Visit our newest sister site!
Hundreds of free aircraft flight manuals
Civilian • Historical • Military • Declassified • FREE!


TUCoPS :: Unix :: General :: ciach001.txt

Vulnerabilities in bash





             __________________________________________________________

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

                             INFORMATION BULLETIN

                            Vulnerabilities in bash

October 9, 1996 22:00 GMT                                          Number H-01
______________________________________________________________________________
PROBLEM:       Two vulnerabilities have been identified with GNU Project's
               Bourne Again SHell (bash): (1) in yy_string_get() and (2) in
               yy_readline_get().
PLATFORM:      Any UNIX platform or any "free" versions of UNIX such as Linux,
               where bash has been installed as "/bin/sh". The problems affect
               bash versions 1.14.6 and earlier.
DAMAGE:        The user could execute any arbitrary command with the user-id
               and permissions of the user running the server (frequently
               root).
SOLUTION:      Upgrade to bash 1.14.7 and apply the patch indicated below.
______________________________________________________________________________
VULNERABILITY  The bash shell is the default shell for most (if not all) Linux
ASSESSMENT:    distributions, so most Linux systems are subject to this 
               attack. The nature of the vulnerability makes Web servers 
               running CGI scripts susceptible to outside attack -- it is
               not necessary to have a user account to exploit this 
               vulnerability. It is highly recommended that this vulnerability
               be addressed on all Linux systems, particularly those with 
               Web servers running CGI scripts.
______________________________________________________________________________

[ Start of CERT Advisory ]

=============================================================================
CERT(sm) Advisory CA-96.22
Original issue date: October 8, 1996
Last revised: --

Topic: Vulnerabilities in bash
-----------------------------------------------------------------------------
                The original technical content for this advisory
                was published by the IBM-ERS response team and
                is used here with their permission.

This advisory describes two problems with the GNU Project's Bourne Again
SHell (bash): one in yy_string_get() and one in yy_readline_get().

The vulnerability in yy_string_get() allows the character with value 255
decimal to be used as a command separator. When used in environments where
users provide strings to be used as commands or arguments to commands, bash
can be tricked into executing arbitrary commands.

It is not clear whether the problem with yy_readline_get() results in an
exploitable vulnerability, though you may want to address both problems for
completeness.

The problems affect bash versions 1.14.6 and earlier.

The CERT/CC team recommends that you upgrade to bash 1.14.7 as soon as
possible, as discussed in Section III.A below. Section III.B contains a
patch for 1.14.7, which we recommend using to address the yy_readline_get()
problem.

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

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

I. Description

   A. Introduction

      The GNU Project's Bourne Again SHell (bash) is a drop-in replacement
      for the UNIX Bourne shell (/bin/sh). It offers the same syntax as the
      standard shell, and it also includes additional functionality such as
      job control, command line editing, and history.

      Although bash can be compiled and installed on almost any UNIX
      platform, its most prevalent use is on "free" versions of UNIX such as
      Linux, where it has been installed as "/bin/sh" (the default shell for
      most uses).

      The bash source code is freely available from many sites on the
      Internet.

   B. Vulnerability Details

      1. Vulnerability in yy_string_get()

         There is a variable declaration error in the "yy_string_get()"
         function in the "parse.y" module of the "bash" source code. This
         function is responsible for parsing the user-provided command line
         into separate tokens (commands, special characters, arguments, etc.).
         The error involves the variable "string", which has been declared to
         be of type "char *".

         The "string" variable is used to traverse the character string
         containing the command line to be parsed. As characters are
         retrieved from this pointer, they are stored in a variable of type
         "int". On systems/compilers where the "char" type defaults to
         "signed char" this value will be sign-extended when it is assigned
         to the  "int" variable. For character code 255 decimal (-1 in two's
         complement form), this sign extension results in the value (-1)
         being assigned to the integer.

         However, (-1) is used in other parts of the parser to indicate the
         end of a command. Thus, the character code 255 decimal (377 octal)
         will serve as an unintended command separator for commands given to
         bash via the "-c" option. For example,

         bash -c 'ls\377who'

         (where "\377" represents the single character with value 255 decimal)
         will execute two commands, "ls" and "who".

      2. Possible vulnerability in yy_readline_get()

         A similar problem exists with the "yy_readline_get()" function, which
         is also in the file "parse.y" and which is used to read commands in
         interactive shells (ones that print a prompt and read from the
         keyboard, a shell script, or a pipe).

         It is not clear that this problem produces any exploitable
         vulnerabilities in the bash program; however, you may wish to
         address the problem for the sake of completeness.

II.  Impact

     This unexpected command separator can be dangerous, especially on systems
     such as Linux where bash has been installed as "/bin/sh," when a program
     executes a command with a string provided by a user as an argument using
     the "system()" or "popen()" functions (or by calling "/bin/sh -c string"
     directly).

