CIAC A-14

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The Computer Incident Advisory

 · 4 years ago

  

________________________________________________________________________ 
                THE COMPUTER INCIDENT ADVISORY CAPABILITY 

                                 CIAC 

                        INFORMATION BULLETIN 
________________________________________________________________________ 

Additional information on the vulnerability in the UNIX DECODE alias 

 
January 23, 1990, 1130 PST                                      Number A-14 

CIAC information bulletin A-13 described preliminary information about 
a vulnerability in some versions of the UNIX operating system.  This 
bulletin gives additional information and a procedure for patching 
this vulnerability. 

The UNIX operating system maintains a global mail aliases data base 
used by the "sendmail" program to re-route electronic mail.  This 
database file is contained in /usr/lib/aliases for most UNIX systems 
(with exceptions noted below).  One standard alias delivered with some 
versions of UNIX is "decode."  When mail is sent to "decode" at a UNIX 
host, the message is re-routed to the program "uudecode", which will 
translate a file that has been encoded with "uuencode".  There is a 
vulnerability associated with this default alias, and CIAC maintains 
that there is a strong possibility that this vulnerability has been or 
is currently being exploited. 

To determine if your UNIX system has this vulnerability, CIAC 
recommends the following procedure: 

1.      Find the global aliases file for your UNIX system. 
Traditionally this file is kept in /usr/lib/aliases, but for some 
systems such as SUN OS 4.X and ULTRIX 3.X systems it may be in 
/etc/aliases.  If you do not have either of these files, it is 
possible that you are not running the SENDMAIL program, and thus do 
not have this vulnerability.  The global aliases file will be referred 
to as <aliases> in the following steps. 

2.      Determine if the decode alias is present in your global 
aliases file.  To do this execute the command "grep decode <aliases>" 
If this command results in nothing being displayed, your system does 
not have a decode alias, and probably does not have this 
vulnerability.  If you see a line such as  
'decode: "|/usr/bin/uudecode" ' or a similar line, proceed to step 3. 

3.      Become a super-user for your system if you are not already 
running as root.  Create a backup copy of the aliases file found in 
step 1, and edit this file.  Insert a "#" at the beginning of the line 
containing the decode alias.  The line should now read:  
'#decode: "|/usr/bin/uudecode" ' Save the file and exit. 

4.      Assure that the ownership and permissions of this aliases file 
are still set properly, by executing the command "ls -l <aliases>" The 
line should begin with "-rw--r--r--" If this is not the case, run the 
command "chmod 644 <aliases>" 

5.      Once the aliases file has been altered, run the command 
"newaliases" so that the changed aliases file will take effect.  The 
vulnerability has now been closed. 

If you do not wish to disable the DECODE alias, you can redirect 
DECODE to postmaster.  In step 3 above, change the decode alias to 
"decode: postmaster" Now mail to decode will be forwarded to 
postmaster, allowing the designated postmaster to manually uudecode 
the file if desired.  If neither of these solutions is appropriate for 
your system, you may call CIAC for additional alternatives. 

If you have questions, please contact CIAC. 
  
        Tom Longstaff 
        (415) 423-4416 or (FTS) 543-4416 
        FAX: (FTS) 543-0913 or (415) 294-5054   

CIAC's business hours phone number is (415) 422-8193 or (FTS) 532-8193.   

CIAC's 24-hour emergency hot-line number is (415) 971-9384 

or send e-mail to:  ciac@tiger.llnl.gov 
  
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