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TUCoPS :: Unix :: General :: ciacb044.txt

Automated Tftp Probes

             The Computer Incident Advisory Capability
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                           Advisory Notice 

 Automated tftp Probe Attacks on UNIX Systems Connected to the Internet

September 27, 1991, 1700 PDT	                             Number B-44

PROBLEM:  Many systems connected to the Internet have been probed by tftp 
	to allow intruders to copy the /etc/passwd and /etc/rc files.
PLATFORM: UNIX systems supporting tftp.
DAMAGE: Potential disclosure of easily-guessed passwords leading to 
	unauthorized access to user accounts.
SOLUTIONS: Either disable tftp if possible or limit the access of tftp if 
	this utility is required.  Assure that there are no easily guessed 
	passwords on your systems.    
	    Critical Facts about the Automated tftp Probes

CIAC has learned of a series of Internet-based probes involving the
tftp (trivial file transfer protocol) facility available on many UNIX
platforms.  An unpatched vulnerability in this facility can be used to
obtain a copy of the /etc/passwd and /etc/rc files of remote systems
(see CIAC bulletin of June 22, 1989).  If successful, these probes
fetch the /etc/passwd file (and potentially the /etc/rc file) on victim
systems to crack passwords, resulting in unauthorized access to
systems.  There have been a large number of these attacks reported to
CIAC, since an automated probe program is generating these attacks.
Thus, it is possible for many systems at a site to be probed in a short

If your system is connected to the Internet, you should assure that
the tftp service is disabled on systems that do not require this
functionality. (Typically, tftp is useful mainly for boot servers of
diskless machines at boot-up).  To disable tftp service, comment out
the tftp entry in the /etc/inetd.conf file (or similar configuration
file used by your UNIX operating system) by pre-pending a pound "#"
sign to the line beginning "tftp..."  Consult your operating system
documentation concerning tftpd for additional details in disabling
this service.

If it is necessary for your system to support tftp, you should
restrict tftp to a secure home directory.  On many systems this is
done automatically when the tftp daemon is invoked.  For example, the
tftp -s option within SunOS 4.X is used to ensure that a change to the
home directory is successful and will also change its root directory
to the home directory (chroot) to limit access to the remainder of the
file system.

In order to detect this form of attack, we recommend that you use a
monitoring package that will log tftp and other service requests.  The
type of package appropriate to your site will depend on your specific
network architecture.  If you suspect your system has been probed
(with unrestricted tftp), you should check your password file with a
password guessing utility such as the Security Profile Inspector
(SPI--available only to DOE sites), CRACK or COPS packages.  We also
recommend that you require a change of passwords on the root and user

If you are an employee or contractor at a DOE site, you may have been
contacted about these probes by other agencies' response teams.  We
request that any replies to these contacts be made directly to CIAC
instead of other agencies' teams so that we can coordinate responding
both within the DOE community and with other agencies' response teams.
For additional information or assistance (including assistance on
installing a monitoring package), please contact CIAC:

David S. Brown	                             Tom Longstaff
(510) 423-9878**/(FTS) 543-9878       Or     (510) 423-4416**/(FTS) 543-4416      	           

Send e-mail to or call CIAC at (510) 422-8193**/(FTS)
532-8193.  FAX messages to: (510) 423-8002**/(FTS) 543-8002.

**Note area code has changed from 415

CIAC would like to thank Doug Mildran and Craig Leres for their
assistance.  DARPA's Computer Emergency Response Team Coordination
Center also provided some of the information used in this bulletin.
Neither the United States Government nor the University of California
nor any of their employees, makes any warranty, expressed or implied,
or assumes any legal liability or responsibility for the accuracy,
completeness, or usefulness of any information, product, or process
disclosed, or represents that its use would not infringe privately
owned rights.  Reference herein to any specific commercial products,
process, or service by trade name, trademark manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation, or favoring by the United States Government or the
University of California.  The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government nor the University of California, and shall not be used for
advertising or product endorsement purposes.

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