Title

The IPsec VPN Gateway must specify Perfect Forward Secrecy (PFS) during Internet Key Exchange (IKE) negotiation. (Cat I impact)

Discussion

PFS generates each new encryption key independently from the previous key. Without PFS, compromise of one key will compromise all communications. The phase 2 (Quick Mode) Security Association (SA) is used to create an IPsec session key. Hence, its rekey or key regeneration procedure is very important. The phase 2 rekey can be performed with or without PFS. With PFS, every time a new IPsec Security Association is negotiated during the Quick Mode, a new Diffie-Hellman (DH) exchange occurs. The new DH shared secret will be included with original keying material (SYKEID_d, initiator nonce, and responder nonce) from phase 1 for generating a new IPsec session key. If PFS is not used, the IPsec session key will always be completely dependent on the original keying material from the Phase-1. Hence, if an older key is compromised at any time, it is possible that all new keys may be compromised. The DH exchange is performed in the same manner as was done in phase 1 (Main or Aggressive Mode). However, the phase 2 exchange is protected by encrypting the phase 2 packets with the key derived from the phase 1 negotiation. Because DH negotiations during phase 2 are encrypted, the new IPsec session key has an added element of secrecy.

Check Content

Verify the IPsec VPN Gateway specifies PFS during IKE negotiation. If the IPsec VPN Gateway does not specify PFS during IKE negotiation, this is a finding.

Fix Text

Configure the IPsec VPN Gateway to specify PFS during IKE negotiation.

Additional Identifiers

Rule ID: SV-207244r916233_rule

Vulnerability ID: V-207244

Group Title: SRG-NET-000371

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.