IPsec/ICMP active attack and crypto rules of thumb

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NISCC has published a vulnerability which could allow the disclosure of information in IPsec. The attack works when data is encrypted but not authenticated. The attacker intercepts an encrypted message, damages it,1 and forwards it to the recipient. When the recipient receives the damaged packet, it responds with an ICMP message containing the beginning of the decrypted (but damaged) packet.

This only works under two conditions:

  1. When the IPsec implementation is configured not to encrypt ICMP messages.
  2. When authentication isn't being used.
The key point is the second. When authentication/message integrity is used, the IPsec stack is supposed to discard damaged packets without generating an ICMP message, in order to protect against exactly this kind of attack (this is a classic crypto error).

One of the general rules of thumb in designing communication security protocols is that you should always use authentication when you use encryption. It's not that there is no safe way to use encryption without authentication/integrity, but just that there are a number of ways it can go wrong, so it's better to be safe. IPsec doesn't require you to use authentication/message integrity, but it's recommended practice.

That said, it's not clear how great the impact of this attack is. All IPsec stacks should let you turn on authentication/integrity and in my experience most people do use it. So, I wouldn't expect this to be a big source of disclosure of secret information.

1. Damaging it correctly turns out to be kind of tricky.

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