Zyxel security advisory for IKEv1 protocol vulnerability


CVE: CVE-2018-5389



We suggest that users use long and complex pre-shared keys or certificates for IKEv1 authentication to ensure optimal protection.


What’s the vulnerability?

A vulnerability was found in the Internet Key Exchange v1 (IKEv1) protocol used to negotiate keys between two parties for IPsec connections. One of the specified modes of authentication uses pre-shared keys (PSKs), i.e., passwords. If a PSK is weak, an attacker may use a dictionary or brute force attack to identify it and decrypt the connection.


What should you do?

As this is not a vulnerability in the IKEv1 implementation of Zyxel products but rather a weakness of the PSKs in the IKEv1 protocol, for the time being it cannot be fixed through coding changes without violating the current RFC 2409 standard. We strongly recommend that users:

  • Use long and complex PSKs (passwords) that are more difficult to crack.
  • If possible, use certificates instead of PSKs for authentication, as certificates normally provide a higher level of security with longer public and private keys.


Got a question or a tipoff?

Please contact your local service rep for further information or assistance. If you’ve found a vulnerability, we want to work with you to fix it—contact  security@zyxel.com.tw and we’ll get right back to you.



Thanks to US-CERT Coordination Center for reporting this vulnerability to us.


Revision history

Initial release 2018-08-15