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CIAC G-17 March 12, 1996: Vulnerabilities in sample HTTPD CGIs




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 Msg  : 35 of 55                                                                
 From : Jack Burroughs              1:340/13                21 Mar 96  14:07:32 
 To   : All                                                 22 Mar 96  10:13:28 
 Subj : CIAC Bulletin G-17: Vulnerabilities in Sample HTTPD CGIs                
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From: got@popd.ix.netcom.com (Jack Burroughs)
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             __________________________________________________________

                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

                 Vulnerabilities in Sample HTTPD CGIs

March 12, 1996 21:00 GMT                                           Number G-17
______________________________________________________________________________
PROBLEM:       C-language based Common Gateway Interface (CGI) programs
               that call a shell to execute other programs.
PLATFORM:      Any CGI program built using the sample code distributed with
               NCSA HTTPD Version 1.5A-Export and earlier or Apache HTTPD
               version 1.0.3 and earlier that allows input from user and
               passes that input as arguments to a shell command.
DAMAGE:        Unauthorized access to the server host may allow an intruder
               to read, modify, or destroy files.
SOLUTION:      Check for vulnerability and install the proper patch.
______________________________________________________________________________
VULNERABILITY  Knowledge of how to exploit this vulnerability is becoming
ASSESSMENT:    widely known. This is not an HTTPD server program
               vulnerability.
______________________________________________________________________________


CIAC has obtained information from IBM-ERS pertaining to vulnerabilities
with software distributed with NCSA HTTPD and Apache HTTPD. CIAC recommends
that you install the proper patch and/or follow the solutions describe below.

[ Start of IBM Bulletin ]


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                           EMERGENCY RESPONSE SERVICE
                          SECURITY VULNERABILITY ALERT

26 February 1996 14:00 GMT                       Number: ERS-SVA-E01-1996:002.1
===============================================================================
                             VULNERABILITY  SUMMARY

VULNERABILITY:  Some C-language based Common Gateway Interface programs that
                call a shell to execute other programs can be tricked into
                executing any arbitrary command.

PLATFORMS:      Any CGI program built using the sample code distributed with
                NCSA HTTPD Version 1.5A-Export and earlier or Apache HTTPD
                Version 1.0.3 and earlier that accepts input from the user
                and passes that input as arguments to a shell command.

SOLUTION:       If using the sample code, apply the patch described below.
                If using other code, check for the vulnerability as
                described in the following text.

THREAT:         Many World Wide Web sites have been built using the programs
                in question.  Therefore, the potential security exposure may
                be widespread.

===============================================================================
                                      NOTE

The vulnerability described in this alert is contained in ancillary, example
software distributed with NCSA HTTPD and Apache HTTPD.

            THIS IS NOT AN HTTPD SERVER PROGRAM VULNERABILITY.

Commercial HTTPD server products developed and sold by IBM do not include
this sample code on their distribution media.  Commercial HTTPD server
products developed by Netscape Communications Corporation and sold by
Netscape and IBM do not include this sample code on their distribution media.

If you have purchased one of these products and have not installed any
programs other than those on the distribution media, you are not vulnerable
to this problem.  Note, however, that if you previously had NCSA HTTPD or
Apache HTTPD installed, you may have remnants of the installation left over
which may be vulnerable.

If you have purchased another commercial HTTPD server product, you may wish
to contact your vendor to see if any action is necessary.

===============================================================================
                              DETAILED INFORMATION

I. Description

  A. Background Information

     The Common Gateway Interface (CGI) specifies a protocol by which a World
     Wide Web server can interface with external programs run on the server
     host.  These external programs perform tasks on behalf of the user such
     as sending mail, accessing databases, and so forth.  Many of these
     programs accept input from the user via an interactive form containing
     dialog boxes, menus, buttons, etc.  Upon completion of the task, the
     programs may return the results of the operation to the user, usually in
     the form of a Hypertext Markup Language (HTML) document.  Programs which
     implement the Common Gateway Interface are usually called "CGI scripts,"
     although they are also referred to as "CGI-BIN programs," after the name
     of the directory in which they are commonly stored on UNIX systems.

     CGI programs, because they often accept input from the user and then
     pass that input to other programs as command-line arguments, must be
     written with great care.  Because the languages used to implement these
     programs (Perl, Bourne shell, Korn shell, C shell, etc.) attribute
     special meaning to a large variety of characters ('>', ';', '&', '$',
     etc.), the program must check its input and remove these characters
     before passing them on to other commands.  If this is not done, or is
     done incorrectly, an attacker can provide "bogus" input to these
     programs that causes them to behave in unexpected ways, often resulting
     in allowing the attacker to gain unauthorized access to the server
     host, or to execute unauthorized commands on the host.

