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

Danny Mayer <mayer@pdmconsulting.net> Tue, 04 June 2019 03:09 UTC

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To: Watson Ladd <watson=40cloudflare.com@dmarc.ietf.org>, NTP WG <ntp@ietf.org>
References: <CAN2QdAGS20q=7+r+qMFEBBu4gNmSDR9-vYDbvgC=ZnqWLEU-6w@mail.gmail.com>
From: Danny Mayer <mayer@pdmconsulting.net>
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Date: Mon, 03 Jun 2019 23:09:50 -0400
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Subject: Re: [Ntp] Details of the fragmentation attacks against NTP and port randomization
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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.

Danny

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