Critical severity · published Apr 3, 2026
CVE-2021-4477
Hirschmann HiLCOS OpenBAT BAT450 IPv6 IPsec Firewall Bypass
9.1
CVSS
0.3%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 25% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
HH
Hirschmann HiLCOS OpenBAT · Belden
3.80-REL; 8.90-REL; 9.00-REL; 9.00-RU1; 9.10-REL; 9.12-REL; +13 more · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-4477
CVE.org
https://www.cve.org/CVERecord?id=CVE-2021-4477
assets.belden.com
https://assets.belden.com/m/5fd1a50fa50cb252/original/Belden-Security-Bulletin-BSECV-1v0-2019-09.pdf
www.vulncheck.com
https://www.vulncheck.com/advisories/hirschmann-hilcos-openbat-bat450-ipv6-ipsec-firewall-bypass