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TUCoPS :: Unix :: General :: cert0098.txt

CERT Advisory CA-95:10 ghostscript





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=============================================================================
CERT(sm) Advisory CA-95:10 
Original issue date: August 31, 1995
Last revised: August 30, 1996
              Information previously in the README was inserted
              into the advisory, with some editing.

              A complete revision history is at the end of this file.
                    
Topic:  ghostscript Vulnerability
- -----------------------------------------------------------------------------
                    
             A large portion of the technical content of this
             advisory was provided by the DFN-CERT and NASIRC
             response teams, and is used with their permission.

There is a vulnerability in older versions of ghostscript (gs) that enables
users to execute commands and thus modify files. This problem involves the
- -dSAFER option and is present in all versions of ghostscript from 2.6 through
3.22 beta.

We recommend that you apply the solution in Section III below to fix the
- -dSAFER PostScript code or install the latest version of ghostscript (version
4.01). In both cases, we urge you to make -dSAFER the default mode for
all versions of ghostscript starting with version 2.6.

We will update this advisory as we receive additional information.
Please check advisory files regularly for updates that relate to your site.

- -----------------------------------------------------------------------------

I.   Background    

     The PostScript language, which was designed for the expression of 
     graphical data, is widely used for transferring images and preformatted
     text across the Internet. The language includes primitives for file 
     operations, which were intended to be useful in the expression of images.
     Unfortunately the operations can be abused by people intentionally 
     embedding commands within an otherwise harmless image so that when 
     displaying that image the PostScript viewer may perform malicious
     file creations or deletions.
    
     This is a potentially serious problem because many images transferred
     on the World Wide Web are sent in PostScript. For example, a malicious
     person could install a booby-trapped image on a web-page, buried among
     useful or interesting data.
    
     The viewer "ghostscript," a PostScript interpreter, recognizes the
     command-line option: "-dSAFER". This option is intended to disable
     the file operations and the %pipe PostScript operator that could be 
     abused to do damage. This option is intended to protect you from this 
     type of sabotage when viewing images from untrusted sources.

II.  Problem Description

     Problems exist with the ghostscript program, which supports the kind
     of commands discussed above. 

     Older versions of ghostscript do not completely disable the pipe operator
     that can be used execute commands that can modify files. Therefore the
     option -dSAFER does not provide full protection.
     
     This problem is present in all versions of ghostscript between
     2.6 (when the %pipe operator was added) and 3.22beta (when a fix
     was made). 

III. Impact

     Attackers who have inserted malicious code into a PostScript file can 
     cause commands to be executed and files to be modified on any system 
     where that PostScript file is viewed with ghostscript.

IV. Solutions

     We recommend either fixing the -dSAFER PostScript code or installing
     version 4.01 of ghostscript (see Sections IV.A and IV.B). In addition, 
     we urge you to enable the -dSAFER option as the default (see Section
     IV.C). 

     A. Fixing the -dSAFER PostScript code

        The following fix is in the form of "diff" output, which is
        suitable for use with the GNU patch program.  This patch brings the
        code into conformance with the version of gs_init.ps distributed
        with the latest version of ghostscript (4.01) and can be
        applied to the GNU versions 2.6, 2.6.1, and 2.6.2.  The file
        to be patched is in the ghostscript library. As an example, 
        gs_init.ps could be installed in:

          /usr/local/lib/ghostscript/gs_init.ps
    
        Here is the patch. Please see the Updates section at the end of this
        file for cautions and for corrections to be applied in some situations.

- --------------------------------cut here--------------------------------------

*** gs_init.ps.orig     Fri Aug 25 10:42:51 1995
- --- gs_init.ps  Fri Aug 25 11:16:24 1995
***************
*** 302,308 ****
  % If we want a "safer" system, disable some obvious ways to cause havoc.
  SAFER not { (%END SAFER) .skipeof } if
  /file
!  { dup (r) eq
      { file }
      { /invalidfileaccess signalerror }
     ifelse
- --- 302,308 ----
  % If we want a "safer" system, disable some obvious ways to cause havoc.
  SAFER not { (%END SAFER) .skipeof } if
  /file
! { dup (r) eq 2 index (%pipe*) .stringmatch not and
      { file }
      { /invalidfileaccess signalerror }
     ifelse
- --------------------------------cut here--------------------------------------

        The key is to change the line that says:

          { dup (r) eq

        to one that says:

          { dup (r) eq 2 index (%pipe*) .stringmatch not and

        Here are the relevant lines in the gs_init.ps file for version 2.6.2
        of ghostscript before the patch:

