CERT-SEI

Vulnerabilities in bash

Original issue date: October 8, 1996
Last revised: September 24, 1997
Updated copyright statement

A complete revision history is at the end of this file. 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.

When the advisory was first published, was not clear whether the problem with yy_readline_get() resulted in an exploitable vulnerability. As of November 1996, it appears that the problem is not exploitable in yy_readline_get.

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".

Note about 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).

However, it appears that this problem does not produce an exploitable vulnerability.

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

Install the most current version of bash. On 27 August 1996, Version 1.14.7 of bash was released; It is available from

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

This version addresses the vulnerability in yy_string_get.

When this advisory was first released, we included a patch for yy_readline_get. It now appears that the patch is unnecessary as the problem is not exploitable in yy_readline_get. Upgrading to the current version of bash is sufficient.


Appendix A

The following is vendor-supplied information.
For the most up-to-date information, contact your vendor.

IBM Corporation

AIX does not ship with the bash shell.

IBM and AIX are registered trademarks of International
Business Machines Corporation.

Silicon Graphics, Inc.

SGI has distributed bash (version 1.14.6) as part of the Freeware 1.0 CDROM. This collection of software has been compiled for IRIX as a service to our customers, but is furnished without formal SGI support.

The problem identified by IBM in bash is present in the version of bash on the Freeware 1.0 CDROM. This CDROM included both the source code for bash an compiled versions of it.

SGI urges customers to recompile bash after making the changes in parse.y suggested by IBM.

As a service similar to that of the original Freeware 1.0 CDROM, SGI intends to make available a compiled version of bash and its source in the near future. This action does not necessarily imply a commitment to any future support actions for the programs found on the Freeware 1.0 CDROM.

Linux

Patches for the following Linux versions are available.

SuSE 4.2

ftp://ftp.suse.de/suse_update/suse42/a1/bash.tgz

Red Hat 3.0.3

ftp://ftp.redhat.com/pub/redhat/updates/3.0.3/{architecture}/bash-1.14.6.8.{architecture}.rpm

Yggdrasil

Patched bash source and binary tar files are now FTPable from
ftp://ftp.yggdrasil.com/pub/support/fall95 WGS Linux Pro

ftp://ftp.wgs.com/pub/linux/redhat/updates/3.0.3/i386/bash-1.14.6-8.i386.rpm

Caldera

Have built the new bash-1.14.7 code from prep.ai.mit.edu -/pub/gnu. Tested only insofar as to ascertain that the security bug is fixed. Binary and source RPMs live in ftp -

ftp://ftp.caldera.com/pub/cnd-1.0/updates/bash-1.14.7-1.i386.rpm

ftp://ftp.caldera.com/pub/cnd-1.0/updates/bash-1.14.7-1.src.rpm


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.


Copyright 1996 Carnegie Mellon University.


Revision History
Sep. 24, 1997  Updated copyright statement
Nov. 13, 1996  Noted that yy_readline_get does not require the patch included
               in the original advisory. Removed the patch from Sec. III.
Oct. 14, 1996  Added Appendix A - vendor information.