4169493 1999-07-28 03:23 /176 rader/ Postmaster
Mottagare: Bugtraq (import) <7090>
Ärende: Linux 2.2.10 ipchains Advisory
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Message-ID: <199907271656.SAA27464@malaka.dbs.informatik.uni-muenchen.de>
Date: Tue, 27 Jul 1999 18:56:22 +0200
Reply-To: Thomas Lopatic <lopatic@MALAKA.DBS.INFORMATIK.UNI-MUENCHEN.DE>
Sender: Bugtraq List <BUGTRAQ@SECURITYFOCUS.COM>
From: Thomas Lopatic <lopatic@MALAKA.DBS.INFORMATIK.UNI-MUENCHEN.DE>
X-To: bugtraq@securityfocus.com
To: BUGTRAQ@SECURITYFOCUS.COM
Linux ipchains Firewall Vulnerability
data protect GmbH - Advisory #2
July 27, 1999
Authors: Thomas Lopatic <tl@dataprotect.com>
John McDonald <jm@dataprotect.com>
Overview
--------
data protect has discovered a potential vulnerability in the Linux
ipchains firewall implementation. In certain situations, it is
possible for an attacker to bypass the packet filter when
communicating with machines that allow incoming packets to specific
ports. This attack is a variation of previously discussed
fragmentation attacks, where the attacker uses fragments to rewrite
parts of the TCP or UDP protocol header. In this case port
information is rewritten in order to gain access to ports that should
be blocked by the firewall.
Included in this advisory is a patch to the 2.2.10 Linux kernel that
corrects this vulnerability, and a pointer to example code that
demonstrates the problem.
Problem Description
-------------------
The Linux ipchains firewall code has special provisions for IP
fragments that do not contain enough information for transport
protocol header analysis. Fragments that start at offset 0, and are
not long enough to provide complete transport header information are
treated like fragments with an offset > 0 (> 1 in the TCP case). This
is the relevant code from ip_fw.c:
if (offset == 0) {
unsigned int size_req;
switch (ip->protocol) {
case IPPROTO_TCP:
/* Don't care about things past flags word */
size_req = 16;
break;
case IPPROTO_UDP:
case IPPROTO_ICMP:
size_req = 8;
break;
default:
size_req = 0;
}
offset = (ntohs(ip->tot_len) < (ip->ihl<<2)+size_req);
}
As mentioned above, fragments with an offset of 0, that are too short
to provide a full transport protocol header, are treated like
non-first fragments. This allows an attacker to perform the
following port rewriting attack:
1. Attacker sends a fragment, with offset 0, a set IP_MF bit, and a full
transport protocol header which meets the packet filter and is passed to
the victim machine.
2. Attacker sends a fragment, with offset 0, a set IP_MF bit, and a length of
4 bytes. This contains the (blocked) ports that the attacker wishes to
access on the victim machine. This fragment will be accepted by the
firewall and overlap - in the victim machine's reassembly chain - the port
information contained in the fragment sent in step 1.
3. Attacker sends a fragment with a cleared IP_MF bit, starting where the first
fragment left off, that completes the set of fragments.
Depending on the defragmentation strategy of the victim machine's
operating system, it might be necessary to swap steps 1 and 2.
It is important to note that there are two conditions that must be
met for a particular ipchains packet filter to be vulnerable:
1. The packet filter must not be configured with the Linux kernel option
CONFIG_IP_ALWAYS_DEFRAG. If the packet filter reassembles the fragments
before doing the firewall checks, then this attack will fail.
2. The packet filter must have a rule to allow non-first fragments to pass.
The Linux ipchains how-to suggests that either an administrator selects
CONFIG_IP_ALWAYS_DEFRAG, or implements such a rule. This rule was considered
to be safe because fragments with an offset of 1 are blocked by the packet
filter, which prevents attacks based on rewriting the TCP flags.
Fix Information
---------------
The following Linux kernel patch (against version 2.2.10) will close
this vulnerability by blocking packets that could be used to rewrite
header information in this fashion.
It is also possible to reconfigure the ipchains machine to always
defragment packets, or to remove any rule which passes non-first IP
fragments through the firewall ("-f" option of the "ipchains"
command). The latter, however, might introduce incompatibilities,
e.g. with applications that transmit large UDP datagrams across the
firewall and hence cause IP fragmentation.
*** linux.old/net/ipv4/ip_fw.c Wed Jun 9 05:33:07 1999
--- linux/net/ipv4/ip_fw.c Fri Jul 23 19:20:45 1999
***************
*** 37,42 ****
--- 37,45 ----
* 19-May-1999: Star Wars: The Phantom Menace opened. Rule num
* printed in log (modified from Michael Hasenstein's patch).
* Added SYN in log message. --RR
+ * 23-Jul-1999: Fixed small fragment security exposure opened on
15-May-1998.
+ * John McDonald <jm@dataprotect.com>
+ * Thomas Lopatic <tl@dataprotect.com>
*/
/*
***************
*** 644,650 ****
default:
size_req = 0;
}
! offset = (ntohs(ip->tot_len) < (ip->ihl<<2)+size_req);
}
src = ip->saddr;
--- 647,666 ----
default:
size_req = 0;
}
!
! /* If it is a truncated first fragment then it can be
! * used to rewrite port information, and thus should
! * be blocked.
! */
!
! if (ntohs(ip->tot_len) < (ip->ihl<<2)+size_req)
! {
! if (!testing && net_ratelimit()) {
! printk("Suspect short first fragment.\n");
! dump_packet(ip,rif,NULL,NULL,0,0,0,0);
! }
! return FW_BLOCK;
! }
}
src = ip->saddr;
Demonstration Code
------------------
fragrouter, a component of Nidsbench, has been updated to perform
this attack transparently. This is an excellent open source tool for
testing intrusion detection systems and packet filters provided by
Anzen Computing. The version of fragrouter that performs this attack
should be available shortly, at
http://www.anzen.com/research/nidsbench/.
Additional Information
----------------------
data protect would like to thank Dug Song <dugsong@anzen.com> for his
help in implementing this attack.
For information regarding this advisory, please contact Thomas
Lopatic <tl@dataprotect.com> or John McDonald <jm@dataprotect.com>.
The contents of this advisory are Copyright (C) 1999 data protect
GmbH, and may be distributed freely provided that no fee is charged
for distribution, and that proper credit is given.
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