I would like to know more about EIGRP MD5 authentication in the following situation: an attacker captured the MD5 hashed key and send to the target router to fake the authentication, is it possible?

Thx for any comment.

  • This question would do better over at the Information Security Stack.
    – Ryan Foley
    Feb 22, 2014 at 8:54
  • 1
    This sounds like an important network engineering question to me, although I think the simple answer is that the attack vector mentioned in the question will not be successful. If it was successful, md5 would be no better than plain-text Feb 22, 2014 at 9:29

1 Answer 1


No, that isn't possible unless they know (or obtain) the shared secret. Cisco outlines how the authentication takes place.1

The device sending a packet calculates the hash to be sent based on the following:

  • Key part 1—the configured shared secret.
  • Key part 2—the local interface address from which the packet will be sent.
  • Data—the EIGRP packet to be sent (prior to the addition of the IP header).

The instance you're talking about is called Pass the hash, which involves sniffing a hash, and then sending it back with the rest of the modified data without any actual knowledge of what was used to generate the hash. EIGRP would be vulnerable to this if if didn't use the local interface and the data inside the packet to generate its hashes.

So no, an attacked really couldn't perform any of this unless he had prior knowledge of the shared secret. He could, however, capture a hello packet between neighbors, and then attempt to crack it. If you're key-length and complexity are high, though, you won't have much to worry about.

1Note: this is based on HMAC-SHA-256 authentication, but the same process should still be true with MD5.

  • 1
    I recommended capturing a hello packet because that is the smallest packet size with a hash that EIGRP uses. Having a smaller packet will increase your speed in cracking the hash.
    – Ryan Foley
    Feb 22, 2014 at 10:39
  • Thanks. From your comment, here is my understanding: The attacker cannot perform a success replay attack by using Pass the hash because the message digest is different for every packet because the data content is different. However, if the attacker can crack to shared secret, he may fake the opposite device because he has the shared secret + local interface address (which can be faked too) + data. Is this correct? Thx so much!! Feb 22, 2014 at 14:03
  • @user3162764 Yep, you sound like you've got it. Someone with malicious intent could not perform a successful attack on your network unless they discover your shared secret. If they did, then they could modify data, rehash, and then inject it into your network. A password with sufficiently high entropy would likely eliminate this is a possibility altogether. In linux, you could generate one with openssl rand -base64 32 which would produce a sufficiently complex password for everyone to authenticate with.
    – Ryan Foley
    Feb 22, 2014 at 14:16
  • @user3162764 Is there anything you need additional clarification on? If not, users are encouraged to accept the best answer or expand on the original post to include any additional questions.
    – Ryan Foley
    Feb 23, 2014 at 8:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.