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NetBSD Security Advisory 2019-004: IPv6 neighbor cache leak on expiration
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NetBSD Security Advisory 2019-004
Topic: IPv6 neighbor cache leak on expiration
Version: NetBSD-current: affected up to 8.99.51
NetBSD 8.1: affected
NetBSD 7.2: not affected
NetBSD 7.1.2: not affected
NetBSD 7.0.2: not affected
Severity: Remote DoS attack can cause the kernel to run out of memory
Fixed: NetBSD-current: July 26, 2019
NetBSD-9 branch: not affected
NetBSD-8 branch: July 26, 2019
NetBSD-7 branch: not affected
NetBSD-7-2 branch: not affected
NetBSD-7-1 branch: not affected
NetBSD-7-0 branch: not affected
Please note that NetBSD releases prior to 7.1 are no longer supported.
It is recommended that all users upgrade to a supported release.
An unprivileged user can cause kernel resource exhaustion by creating many
IPv6 neighbor entries, because the GC mechanism will not delete them due to
a reference counting bug. An explicit cache flush will delete them,
recovering the used memory.
The garbage collection (GC) mechanism of IPv6 neighbor caches has a bug in
that it doesn't free expired neighbor caches (llentry) because of a missing
The leak occurs when:
1) giving up a neighbor discovery for a cache,
2) giving up a reachability confirmation for a cache,
3) a cache expires, or
4) a forced GC runs.
A forced GC destroys cache entries that have overflowed its limit
The third and fourth cases can be triggered on receiving NS packets so it
can be used as a DoS attack that can originate from machines on the same link.
Solutions and Workarounds
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/netinet6/nd6.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:
Internet Initiative Japan Inc. for finding and fixing the vulnerability.
Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
Information about NetBSD and NetBSD security can be found at
Copyright 2019, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.
$NetBSD: NetBSD-SA2019-004.txt.asc,v 1.1 2019/08/08 10:18:29 christos Exp $
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