security or firewall configuration section to verify if there are explicit rules filtering IPv6 IPsec traffic, or use 'show firewall rules ipv6' commandAffected if Firewall rules exist but may not be applied to IPv6 IPsec traffic due to the bypass vulnerability
The device is affected if it is a Hirschmann OpenBAT or BAT450 with an unpatched HiLCOS version that has both IPv6 and IPv6 IPsec VPN enabled, allowing VPN traffic to bypass configured firewall rules.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataApply vendor-provided firmware patches when available. Until then, consider disabling IPv6 IPsec if not required, or implement additional network-level controls to monitor and filter IPv6 IPsec traffic.
- Consultation4.0 h
- Implementation3.0 h
- Testing8.0 h
- Review / QA3.0 h
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-4477 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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