So what are inter-session Replay attacks?

Inter-session replay attacks are extremely hard to fix and most IETF routing and signaling protocols are vulnerable to them. Lets first understand what an inter-session replay attack is before we delve deeper into how we can fix them.

A reply attack is a type of an attack where the attacker captures the packets exchanged between two routers and later retransmits, or “replays”, this same packet back to the routers and thereby deceiving them into believing that this is a legitimate packet sent by their remote neighbor. Lets see how this will work:

Assume router A is sending an integrity protected (via some authentication mechanism) protocol packet to router B. The attacker can record the packet that A is sending. The attacker now waits for some time and retransmits this packet without any modification, back to B. B upon receiving this packet will as usual first try to verify the contents for any tampering. It will do this by verifying the authentication digest (usually Keyed-MD5 or HMAC-SHA) that the packet carries. Since the attacker has not modified the packet it will pass the integrity check as long as the key exchanged between the two routers remains unchanged. The integrity checks will pass on Router B and it will accept this packet as a legitimate packet sent by A.

This is a replay attack – So, how can it harm you?

Assume A was not advertising any route, or any neighbor reachability when the attacker had recorded this control packet.  In OSPF parlance, this could be a Hello without any neighbors or a RIPv2 packet without any routing information. Later when A learns some routes or neighbors it sends an updated protocol packet listing this information. B receives this packet and updates its protocol state and routing tables based on the information that A provides. Now the attacker replays the earlier recorded packet. B, upon receiving this “new” packet believes this to have come from A and updates its routing tables accordingly. This is incorrect as B will now update its forwarding tables based on stale information. If the replayed packet is an old OSPF Hello when A did not have any neighbors, B will, upon receiving this packet assume that A has now lost all its neighbors and will delete all routes via A. I had co-authored RFC 6039 some time back which describes many such replay attacks in great detail.

So, how do IETF protocols protect themselves from such attacks?

Most protocols packets carry a Cryptographic Sequence Number that increases as each packet is sent. The receivers only accept a packet if it carries a sequence number that is higher than what it had received earlier from the same neighbor.

This fixes the problem that i had described earlier as the replayed packet will carry a sequence number that will be lower than what B would have last heard from A. B, upon receiving this replayed packet will not accept it and would thus prevent itself from such replay attacks.  Its appears that we have a solution against all replay attacks – do we?

Well it turns out that the answer to this question is a big NO!

The cryptographic sequence number can protect us from what i call the intra-session replay attacks. However, it cannot protect us against inter-session replay attacks. Let see why?

Assume that the cryptographic sequence number currently being used by router A for some specific routing protocol is 1000. This means that B will not accept any protocol packet if it comes with a sequence number less than 1000. This is fine, and this will protect us against some attacks. Now assume that the attacker captures and records this packet with sequence number 1000. No one will know about this as the attacker has silently recorded this packet.

Now the attacker has to wait patiently till the current session between the Router A and Router B goes down and a new one is established. This can happen if one of the routers reboots (could be planned or unplanned). When this happens the routers reset their cryptographic sequence number to 0 and start all over again. If the password key  between the two routers has not changed, and it usually doesn’t, then the packet that the attacker has captured is carrying a valid cryptographic digest. The attacker can replay this packet any time and this will get accepted by B if the current sequence number that its seeing in the new session from A is less than 1000. This is an inter-session replay attack and is extremely difficult to fix with the current IETF security and authentication mechanisms. Note that a trivial way to protect against inter-session replay attacks is by changing the key each time a new session is established. However changing the key requires manual intervention and thus cannot be easily done all the time.

So, how do you fix this issue?

Sam Hartman (Huawei), Dacheng (Huawei) and I have submitted two proposals in the IETF to fix this inter-session replay attacks that i have described above.

The first is extremely simple.

We propose to change the current cryptographic sequence number space from 32 bits to 64.  The least significant 32 bits would be the usual cryptographic sequence number that will monotonically increase with each fresh packet transmitted. The most significant 32 bits would indicate the number of times this router has cold booted. Thus when the router initially comes up for the first time its value would be 0. Next time when it reboots and comes up, its value would be 1.

Consider a state when the router has cold booted “n” times and its current cryptographic sequence number is “m”. The aggregated cryptographic sequence number that will be used by the routing protocols would be:

m << 32 || n, where << is the left shift operator and || is the bit-wise OR operation.

Now this router reboots (again planned or unplanned).

Now its cryptographic sequence space starts from:

(m+1) << 32

Its trivial to see that the ((m+1) << 32) > ((m << 32) || n) for all values of m and n where each m and n > 0.

This mechanism will solve the inter-session replay attacks that have been described above. I will describe the second method in some other post. We have defined a generic mechanism that all protocols can use here in this KARP draft.


2 thoughts on “So what are inter-session Replay attacks?

  1. Isn’t it the other way around, i.e. “m” is the count of router cold boots and “n” is current cryptographic sequence number?


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