     Because these problems are so difficult to avoid when using these
     powerful scripting languages, many experts now recommend that CGI
     programs be written in the C programming language instead.  Because C
     does not attempt to evaluate character strings, a program written in C
     is less vulnerable to this type of attack.  However, if a CGI program
     written in C uses the input it receives to execute another command
     using the system() or popen() library functions, the benefits of using
     C are negated, and the program must once again carefully check its
     inputs before passing them as command line arguments to other programs.

  B. Vulnerability Details

     NCSA HTTPD and Apache HTTPD, two of the most popular freely-available
     World Wide Web server implementations, come with several CGI programs
     that are compiled by the default build procedure.  These programs,
     because they are meant to serve as examples, are frequently installed
     at a site even if they are not being used.  Furthermore, because the
     source code for these programs is meant to be an example of how to
     implement CGI programs, it is frequently copied into other CGI programs
     programs as well.

     One of the utility functions offered by the CGI example source code is
     called escape_shell_cmd().  It is intended to help programmers avoid the
     vulnerability described above.  This function, when given an input
     string received from the user, scans the string for characters that have
     special meaning to the UNIX shell, and inserts escapes in front of these
     characters to remove their special meaning.  However, the list of
     special characters used by escape_shell_cmd():

         &;`'"|*?~<>^()[]{}$\

     is incomplete: it is missing the newline ('\n', octal 012, hex 0x0a)
     character.

     The CGI example source code also includes a program called "phf," which
     implements a form-based interface to a local CCSO Name Server.  (The
     CCSO Name Server is a white pages service used for looking up name and
     address information about people.)  The "phf" demonstration program
     uses the escape_shell_cmd() function to check its inputs, and is thus
     vulnerable to attack as described above.

     Any World Wide Web server host that has been configured to allow access
     to and execution of Common Gateway Interface programs and

       1. Has installed the "phf" program from the NCSA HTTPD distribution or
          the Apache HTTPD distribution in a CGI-BIN directory (even if the
          program is not being used)

       and/or

       2. Has installed programs obtained from the Internet or elsewhere that
          are using the escape_shell_cmd() function contained in the sample
          CGI-BIN source code distributed with NCSA HTTPD or Apache HTTPD

       and/or

       3. Has installed programs developed internally using the
          escape_shell_cmd() function contained in the sample CGI-BIN source
          code distributed with NCSA HTTPD or Apache HTTPD

       and/or

       4. Has installed CGI programs that do not adequately check their
          inputs for the special characters listed above (including newline)
          before passing these inputs to a shell command

     is vulnerable to attack in this manner.

II. Impact

An attacker who knows how to exercise this vulnerability may have the
ability to:

  1. Execute arbitrary commands on the server host using the same user-id as
     the user running the "httpd" server.  If "httpd" is being run as
     "root," the attacker's commands are also run as "root."

  2. Access any file on the system that is accessible to the user-id that is
     running the "httpd" server.  If the "httpd" server user-id has read
     access to the file, the attacker can also read the file.  If the
     "httpd" server user-id has write access to the file, the attacker can
     change or destroy the contents of the file.  If the "httpd" server is
     being run as "root," the attacker can read, modify, or destroy any file
     on the server host.

  3. Given an X11-based terminal emulator ("xterm" or equivalent) installed
     on the "httpd" server host, gain full interactive access to the server
     host just as if he were logging in locally.

III. Solutions

  A. Suggested actions

     The IBM Emergency Response Service (IBM-ERS) team recommends that you
     consider taking the following actions (subject to any licensing
     restrictions that may apply to your copies of the programs):

       1. If you have installed the "phf" program from the NCSA or Apache
          HTTPD binary or source distributions and are not using it, remove
          it.

       2. If you have installed the "phf" program from the NCSA or Apache
          HTTPD source distributions and are using it, apply one of the
          patches below, rebuild the "phf" program, and install the new
          version.

       3. If you have installed the "phf" program from the NCSA or Apache
          HTTPD binary distributions and are using it, obtain the NCSA or
          Apache source distributions:

          NCSA:
            ftp://ftp.ncsa.uiuc.edu/Web/httpd/Unix/ncsa_httpd/httpd_1.5/
                  httpd_1.5a-export_source.tar.Z

          Apache:
            ftp://ftp.apache.org/apache/dist/apache_1.0.3.tar.gz

          Then apply one of the patches below, rebuild the "phf" program, and
          install the new version.

