TUCoPS :: Unix :: Various Flavours :: 9121.txt

IBM AIX TFTP Daemon Vulnerability

Security Bulletin 9121              DISA Defense Communications System
21 October 1991         Published by: DDN Security Coordination Center
                                  (SCC@NIC.DDN.MIL)   1-(800) 365-3642

                        DEFENSE  DATA  NETWORK
                          SECURITY  BULLETIN

The DDN  SECURITY BULLETIN is distributed  by the  DDN SCC  (Security
Coordination Center) under DISA contract as  a means of  communicating
information on network and host security exposures, fixes, &  concerns
to security & management personnel at DDN facilities.  Back issues may
be  obtained  via  FTP  (or  Kermit)  from  NIC.DDN.MIL  []
using login="anonymous" and password="guest".  The bulletin pathname is
SCC:DDN-SECURITY-yynn (where "yy" is the year the bulletin is issued
and "nn" is a bulletin number, e.g. SCC:DDN-SECURITY-9121).
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!                                                                       !
!     The following important  advisory was  issued by the Computer     !
!     Emergency Response Team (CERT)  and is being relayed unedited     !
!     via the Defense Information Systems Agency's Security             !
!     Coordination Center  distribution  system  as a  means  of        !
!     providing  DDN subscribers with useful security information.      !
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CA-91:19                        CERT Advisory
                              October 17, 1991
                        AIX TFTP Daemon Vulnerability


The Computer Emergency Response Team/Coordination Center (CERT/CC) has
received information concerning a vulnerability in the TFTP daemon in
all versions of AIX for IBM RS/6000 machines.

IBM is aware of this problem and a fix is available as apar number "ix22628".
This patch is available for all AIX releases from "GOLD" to the current

a security hole in the original patch.  The SCCS id of the correct patch
is tftpd.c (*not* or earlier versions).  This can be 
checked using the following "what" command.

    % what /etc/tftpd
       56  tftpd.c, tcpip, tcpip312 10/10/91 09:01:48
       tftpsubs.c      1.2  com/sockcmd/tftpd,3.1.2,9048312 10/8/89 17:40:55

IBM customers may call IBM Support (800-237-5511) and ask that the fix
be shipped to them.  The fix will appear in the upcoming 2009 update
and the next release of AIX.


I.   Description

     Previous versions of tftpd did not provide a method for restricting 
     TFTP access.

II.  Impact

     If TFTP is enabled at your site, anyone on the Internet can retrieve
     copies of your site's world-readable files, such as /etc/passwd.

III. Solution
     A. Sites that do not need to allow tftp access should disable it.
	This can be done by editing /etc/inetd.conf and deleting or
	commenting out the tftpd line:

	#tftp     dgram     udp    wait    nobody  /etc/tftpd     tftpd -n

	and then, as root, restarting inetd with the "refresh" command.

	    # refresh -s inetd

	For more details on starting/stopping tftp, refer to documentation
	for the System Resource Controller (SRC) or the System Management
	Interface Tool (SMIT).

     B. Sites that must run tftpd (for example, to support X terminals)
	should obtain and install the above patch AND create a
	/etc/tftpaccess.ctl file to restrict the files that are accessible.
        The /etc/tftpaccess.ctl file should be writable only by root.
	Although the new /etc/tftpaccess.ctl mechanism provides a very general
	capability, the CERT/CC strongly recommends that sites keep this
	control file simple.  For example, the following tftpaccess.ctl file
	is all that is necessary to support IBM X terminals:

	# /etc/tftpaccess.ctl
	# By default, all files are restricted if /etc/tftpaccess.ctl exists.
	# Allow access to X terminal files.

	NOTE: Be CERTAIN to create the /etc/tftpaccess.ctl file.
	If it does not exist then all world-readable files are accessible
	as in the current version of tftpd.

        Installation Instructions:

        1.  Create an appropriate /etc/tftpaccess.ctl file.

        2.  From the directory containing the new tftpd module, issue
            the following commands as root.
            # chmod 644 /etc/tftpaccess.ctl
            # chown root.system /etc/tftpaccess.ctl
	    # mv /etc/tftpd /etc/tftpd.old
	    # cp tftpd /etc
	    # chmod 755 /etc/tftpd
	    # chown root.system /etc/tftpd
	    # refresh -s inetd

The CERT/CC wishes to thank Karl Swartz of the Stanford Linear Accelerator
Center for bringing this vulnerability to our attention.

If you believe that your system has been compromised, contact CERT/CC via
telephone or e-mail.

Computer Emergency Response Team/Coordination Center (CERT/CC)
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

Internet E-mail: cert@cert.sei.cmu.edu
Telephone: 412-268-7090 24-hour hotline:
           CERT/CC personnel answer 7:30a.m.-6:00p.m. EST/EDT,
           on call for emergencies during other hours.

Past advisories and other computer security related information are available
for anonymous ftp from the cert.sei.cmu.edu ( system.

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