security advisory linux 2.4.x ip_conntrack_irc

[Harald Welte <laforge () gnumonks org>]

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Important security announcement of the netfilter project, 25 Feb 2002
(http://www.netfilter.org/security/2002-02-25-irc-dcc-mask.html).

SUBJECT:  IRC connection tracking helper module 
SUMMARY:  IRC connection tracking opens unwanted ports
SYSTEM:   All Linux kernel versions from 2.4.14 to 2.4.18-pre8
SOLUTION: Apply attached patch
CREDITS: Jozsef Kadlecsik <kadlec () netfilter org>, 
         Harald Welte <laforge () netfilter org>

The Common Vulnerabilities and Exposures project (cve.mitre.org) has           
assigned the name CAN-2002-0060 to this issue.

DESCRIPTION
===========
The netfilter subsystem in Linux kernels >= 2.4.14 contains a connection
tracking helper module for the IRC DCC protocol.  The purpose of this module is
to monitor outgoing DCC CHAT/SEND requests and issue so-called 'conntrack
expectations' about the respective inbound DCC connections.

A bug in the implementation of this module causes the conntrack expectation to
be less precise than it should, resulting in unwanted ports for inbound
connections opened on the firewall.

The conntrack expectation is described by a tuple (layer 4 protocol, source ip,
source port, destination ip, destination port) and mask indicating which parts
of the tuple need to match with a new connection in order to be fulfilled.

With IRC DCC, we can only tell the destination IP and port, thus we need
an expectation "expect related connection from any ip / any port to this
particular port number X at this particular IP address Y".

Due to the implementation bug, however, the mask was to wide. The conntrack
helper really says "expect related connection from any ip / any port to
this particular port X at ANY IP".

As a result, incoming connection requests are only matched on the destination
port number, and nothing else. 

This does not always need to result in this unwanted incoming connection
request to be allowed.  It always depends on the ruleset, since connection
tracking only decides on the state of a packet.  


IMPLICATIONS
============
The implications depend on the ruleset, since connection tracking only
assigns state to packets.  What to do with this state information is up
to the user.  

However, a big number of installation seem to have a very
permissive "-m state --state RELATED -j ACCEPT" rule.  In this case,
as soon as somebody from inside the private network issues a IRC DCC 
request, a single connection from the outside network to the port number
stated in the DCC request on any (internal) IP adddres will get accepted.


SOLUTION
- --------
Update to a >= 2.4.18-pre9 kernel OR apply the following patch:

- --- linux-2.4.18-pre8-plain/net/ipv4/netfilter/ip_conntrack_irc.c     Sat Dec 22 18:52:16 2001
+++ linux-2.4.18-pre8-nfpom/net/ipv4/netfilter/ip_conntrack_irc.c       Tue Feb  5 15:55:29 2002
@@ -1,8 +1,8 @@
- -/* IRC extension for IP connection tracking, Version 1.20
- - * (C) 2000-2001 by Harald Welte <laforge () gnumonks org>
+/* IRC extension for IP connection tracking, Version 1.21
+ * (C) 2000-2002 by Harald Welte <laforge () gnumonks org>
  * based on RR's ip_conntrack_ftp.c    
  *
- - * ip_conntrack_irc.c,v 1.20 2001/12/06 07:42:10 laforge Exp
+ * ip_conntrack_irc.c,v 1.21 2002/02/05 14:49:26 laforge Exp
  *
  *      This program is free software; you can redistribute it and/or
  *      modify it under the terms of the GNU General Public License
@@ -112,9 +112,9 @@
 
        struct ip_ct_irc *info = &ct->help.ct_irc_info;
 
- -     memset(&mask, 0, sizeof(struct ip_conntrack_tuple));
- -     mask.dst.u.tcp.port = 0xFFFF;
- -     mask.dst.protonum = 0xFFFF;
+       mask = ((struct ip_conntrack_tuple)
+               { { 0, { 0 } },
+                 { 0xFFFFFFFF, { 0xFFFF }, 0xFFFF }});
 
        DEBUGP("entered\n");
        /* Can't track connections formed before we registered */

CREDITS
=======
Jozsef Kadlecsik has discovered this bug initially, Harald Welte has
written the patch.


COPYRIGHT
=========
This advisory is copyright (C) 2002 by the netfilter core team.
Redistribution is permitted after 25 Feb 2002, provided the contents of the
advisory is not modified in any way.

Live long and prosper
- - Harald Welte / laforge () gnumonks org               http://www.gnumonks.org/
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Live long and prosper
- Harald Welte / laforge () gnumonks org               http://www.gnumonks.org/
============================================================================
GCS/E/IT d- s-: a-- C+++ UL++++$ P+++ L++++$ E--- W- N++ o? K- w--- O- M+ 
V-- PS++ PE-- Y++ PGP++ t+ 5-- !X !R tv-- b+++ !DI !D G+ e* h--- r++ y+(*)