[CARTSA-20020402] Linux Netfilter NAT/ICMP code information leak

[Philippe Biondi <biondi () cartel-securite fr>]

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               Cartel Sécurité --- Security Advisory

Advisory Number: CARTSA-20020402
Subject:         Linux Netfilter NAT/ICMP code information leak
Author:          Philippe Biondi <biondi () cartel-securite fr>
Discovered:      April 2, 2002
Published:       May 8, 2002
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Problem description
===================

The following bug exists in the netfilter NAT implementation: When the
first packet of a connection is hitting a NAT rule, and this packet
causes the NAT box itself to reply with an ICMP error message, the
inner IP packet inside the ICMP error message is not un-NAT'ed
correctly.  This leads to the ability to discover which ports of a
host are NATed and where the packet will really go. This can also lead to
those ICMP error packets being dropped by stateful firewalls not
recognizing the related connection.


Vulnerable versions
===================

All kernel patches from iptables package < ipables-1.2.6a are vulnerable.
All versions of kernel >= 2.4.4 and up to (at least) 2.4.19-pre6 use a
vulnerable version.

Vendor status
=============

The netfilter team has solved this bug with a patch that has been refused
for inclusion in the linux kernel. They are working on a new patch.


Solutions
=========

* Upgrade your kernel using the patch at
  http://www.netfilter.org/security/2002-04-02-icmp-dnat.html
  (link active starting with May 8)
  or http://www.cartel-securite.fr/pbiondi/2.4.19-pre6_icmp-nat.patch
* Use a workarround until the final solution to this bug is implemented
  and included in the linux kernel source


Workarounds
===========

Filter out untracked local packets:
iptables -A OUTPUT -m state -p icmp --state INVALID -j DROP


Example
=======

Let's take a machine (172.16.1.40) that DNAT port 666 to 172.16.3.26:22 :
iptables -t nat -A PREROUTING -p tcp --dport 666 -j DNAT --to
172.16.3.26:22

Then if a host sends a packet that will die on 172.16.1.40 :
hping  -t 1 --syn -p 666  172.16.1.40

This is the icmp packet we'll get from 172.16.1.40 :
17:07:46.709230 172.16.1.40 > 172.16.1.28: icmp: time exceeded in-transit
0x0000   45c0 0044 eaa6 0000 ff01 75f1 ac10 0128        E..D......u....(
0x0010   ac10 0118
                   0b00 516d 0000 0000
                                       4500 0028        ......Qm....E..(
0x0020   b0f3 0000 0106 ac8a ac10 0118 ac10 031a <-+    ................
0x0030   04bd 0016 3206 3ec0 0490 00b4 5002 0200   |    ....2.>.....P...
0x0040   d6b2 00^0                                 |    ....
                |                            172.16.3.26
                +-- port 22


You can also try a patch to nmap that does that and much more :
http://www.cartel-securite.fr/pbiondi/nmap/

# ./nmap -sS -P0 xxx.xxx.xxx.xxx -p 22,23,666,667 -t 9

Starting nmap V. 2.54BETA32 ( www.insecure.org/nmap/ )
Interesting ports on xxx.xxx.xxx.xxx:
Port       State       Service
22/tcp     open        ssh
23/tcp     filtered    telnet
666/tcp    UNfiltered  unknown                  DNAT to 192.168.8.10:22
667/tcp    UNfiltered  unknown                  DNAT to 192.168.26.10:22


Nmap run completed -- 1 IP address (1 host up) scanned in 2 seconds

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-- 
Philippe Biondi <biondi@ cartel-securite.fr> Cartel Sécurité
Security Consultant/R&D                      http://www.cartel-securite.fr
Phone: +33 1 44 06 97 94                     Fax: +33 1 44 06 97 99
PGP KeyID:3D9A43E2  FingerPrint:C40A772533730E39330DC0985EE8FF5F3D9A43E2