302  % If we want a "safer" system, disable some obvious ways to cause havoc. 
303  SAFER not { (%END SAFER) .skipeof } if
304  /file
305   { dup (r) eq
306      { file }
307      { /invalidfileaccess signalerror }
308     ifelse
309   } bind odef
310  /renamefile { /invalidfileaccess signalerror } odef
311  /deletefile { /invalidfileaccess signalerror } odef
312  %END SAFER

        Here are the same lines after the patch has been applied:

302  % If we want a "safer" system, disable some obvious ways to cause havoc.
303  SAFER not { (%END SAFER) .skipeof } if
304  /file
305  { dup (r) eq 2 index (%pipe*) .stringmatch not and
306      { file }
307      { /invalidfileaccess signalerror }
308     ifelse
309   } bind odef
310  /renamefile { /invalidfileaccess signalerror } odef
311  /deletefile { /invalidfileaccess signalerror } odef
312  %END SAFER


     B. Installing version 4.01
  
        You may wish to install Aladdin Ghostscript version 4.01.
        The latest version of ghostscript is version 4.01 and
        is available at the locations noted below.  

        This version of ghostscript is provided by Aladdin Enterprises and 
        is subject to their licensing agreements.  Please read the "Aladdin
        Ghostscript Free Public License" (included in the source code
        distribution) which differs from the "GNU Public License."

        Please note that this version is not the GNU version.  The latest GNU
        version, which is version 2.6.2, does not fix this problem.
             
          ftp://ftp.cs.wisc.edu/ghost/aladdin/ghostscript-4.01.tar.gz
                MD5=21a0fe505bbaf75e2e6aeb4e07689fb6

          ftp://ftp.cs.wisc.edu/ghost/aladdin/ghostscript-4.01jpeg.tar.gz
                MD5=5360e0aa47b415daa44623196f7e6160

          ftp://ftp.cs.wisc.edu/ghost/aladdin/ghostscript-4.01zlib.tar.gz
                MD5=8eb230a39275b0759f06fa100250fc00

        Optionally, you may need the font files for this release.  They are
        available at these locations:
     
         ftp://ftp.cs.wisc.edu/pub/aladdin/ghostscript-fonts-std-4.01.tar.gz
                MD5=1e0fe2149affd80deaaae144227049b9

         ftp://ftp.cs.wisc.edu/pub/aladdin/ghostscript-fonts-other-4.01.tar.gz
                MD5=afe46faf7fde6518ae004a7e8d9a4af4

     C. Making -dSAFER the default
     
        To make -dSAFER the default mode for ghostscript for all versions
        of ghostscript starting with version 2.6, the file gs_init.ps must
        again be changed.  The PostScript commands which check the actual
        interpreted command are collected in one single if statement in the
        gs_init.ps file. By commenting out the begin and end lines of this
        if statement, the check is always applied meaning that the -dSAFER
        option is always enabled.

        NOTE: If you make this change, all file and %pipe operations are
        disabled and cannot be re-enabled.

        The lines which must be changed are:

              303  SAFER not { (%END SAFER) .skipeof } if
        and
              312  %END SAFER

        These two lines should be commented out and made to look like this:

              303  % SAFER not { (%END SAFER) .skipeof } if
        and
              312  % %END SAFER

        If you are using ghostscript 2.6.2, the code will look like the
        following when both patches noted above are installed:

302  % If we want a "safer" system, disable some obvious ways to cause havoc.
303  % SAFER not { (%END SAFER) .skipeof } if
304  /file
305  { dup (r) eq 2 index (%pipe*) .stringmatch not and
306      { file }
307      { /invalidfileaccess signalerror }
308     ifelse
309   } bind odef
310  /renamefile { /invalidfileaccess signalerror } odef
311  /deletefile { /invalidfileaccess signalerror } odef
312  % %END SAFER


- ---------------------------------------------------------------------------
The CERT Coordination Center staff thanks the DFN-CERT and NASIRC response
teams for providing a large portion of the technical content of this advisory,
and we thank Wolfgang Ley for his assistance.
- ---------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident
Response and Security Teams (FIRST).

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the email be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on info.cert.org), or PEM (contact
CERT staff for details).

Internet email: cert@cert.org
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
           and are on call for emergencies during other hours.
Fax: +1 412-268-6989

Postal address:  CERT Coordination Center
                 Software Engineering Institute
                 Carnegie Mellon University
                 Pittsburgh, PA 15213-3890
                 USA

CERT advisories and bulletins are posted on the USENET newsgroup
comp.security.announce. If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send mail
to cert-advisory-request@cert.org.