       4. If you have installed other C-language CGI programs obtained from
          the Internet or elsewhere and have the source code for these
          programs, examine them closely and make sure that they are
          properly checking their inputs before passing them to a shell.

          If these programs are using the escape_shell_cmd() function from
          the NCSA or Apache distributions, the patches below can be adapted
          to these programs.

          Once you have identified any problem areas and developed fixes for
          them, rebuild the programs and install the new versions.

       5. If you have developed your own C-language CGI programs using the
          sample code contained in the NCSA or Apache distributions, adapt
          the patches below to your code, rebuild these programs, and
          install the new versions.

       6. If you have purchased CGI programs from third-party vendors,
          contact your vendor to determine whether or not these programs
          contain this vulnerability.

       7. If you are running your "httpd" server as "root," consider running
          it as an unprivileged user instead.

  B. Patch for escape_shell_cmd()

     The escape_shell_cmd() function is contained in the "util.c" file in the
     "cgi-src" directory of the NCSA HTTPD and Apache HTTPD source
     distributions.

     The patch below can be applied to the NCSA version of "util.c" to fix
     this vulnerability:

- - ---------------------------------- cut here
----------------------------------
*** httpd_1.5a-export/cgi-src/util.c.old        Tue Nov 14 11:38:40 1995
- - --- httpd_1.5a-export/cgi-src/util.c        Thu Feb 22 20:37:07 1996
***************
*** 139,145 ****

      l=strlen(cmd);
      for(x=0;cmd[x];x++) {
!         if(ind("&;`'\"|*?~<>^()[]{}$\\",cmd[x]) != -1){
              for(y=l+1;y>x;y--)
                  cmd[y] = cmd[y-1];
              l++; /* length has been increased */
- - --- 139,145 ----

      l=strlen(cmd);
      for(x=0;cmd[x];x++) {
!         if(ind("&;`'\"|*?~<>^()[]{}$\\\n",cmd[x]) != -1){
              for(y=l+1;y>x;y--)
                  cmd[y] = cmd[y-1];
              l++; /* length has been increased */
- - ---------------------------------- cut here
----------------------------------

     The patch below can be applied to the Apache version of "util.c" to fix
     this vulnerability:

- - ---------------------------------- cut here
----------------------------------
*** apache_1.0.3/cgi-src/util.c.old     Sat Feb 17 03:32:14 1996
- - --- apache_1.0.3/cgi-src/util.c     Fri Feb 23 10:48:43 1996
***************
*** 135,141 ****

      l=strlen(cmd);
      for(x=0;cmd[x];x++) {
!         if(ind("&;`'\"|*?~<>^()[]{}$\\",cmd[x]) != -1){
              for(y=l+1;y>x;y--)
                  cmd[y] = cmd[y-1];
              l++; /* length has been increased */
- - --- 135,141 ----

      l=strlen(cmd);
      for(x=0;cmd[x];x++) {
!         if(ind("&;`'\"|*?~<>^()[]{}$\\\n",cmd[x]) != -1){
              for(y=l+1;y>x;y--)
                  cmd[y] = cmd[y-1];
              l++; /* length has been increased */
- - ---------------------------------- cut here
----------------------------------

     To apply the patch, save the text between the two "-- cut here --" lines
     in a file, change directories to your source tree (the directory that
     contains "httpd_1.5a-export" or "apache_1.0.3" as a subdirectory) and
     issue the command:

         patch < filename

     If you do not have the "patch" program, you can obtain it from

         ftp://prep.ai.mit.edu/pub/gnu/patch-2.1.tar.gz

     or you can apply the patch by hand.

     This fix is provied "AS IS" without warranty of any kind, including,
     without limitation, any implied warranties of merchantibility or
     fitness for a particular purpose.  This advisory does not create or
     imply any support obligations or any other liability on the part of
     IBM or its subsidiaries.

  C. Pending fix from NCSA

     NCSA states that they have fixed this problem in HTTPD Version 1.5.1,
     which is currently in the beta-testing stage.  You can obtain NCSA
     HTTPD 1.5.1 Beta 3 from

         http://hoohoo.ncsa.uiuc.edu/beta-1.5/

     if you wish to experiment with it.  However, NCSA warns that this is
     NOT a stable version of the server.

  D. Pending fix from Apache

     The Apache Project is aware of this problem, and states that they plan
     to provide a fix for it in Apache HTTPD Version 1.1, which is scheduled
     for release in early March.  For more information about the pending
     release of Apache HTTPD Version 1.1, see the Apache Project home page at

         http://www.apache.org/

  E. A second potential vulnerability

     When examining your CGI programs that make use of the escape_shell_cmd()
     function, note that escape_shell_cmd() does not perform any check on the
     length of the buffer it is passed.  Because each character in the buffer
     has the potential to be escaped with a backslash, the resulting string
     can be up to twice as long as the original.

