Re: [IPsec] IKE fragmentation

Yaron Sheffer <> Thu, 14 March 2013 21:13 UTC

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Date: Thu, 14 Mar 2013 23:13:18 +0200
From: Yaron Sheffer <>
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To: Paul Wouters <>
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Cc: "<>" <>, Valery Smyslov <>, Yoav Nir <>, Tero Kivinen <>
Subject: Re: [IPsec] IKE fragmentation
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Hi Paul,

Can't an off-path attacker DoS the gateway if they can guess the SPI 
values? We never mandated that SPIs should be random (except for RFC 
6290, in Sec. 9.3, but this is rarely implemented), so implementations 
are free to use very small integers for the SPIs. In fact I think we 
should reconsider mandating random SPIs once again.


On 03/14/2013 04:51 PM, Paul Wouters wrote:
> On Thu, 14 Mar 2013, Tero Kivinen wrote:
>> As earlier explained not doing that allows very wasy DoS attack, which
>> allows IKEv2 to finish by just sending very few packets, i.e. you send
>> one corrupted fragment to the packet and if you do that before
>> responder gets the correct fragment, the responder stores it for
>> reassembly and after it reassembles the packet it will only then
>> notice that the packet is corrupted, and then it needs to throw the
>> whole packet away. It cannot know which of the fragment is corrupted.
>> This means the initiator needs to retransmit whole packet, i.e. all
>> fragments of it, and attacker can do this again.
> Note that requires an observer that can see your cookies/spi. Which would
> mean a local attacker, whom could just as easilly send you nonsense
> forged from the remote endpoint - as they are guaranteed to answer
> faster. You'd be decrypting thousands of packets to find the needle in
> the haystack. I wonder what the chances then are that you don't end up
> dropping teh valid fragment.
> If the attacker is not local, they need to be in your path to know the
> spi/cookies, and they can just filter out the valid fragments.
> But I see your point, it does raise the bar a little bit. Although I'm
> not convinced it's worth it.
> Paul
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