Past CERT publications, information about FIRST representatives, and
other information related to computer security are available for anonymous
FTP from info.cert.org.


This material may be reproduced and distributed without permission provided it
is used for noncommercial purposes and the CERT Coordination Center, DFN-CERT,
and NASIRC are acknowledged.

CERT is a service mark of Carnegie Mellon University.


=============================================================================
UPDATES

1) We have received information that some tools that convert PostScript to
   other formats break when the SAFER option is the default, as recommended
   in Section III.C above.

   The problem is that these tools need the PostScript /file directive
   that is disabled when the SAFER option is made the default.  To this end,
   there is a fix from Joern Tellkamp (tellkamp@informatik.uni-hamburg.de),
   provided by DFN-CERT that defines an UNSAFER option to ghostscript.
   By default, ghostscript with the fixes listed in Section III.C above
   sets the SAFER option.

   The following patch changes the SAFER option to the UNSAFER option.  By
   default, SAFER is on but it can be turned off with the -dUNSAFER option to
   ghostscript.  This, too, is applied to the original gs_init.ps file.

                        Begin UNSAFER Patch
- ------------------------------------------------------------------------------

*** gs_init.ps          Fri Aug 25 10:42:51 1995
- --- gs_init.ps.unsafer  Fri Oct 20 13:57:37 1995
***************
*** 66,72 ****
     currentdict /OUTPUTFILE undef
   } if
  currentdict /QUIET known   /QUIET exch def
! currentdict /SAFER known   /SAFER exch def
  currentdict /WRITESYSTEMDICT known   /WRITESYSTEMDICT exch def
  
  % Acquire environment variables.
- --- 66,72 ----
     currentdict /OUTPUTFILE undef
   } if
  currentdict /QUIET known   /QUIET exch def
! currentdict /UNSAFER known /UNSAFER exch def
  currentdict /WRITESYSTEMDICT known   /WRITESYSTEMDICT exch def
  
  % Acquire environment variables.
***************
*** 299,308 ****
  /.run /run load def
  /run /run0 load def
  
! % If we want a "safer" system, disable some obvious ways to cause havoc.
! SAFER not { (%END SAFER) .skipeof } if
  /file
!  { dup (r) eq
      { file }
      { /invalidfileaccess signalerror }
     ifelse
- --- 299,308 ----
  /.run /run load def
  /run /run0 load def
  
! % If we want an "unsafer" system, enable some obvious ways to cause havoc.
! UNSAFER { (%END UNSAFER) .skipeof } if
  /file
! { dup (r) eq 2 index (%pipe*) .stringmatch not and
      { file }
      { /invalidfileaccess signalerror }
     ifelse
***************
*** 309,315 ****
   } bind odef
  /renamefile { /invalidfileaccess signalerror } odef
  /deletefile { /invalidfileaccess signalerror } odef
! %END SAFER
  
  % Create the error handling machinery.
  % The interpreter has created the ErrorNames array.
- --- 309,315 ----
   } bind odef
  /renamefile { /invalidfileaccess signalerror } odef
  /deletefile { /invalidfileaccess signalerror } odef
! %END UNSAFER
  
  % Create the error handling machinery.
  % The interpreter has created the ErrorNames array.
- ------------------------------------------------------------------------------
                        End UNSAFER Patch

   Once applied, all of the aforementioned tools need to be changed to add the
   -dUNSAFER option to the rest of the arguments given to gs, the ghostscript
   interpreter.

2) We received a report that adding any of the above-mentioned patches
   may cause the gs interpreter to fail (in version 2.6.0). Should this
   be the case, changing '.stringmatch' to 'stringmatch' fixes this problem
   (see below).  Upgrading to ghostscript version   2.6.1 also will address
    the problem. 

        If you have a problem with

             ! { dup (r) eq 2 index (%pipe*) .stringmatch not and

        change to

             ! { dup (r) eq 2 index (%pipe*) stringmatch not and

3) Since it is unknown at this time whether the Macintosh and DOS/Windows
   versions of ghostscript are vulnerable, we suggest that you apply the
   patch.

4) Version 3.33 with appropriate patches will address the vulnerabilities
   outlined in advisory CA-95:10. As of Nov. 8, 1995, the most recent release
   of ghostscript is Version 3.51. 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Revision history

Aug. 30, 1996  Information previously in the README was inserted
                into the advisory, with editing in the Updates section.
Nov. 08, 1995  Updates section - added SAFER and UNSAFER patches and
                instructions for applying them. Included a note for Macintosh
                and DOS/Windows users. Noted a recent release of ghostscript
                version 3.51. 




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