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FreeBSD Security Advisory FreeBSD-SA-13:05.nfsserver [REVISED] Apr 29 2013 09:56PM
FreeBSD Security Advisories (security-advisories freebsd org)
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FreeBSD-SA-13:05.nfsserver Security Advisory
The FreeBSD Project

Topic: Insufficient input validation in the NFS server

Category: core
Module: nfsserver
Announced: 2013-04-29
Revised: 2013-04-29
Credits: Adam Nowacki
Affects: All supported versions of FreeBSD.
Corrected: 2013-04-29 21:10:49 UTC (stable/8, 8.4-PRERELEASE)
2013-04-29 21:10:53 UTC (releng/8.3, 8.3-RELEASE-p8)
2013-04-29 21:11:31 UTC (releng/8.4, 8.4-RC1-p1)
2013-04-29 21:11:31 UTC (releng/8.4, 8.4-RC2-p1)
2013-04-29 21:11:01 UTC (stable/9, 9.1-STABLE)
2013-04-29 21:11:05 UTC (releng/9.1, 9.1-RELEASE-p3)
CVE Name: CVE-2013-3266

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

0. Revision History

v1.0 2013-04-29 Initial release.
v1.1 2013-04-29 Corrected patch URL.
Additional workaround information.

I. Background

The Network File System (NFS) allows a host to export some or all of its
file systems so that other hosts can access them over the network and mount
them as if they were on local disks. FreeBSD includes server and client
implementations of NFS.

FreeBSD 8.0 and onward has two NFS implementations: the original CSRG
NFSv2 and NFSv3 implementation and a new implementation which also
supports NFSv4.

FreeBSD 9.0 and onward uses the new NFS implementation by default.

II. Problem Description

When processing READDIR requests, the NFS server does not check that
it is in fact operating on a directory node. An attacker can use a
specially modified NFS client to submit a READDIR request on a file,
causing the underlying filesystem to interpret that file as a
directory.

III. Impact

The exact consequences of an attack depend on the amount of input
validation in the underlying filesystem:

- If the file resides on a UFS filesystem on a little-endian server,
an attacker can cause random heap corruption with completely
unpredictable consequences.

- If the file resides on a ZFS filesystem, an attacker can write
arbitrary data on the stack. It is believed, but has not been
confirmed, that this can be exploited to run arbitrary code in
kernel context.

Other filesystems may also be vulnerable.

IV. Workaround

Systems that do not provide NFS service are not vulnerable. Neither
are systems that do but use the old NFS implementation, which is the
default in FreeBSD 8.x.

To determine which implementation an NFS server is running, run the
following command:

# kldstat -v | grep -cw nfsd

This will print 1 if the system is running the new NFS implementation,
and 0 otherwise.

To switch to the old NFS implementation:

1) Append the following lines to /etc/rc.conf:

nfsv4_server_enable="no"
oldnfs_server_enable="yes"

2) If the NFS server is compiled into the kernel (which is the case
for the stock GENERIC kernel), replace the NFSD option with the
NFSSERVER option, then recompile your kernel as described in
<URL:http://www.FreeBSD.org/handbook/kernelconfig.html>.

If the NFS server is not compiled into the kernel, the correct
module will be loaded at boot time.

3) Finally, reboot the system.

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

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

# fetch http://security.FreeBSD.org/patches/SA-13:05/nfsserver.patch
# fetch http://security.FreeBSD.org/patches/SA-13:05/nfsserver.patch.asc
# gpg --verify nfsserver.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> and reboot the
system.

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI. Correction details

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

Branch/path Revision
- ------------------------------------------------------------------------
-
stable/8/ r250068
releng/8.3/ r250069
releng/8.4/ r250073
stable/9/ r250070
releng/9.1/ r250071
- ------------------------------------------------------------------------
-

VII. References

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-3266

The latest revision of this advisory is available at
http://security.FreeBSD.org/advisories/FreeBSD-SA-13:05.nfsserver.asc
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