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Date: Thu, 12 Jan 2017 13:46:09 -0300
From: Fernando Gont <>
Cc: "" <>
Subject: [FD] ICMPv6 PTBs and IPv6 frag filtering (particularly at BGP peers)


I'm curious about whether folks are filtering ICMPv6 PTB<1280
and/or IPv6 fragments targeted to BGP routers (off-list datapoints are

In any case, you mind find it worth reading to check if you're affected
(from Section 2 of recently-published RFC8021):

---- cut here ----
   The security implications of IP fragmentation have been discussed at
   length in [RFC6274] and [RFC7739].  An attacker can leverage the
   generation of IPv6 atomic fragments to trigger the use of
   fragmentation in an arbitrary IPv6 flow (in scenarios in which actual
   fragmentation of packets is not needed) and can subsequently perform
   any type of fragmentation-based attack against legacy IPv6 nodes that
   do not implement [RFC6946].  That is, employing fragmentation where
   not actually needed allows for fragmentation-based attack vectors to
   be employed, unnecessarily.

   We note that, unfortunately, even nodes that already implement
   [RFC6946] can be subject to DoS attacks as a result of the generation
   of IPv6 atomic fragments.  Let us assume that Host A is communicating
   with Host B and that, as a result of the widespread dropping of IPv6
   packets that contain extension headers (including fragmentation)
   [RFC7872], some intermediate node filters fragments between Host B
   and Host A.  If an attacker sends a forged ICMPv6 PTB error message
   to Host B, reporting an MTU smaller than 1280, this will trigger the
   generation of IPv6 atomic fragments from that moment on (as required
   by [RFC2460]).  When Host B starts sending IPv6 atomic fragments (in
   response to the received ICMPv6 PTB error message), these packets
   will be dropped, since we previously noted that IPv6 packets with
   extension headers were being dropped between Host B and Host A.
   Thus, this situation will result in a DoS scenario.

   Another possible scenario is that in which two BGP peers are
   employing IPv6 transport and they implement Access Control Lists
   (ACLs) to drop IPv6 fragments (to avoid control-plane attacks).  If
   the aforementioned BGP peers drop IPv6 fragments but still honor
   received ICMPv6 PTB error messages, an attacker could easily attack
   the corresponding peering session by simply sending an ICMPv6 PTB
   message with a reported MTU smaller than 1280 bytes.  Once the attack
   packet has been sent, the aforementioned routers will themselves be
   the ones dropping their own traffic.
---- cut here ----

Is this something waiting to be exploited? Am I missing something?

Fernando Gont
SI6 Networks
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492

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