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The U.S. Department of Energy
Computer Incident Advisory Capability
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INFORMATION BULLETIN
Vulnerability in BASH Program
August 29, 1996 15:00 GMT Number G-41
______________________________________________________________________________
PROBLEM: A variable declaration error in the GNU Project's BASH (Bourne
Again SHell) program allows the character with value 255
decimal to be used as a command separator.
PLATFORM: BASH 1.14.6 and any earlier versions.
DAMAGE: 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.
SOLUTION: Apply the patches listed in the vendor bulletin below.
______________________________________________________________________________
VULNERABILITY This vulnerability is becoming widely known.
ASSESSMENT:
______________________________________________________________________________
This is a combination of two bulletins sent out by IBM on the BASH
Vulnerability.
- --ERS-ALERT--ERS-ALERT--ERS-ALERT--ERS-ALERT--ERS-ALERT--ERS-ALERT-ERS-ALERT
- ---EXTERNAL RELEASE---EXTERNAL RELEASE---EXTERNAL RELEASE---EXTERNAL RELEASE
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EMERGENCY RESPONSE SERVICE
SECURITY VULNERABILITY ALERT
21 August 1996 13:00 GMT Number: ERS-SVA-E01-1996:004.1
28 August 1996 18:00 GMT Number: ERS-SVA-E01-1996:004.2
=============================================================================
VULNERABILITY SUMMARY
VULNERABILITY: A variable declaration error in "bash" allows the character
with value 255 decimal to be used as a command separator.
PLATFORMS: Bash 1.14.6 and earlier versions.
SOLUTION: Apply the patch provided below.
THREAT: 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.
=============================================================================
DETAILED INFORMATION
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, but 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
There is a variable declaration error in the "yy_string_get()" function
in the "parser.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
vaule 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."
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. Solutions
How to alleviate the problem
This problem can be alleviated by changing the declaration of the
"string" variable in the "yy_string_get()" function from "char *" to
"unsigned char *."
I. New version of "bash" released
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
(It will also be available from the usual GNU archives in a few days.)
II. Updated patch for second potential problem
IBM-ERS has now received information that a similar problem exists with
the "yy_readline_get()" function, also in the file "parse.y," 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
completeness' sake.
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, as well as many other
sites around the Internet.
- ---------------------------------- 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.
THIS PATCH IS PROVIDED "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.
IV. Acknowledgements
IBM-ERS would like to thank the IBM Global Security Analysis Laboratory at the
IBM T. J. Watson Research Center and Jens Schweikhardt of the DFN Network
Operations Center for their discovery of this vulnerability, bringing it to
our attention, providing the patch to fix it, and assistance in developing
this alert. IBM-ERS would like to thank DFN-CERT for relaying the information
to us.
UNIX is a technology trademark of X/Open Company, Ltd.
===============================================================================
_______________________________________________________________________________
CIAC wishes to acknowledge the contributions of IBM for the
information contained in this bulletin.
_______________________________________________________________________________
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