Re: [Ntp] Details of the fragmentation attacks against NTP and port randomization

[Fernando Gont <[e-mail]>]

On 5/6/19 05:41, Danny Mayer wrote:
> On 6/4/19 2:39 AM, Fernando Gont wrote:
>> On 4/6/19 06:09, Danny Mayer wrote:
>>> On 6/3/19 2:24 PM, Watson Ladd wrote:
>>>
>>>> Dear all,
>>>>
>>>> The debate over client port randomization is missing an important
>>>> fact: off-path attacks against NTP are not prevented by the origin
>>>> timestamp due to the OS handling of fragmentation. In
>>>> http://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf we see that sending
>>>> a properly crafted IP fragment can selectively overwrite NTP packets,
>>>> thus allowing an attacker to modify received data without overwriting
>>>> the origin timestamp. I would recommend we adopt port randomization
>>>> to handle this problem.
>>>>
>>>> Sincerely,
>>>> Watson Ladd
>>> Actually if you read Section VI you will see that the last sentence of
>>> that section states that they do not consider it to be a sufficient defense.
>> Wasn't Your argument that the timestamp was enough to mitigate off-path
>> attacks?
>>
> Yes and no. The draft you wrote requires using random ports to avoid
> off-path attacks.

No.

The draft we wrote says employing predictable identifiers is a bad
practice that has been known for over 20 years. We have been on this
road so many times. You just don't use predictable IDs.

Using them leads to trouble, with no benefit. Using random port numbers
helps mitigate such trouble.



> The paper demonstrates that for at least some O/S's an
> off-path fragmentation attack succeeds regardless of the port number.
> This is at a layer below NTP. So randomizing the ports doesn't matter
> for this attack according to the paper. The proper way to deal with this
> fragmentation attack is to fix the underlying O/S behavior as has
> already been done for most of them. The NTP basic payload is only 48
> bytes so there is no reason to allow fragmentation in the first place.

The IPv4 minimum MTU is 68 bytes. Anything longer than that may need to
be fragmented.



> If you can't attack that way then your off-path attacker cannot guess
> the origin timestamp which is a 64-bit quantity. Furthermore the
> attacker doesn't know the server being used by the NTP client so the IP
> address of that server will be invalid as well. 

In many-many cases, the possible NTP servers are known. We have been
dicussiing this sort of thing *for ages* in the transport area. See e.g.
RFC5927




> So RFC6056 doesn't really apply as a mandate.

It's a BCP from the *transport area* about how to employ ephemeral port
numbers. I can't see what's the rationale to ignore such advice.



> You can certainly ask to
> update draft-ietf-ntp-bcp to recommend randomizing the port for outgoing
> queries.

Yes. That's what our draft does.



> In any case RFC6056 is about randomizing ports and is not
> really a security document. If it were then it would have made a very
> basic recommendation concerning the port: if you have received your
> response and it is valid then close the port immediately and the next
> time you want to make a new request open a new random port to perform
> the request. That way the off-path attacker will have no way of knowing
> what the new port number will be.

The document provides advice for all transport protocols. What you
describe only applies for connection-less protocols, where the app-layer
only does occasional packets exchanges.



> In the case of NTP you don't want to trust a single server, The draft
> NTP BCP recommends at least 3 different servers and preferably 4 to
> allow for the loss of one server.

You keep mixing things up.

There's a lot of things you may want to do (and probably should!) at the
app layer. This document is concerned with what you do at the *transport
layer*. Any good practices at the app layer don't rule out good
practices at the transport layer.


You keep arguing that you want to keep a very bad practice, because
somehow you are doing something else that you thing will relieve you
from the possible pain of the bad practice.

Our draft argues: bad practices are bad practices. Do the right thing.



>  See section 3.2. If you are really
> concerned about security you should arrange with your server providers
> to include either a MAC at the end of the packet or use the new NTS
> security mechanism to validate and secure the packets returned. If you
> are concerned about attacks you should be employing firewalls anyway.
> Lastly you can get yourself a GPS unit and use that to get your clock
> information. For the really paranoid, get an atomic clock!

I guess with the same line of reasoning you could argue that if you're
really concerned about security, you wouldn't keep the servers connected
to the Internet?

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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