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AUSCERT External Security Bulletin Redistribution
ESB-2008.0831 -- [NetBSD]
Malicious PPPoE discovery packet can overrun a kernel buffer
17 September 2008
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AusCERT Security Bulletin Summary
---------------------------------
Product: pppoe
Publisher: NetBSD
Operating System: NetBSD
Impact: Denial of Service
Access: Remote/Unauthenticated
CVE Names: CVE-2008-3584
Original Bulletin:
ftp://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2008-010.txt.asc
Revision History: September 17 2008: Added CVE Reference
August 27 2008: Initial Release
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NetBSD Security Advisory 2008-010
=================================
Topic: Malicious PPPoE discovery packet can overrun a kernel buffer
Version: NetBSD-current: affected
NetBSD 4.0: affected
NetBSD 3.1.*: affected
NetBSD 3.1 affected
NetBSD 3.0.*: affected
NetBSD 3.0: affected
Severity: Remote denial-of-service
Fixed: NetBSD-current: August 08, 2008
NetBSD-4-0 branch: August 08, 2008
(4.0.1 will include the fix)
NetBSD-4 branch: August 08, 2008
(4.1 will include the fix)
NetBSD-3-1 branch: August 08, 2008
(3.1.2 will include the fix)
NetBSD-3-0 branch: August 08, 2008
(3.0.4 will include the fix)
NetBSD-3 branch: August 08, 2008
(3.2 will include the fix)
Abstract
========
A problem has been identified in the pppoe(4) code. A bug in range checking
allows a malicious packet to make the kernel access memory outside of the
allocated buffer and cause a kernel crash. It is currently unclear if this
issue could be exploited any further than denial of service.
Technical Details
=================
The critical code deals with early states of a PPPoE connection, before
a session between client and access concentrator has been established.
Packets in this "discovery" phase may consist of multiple variable length
"tags" packed together in a pppoe packet. Each tag is checked and the length
validated against to total packet size. A bug in this length check allowed
packets to advance the next tag pointer to up to 4 bytes beyond the end
of the packet. This can cause a kernel crash.
The problematic code path is executed even without active pppoe(4) interfaces,
as long as at least one has been created with "ifconfig pppoe0 create". No
further configuration of the pppoe(4) interface is needed.
The attack is not routable, so attackers would have to have access to the
LAN of an affected machine - or the DSL side would need to be configured
like a shared ethernet segment. This is uncommon, but done by some providers.
Solutions and Workarounds
=========================
The pseudo-device pppoe is present in GENERIC kernels. The system must
be configured with a pppoe(4) instance.
For all NetBSD versions, you need to obtain fixed kernel sources,
rebuild and install the new kernel, and reboot the system.
The fixed source may be obtained from the NetBSD CVS repository.
The following instructions briefly summarise how to upgrade your
kernel. In these instructions, replace:
ARCH with your architecture (from uname -m), and
KERNCONF with the name of your kernel configuration file.
To update from CVS, re-build, and re-install the kernel:
# cd src
# cvs update -d -P sys/net/if_spppsubr.c
# ./build.sh kernel=KERNCONF
# mv /netbsd /netbsd.old
# cp sys/arch/ARCH/compile/obj/KERNCONF/netbsd /netbsd
# shutdown -r now
For more information on how to do this, see:
http://www.NetBSD.org/guide/en/chap-kernel.html
Thanks To
=========
Yasuoka Masahiko from Internet Initiative Japan Inc. for discovering the
problem and providing a fix.
Revision History
================
2008-08-26 Initial release
More Information
================
Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
ftp://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2008-010.txt.asc
Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/.
Copyright 2008, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.
$NetBSD: NetBSD-SA2008-010.txt,v 1.1 2008/08/25 22:51:57 adrianp Exp $
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