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FreeBSD-SA-06:10.nfs Security Advisory
The FreeBSD Project

Topic: Remote denial of service in NFS server

Category: core
Module: sys_nfsserver
Announced: 2006-03-01
Credits: Evgeny Legerov
Affects: All FreeBSD releases.
Corrected: 2006-03-01 14:18:11 UTC (RELENG_6, 6.1-PRERELEASE)
2006-03-01 14:18:46 UTC (RELENG_6_0, 6.0-RELEASE-p5)
2006-03-01 14:19:48 UTC (RELENG_5, 5.5-PRERELEASE)
2006-03-01 14:21:01 UTC (RELENG_5_4, 5.4-RELEASE-p12)
2006-03-01 14:24:52 UTC (RELENG_5_3, 5.3-RELEASE-p27)
2006-03-01 14:21:56 UTC (RELENG_4, 4.11-STABLE)
2006-03-01 14:22:30 UTC (RELENG_4_11, 4.11-RELEASE-p15)
2006-03-01 14:23:07 UTC (RELENG_4_10, 4.10-RELEASE-p21)
CVE Name: CVE-2006-0900

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
<URL:http://www.freebsd.org/security/&gt;.

I. Background

The Network File System (NFS) allows a host to export some or all of
its filesystems so that other hosts can access them over the network
and mount them as if they were on local disks. NFS is built on top of
the Sun Remote Procedure Call (RPC) framework.

II. Problem Description

A part of the NFS server code charged with handling incoming RPC
messages via TCP had an error which, when the server received a
message with a zero-length payload, would cause a NULL pointer
dereference which results in a kernel panic. The kernel will only
process the RPC messages if a userland nfsd daemon is running.

III. Impact

The NULL pointer deference allows a remote attacker capable of sending
RPC messages to an affected FreeBSD system to crash the FreeBSD system.

IV. Workaround

1) Disable the NFS server: set the nfs_server_enable variable to "NO"
in /etc/rc.conf, and reboot.

Alternatively, if there are no active NFS clients (as listed by the
showmount(8) utility), simply killing the mountd and nfsd processes
should suffice.

2) Add firewall rules to block RPC traffic to the NFS server from
untrusted hosts.

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to 4-STABLE, 5-STABLE, or 6-STABLE,
or to the RELENG_6_0, RELENG_5_4, RELENG_5_3, RELENG_4_11, or
RELENG_4_10 security branch dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 4.10,
4.11, 5.3, 5.4, and 6.0 systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

[FreeBSD 4.x]

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:10/nfs4.patch

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:10/nfs4.patch.asc

[FreeBSD 5.x and 6.x]

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:10/nfs.patch

fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:10/nfs.patch.asc

b) Apply the patch.

cd /usr/src

patch < /path/to/patch

c) Recompile your kernel as described in
<URL:http://www.freebsd.org/handbook/kernelconfig.html&gt; and reboot the
system.

VI. Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Branch Revision
Path

---

RELENG_4
src/sys/nfs/nfs_socket.c 1.60.2.7
RELENG_4_11
src/UPDATING 1.73.2.91.2.16
src/sys/conf/newvers.sh 1.44.2.39.2.19
src/sys/nfs/nfs_socket.c 1.60.2.6.6.1
RELENG_4_10
src/UPDATING 1.73.2.90.2.22
src/sys/conf/newvers.sh 1.44.2.34.2.23
src/sys/nfs/nfs_socket.c 1.60.2.6.4.1
RELENG_5
src/sys/nfsserver/nfs_srvsock.c 1.92.2.2
RELENG_5_4
src/UPDATING 1.342.2.24.2.21
src/sys/conf/newvers.sh 1.62.2.18.2.17
src/sys/nfsserver/nfs_srvsock.c 1.92.2.1.2.1
RELENG_5_3
src/UPDATING 1.342.2.13.2.30
src/sys/conf/newvers.sh 1.62.2.15.2.32
src/sys/nfsserver/nfs_srvsock.c 1.92.4.1
RELENG_6
src/sys/nfsserver/nfs_srvsock.c 1.94.2.1
RELENG_6_0
src/UPDATING 1.416.2.3.2.10
src/sys/conf/newvers.sh 1.69.2.8.2.6
src/sys/nfsserver/nfs_srvsock.c 1.94.4.1

---

VII. References

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0900

The latest revision of this advisory is available at
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-06:10.nfs.asc
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