AUSCERT External Security Bulletin Redistribution
              ESB-1999.067 -- NetBSD Security Advisory 1999-010
                           ARP table vulnerability
                                 25 May 1999


The NetBSD Foundation, Inc. has released the following advisory concerning
vulnerabilities in the implementation of ARP packet reception. These
vulnerabilities may allow denial of service or traffic hijacking on local

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                 NetBSD Security Advisory 1999-010

Topic:		ARP table vulnerability
Version:	NetBSD-1.3*
Severity:	Denial of service or traffic hijacking from local network
		cable is possible


The implementation of ARP packet reception is vulnerable two attacks:

	- on multihomed hosts, ARP packets from cable A can overwrite
	  ARP entries for cable B.

	- for all hosts, ARP packets can overwrite ARP entries marked
	  as static.

Technical Details

ARP is a protocol used to dynamically obtain IPv4 to Link level address
translation, used for Ethernet, FDDI, Token ring, and ARCnet cables, 
described in RFC 826.

The first vulnerability is specific to hosts with more than one ARP capable
network attached.  The address information of incoming ARP packets is not 
checked to ensure that it corresponds to one of the addresses of the
interface on which the packet arrived.  Thus, it would be able to suppress
or redirect traffic from the attacked host to a different destination.

The second vulnerability is related to so-called "static" arp entries.
The original NetBSD ARP implementation (as that of most other vendors)
allows the creation of "static" or "permanent" ARP entries.  They are
typically used for two reasons:

	- as a security measure, to disallow the redirection of traffic
	  addressed to priviledged hosts by rogue hosts on the cable to
	  themselves or elsewhere,

	- as a cheap routing protocol ("proxy ARP"), mostly when
	  connecting single hosts through point to point links.  To the
	  outside, they occur as if they where on the (e.g.) Ethernet, but
	  traffic destined for them is redirected by the ARP mechanism to
	  the routing host.

The 2nd usage doesn't create specific denial of service possibilities as
the ARP protocol is insecure in itself.

However, if static ARP entries are used to prevent D.O.S. attacks, they need
to be protected from overwriting.

Solutions and Workarounds

NetBSD-1.4, and NetBSD-1.4_BETA after 1999-05-05, are fixed.

A patch is available for NetBSD 1.3.3 to fix this problem.  You may
find this patch on the NetBSD ftp server:


NetBSD-current since 19990506 is not vulnerable.  Users of
NetBSD-current should upgrade to a source tree later than 19990506.

Thanks To

Both vulnerabilities were reported by Olaf "Rhialto" Seibert in NetBSD
PR 7489 and PR 7490.  A fix was provided by Zdenek Salvet in PR 7497,
and integrated into NetBSD by Ignatios Souvatzis.

Revision History

	1999/05/21 - initial version

More Information

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.

Copyright 1999, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA1999-010.txt,v 1.3 1999/05/21 12:47:00 mrg Exp $

Version: 2.6.3ia
Charset: noconv


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