     This is especially true for the CGI programming interface in World Wide
     Web servers, many of which do not strip out characters with value 255
     decimal. If a user sending data to the server can specify the character
     code 255 in a string that is passed to a shell, and that shell is bash,
     the user can execute any arbitrary command with the user-id and
     permissions of the user running the server (frequently "root").

     The bash built-in commands "eval," "source," and "fc" are also
     potentially vulnerable to this problem.

III. Solution

     A. Vulnerability in yy_string_get

        On 27 August 1996, Version 1.14.7 of bash was released. You can
        obtain this new version from:

        ftp://slc2.ins.cwru.edu/pub/dist/bash-1.14.7.tar.gz


     B. Vulnerability in yy_readline_get

        It is not clear that this problem produces any exploitable
        vulnerabilities in the "bash" program; however, you may wish to
        address the problem for the sake of completeness.

        This problem can be alleviated by applying the patch below to the
        bash source code, then recompiling the program, and installing the
        new version.

        The patch below is for Version 1.14.7 of bash. Source code for this
        version can be obtained from the site listed above.

---------------------------------- cut here ---------------------------------
*** parse.y.old Mon Aug 26 11:15:55 1996
- - - --- parse.y   Wed Aug 28 08:49:15 1996
***************
*** 801,807 ****

  #if defined (READLINE)
  char *current_readline_prompt = (char *)NULL;
! char *current_readline_line = (char *)NULL;
  int current_readline_line_index = 0;

  static int
- - - - --- 801,807 ----

  #if defined (READLINE)
  char *current_readline_prompt = (char *)NULL;
! unsigned char *current_readline_line = (unsigned char *)NULL;
  int current_readline_line_index = 0;

  static int
--------------------------------- cut here ----------------------------------

        To apply this patch, save the text between the two "--- cut here ---"
        lines to a file, change directories to the bash source directory,
        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.

        After applying the patch, recompile and reinstall the bash program by
        following the directions in the "INSTALL" file, included as part of
        the bash distribution.

----------------------------------------------------------------------------
The CERT Coordination Center thanks IBM-ERS for permission to reproduce the
technical content in their IBM Emergency Response Service Security
Vulnerability Alerts ERS-SVA-E01-1006:004.1 and ERS-SVA-E01-1006:004.2.
These alerts are copyrighted 1996 International Business Machines Corporation.
----------------------------------------------------------------------------
 
[ End of CERT Advisory ]
_______________________________________________________________________________

CIAC wishes to acknowledge the contributions of CERT and  IBM-ERS 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 emergency backup response team for the National
Institutes 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;
3. SPI-ANNOUNCE for official news about Security Profile Inspector
   (SPI) 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 ListProcessor, which ignores E-mail header subject lines. To
subscribe (add yourself) to one of our mailing lists, send the
following request as the E-mail message body, substituting
CIAC-BULLETIN, CIAC-NOTES, SPI-ANNOUNCE or SPI-NOTES for list-name and
valid information for LastName FirstName and PhoneNumber when sending

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 information on how to change either of them, cancel your
subscription, or get help.

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
containing 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
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 or the University of California, and shall not be used for
advertising or product endorsement purposes.

LAST 10 CIAC BULLETINS ISSUED (Previous bulletins available from CIAC)

G-39: Vulnerability in expreserve
G-40: SGI admin and user Program Vulnerabilities
G-41: Vulnerability in BASH Program
G-42: Vulnerability in WorkMan Program
G-43: Vulnerabilities in Sendmail 
G-44: SCO Unix Vulnerability 
G-45: Vulnerability in HP VUE 
G-46: Vulnerabilities in Transarc DCE and DFS 
G-47: Unix FLEXlm Vulnerabilities 
G-48: TCP SYN Flooding and IP Spoofing Attacks 

RECENT CIAC NOTES ISSUED (Previous Notes available from CIAC)

Notes 07 - 3/29/95     A comprehensive review of SATAN

Notes 08 - 4/4/95      A Courtney update

Notes 09 - 4/24/95     More on the "Good Times" virus urban legend

Notes 10 - 6/16/95     PKZ300B Trojan, Logdaemon/FreeBSD, vulnerability
                       in S/Key, EBOLA Virus Hoax, and Caibua Virus

Notes 11 - 7/31/95     Virus Update, Hats Off to Administrators,
                       America On-Line Virus Scare, SPI 3.2.2 Released,
                       The Die_Hard Virus

Notes 12 - 9/12/95     Securely configuring Public Telnet Services, X
                       Windows, beta release of Merlin, Microsoft Word
                       Macro Viruses, Allegations of Inappropriate Data
                       Collection in Win95

Notes 96-01 - 3/18/96  Java and JavaScript Vulnerabilities, FIRST
                       Conference Announcement, Security and Web Search
                       Engines, Microsoft Word Macro Virus Update


TUCoPS is optimized to look best in Firefox® on a widescreen monitor (1440x900 or better).
Site design & layout copyright © 1986-2014 AOH