     Any buffer that is passed into this function should be at least (2n+1)
     bytes in size, where n is the length of the unescaped string.

     Programs that do not adhere to this requirement are vulnerable to buffer
     overrun attacks, much like those used successfully by the Internet worm
     against the finger daemon, as well as in several more recent attacks
     against other programs.

  F. Additional information

     If you have enabled the Common Gateway Interface in your server, even if
     you are not yet actively using it, IBM-ERS recommends that you learn as
     much as possible about the security issues involved.  We have provided
     URLs for some good sources of information on this topic below:

     CGI Security
     ------------
         http://hoohoo.ncsa.uiuc.edu/cgi/security.html
         http://www.cerf.net/~paulp/cgi-security/

     WWW Security (Including CGI)
     ----------------------------
         http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html

    Please note that the URLs referenced in this advisory are not under IBM's
    control and therefore IBM cannot be responsible for their availability or
    content.  If you have any questions regarding any of these sites, please
    contact that site's administrator or webmaster.

IV. Acknowledgements

IBM-ERS would like to thank the IBM Global I/T Security Consulting Group and
the IBM Global Security Analysis Laboratory at the IBM T. J. Watson Research
Center, for bringing this exposure to our attention, providing the patches
for it, and their assistance in the development of this alert.

[ End of IBM Bulletin ]
_______________________________________________________________________________

CIAC wishes to acknowledge the contributions of IBM Emergency Response
Service for the information contained in this bulletin.
_______________________________________________________________________________

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy (DOE) and the
National Institute of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding member of
FIRST, the Forum of Incident Response and Security Teams, a global organization
established to foster cooperation and coordination among computer security
teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC can be
contacted at:
    Voice:    +1 510-422-8193
    FAX:      +1 510-423-8002
    STU-III:  +1 510-423-2604
    E-mail:   ciac@llnl.gov

For emergencies and off-hour assistance, DOE, DOE contractor sites, and the NIH
may contact CIAC 24-hours a day. During off hours (5PM - 8AM PST), call the
CIAC voice number 510-422-8193 and leave a message, or call 800-759-7243
(800-SKY-PAGE) to send a Sky Page. CIAC has two Sky Page PIN numbers, the
primary PIN number, 8550070, is for the CIAC duty person, and the secondary
PIN number, 8550074 is for the CIAC Project Leader.

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:      http://ciac.llnl.gov/
   Anonymous FTP:       ciac.llnl.gov (128.115.19.53)
   Modem access:        +1 (510) 423-4753 (28.8K baud)
                        +1 (510) 423-3331 (28.8K baud)

CIAC has several self-subscribing mailing lists for electronic publications:
1. CIAC-BULLETIN for Advisories, highest priority - time critical information
   and Bulletins, important computer security information;
2. CIAC-NOTES for Notes, a collection of computer security articles;
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   software updates, new features, distribution and availability;
4. SPI-NOTES, for discussion of problems and solutions regarding the use of
   SPI products.

Our mailing lists are managed by a public domain software package called
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SPI-NOTES for list-name and valid information for LastName FirstName and
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E-mail to       ciac-listproc@llnl.gov:
        subscribe list-name LastName, FirstName PhoneNumber
  e.g., subscribe ciac-notes OHara, Scarlett W. 404-555-1212 x36

You will receive an acknowledgment containing address, initial PIN, and
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PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these communities,
please contact your agency's response team to report incidents. Your agency's
team will coordinate with CIAC. The Forum of Incident Response and Security
Teams (FIRST) is a world-wide organization. A list of FIRST member
organizations and their constituencies can be obtained by sending email to
docserver@first.org with an empty subject line and a message body containingt
the line: send first-contacts.

This document was prepared as an account of work sponsored by an agency of the
United States Government. Neither the United States Government nor the
University of California nor any of their employees, makes any warranty,
express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus, product,
or process disclosed, or represents that its use would not infringe privately
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opinions of authors expressed herein do not necessarily state or reflect those
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be used for advertising or product endorsement purposes.

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Frank R. Swift
Computer Security
Lawrence Livermore National Laboratory
Voice (510) 422-1463  Fax (510) 423-0913
uncl@llnl.gov

PGP Key fingerprint = 1A 14 02 5A 76 B2 BD 47  C0 3E ED 9A C5 3B 81 2D

--- ifmail v.2.8.lwz
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