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

Watson Ladd <watson@cloudflare.com> Mon, 03 June 2019 18:24 UTC

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From: Watson Ladd <watson@cloudflare.com>
Date: Mon, 03 Jun 2019 11:24:25 -0700
Message-ID: <CAN2QdAGS20q=7+r+qMFEBBu4gNmSDR9-vYDbvgC=ZnqWLEU-6w@mail.gmail.com>
To: NTP WG <ntp@ietf.org>
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Subject: [Ntp] Details of the fragmentation attacks against NTP and port randomization
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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

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