============================================================================= 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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