Vulnerability Note VU#10277

Various shells create temporary files insecurely when using << operator

Overview

sh uses /tmp files of a predictable name in creating files for input redirection using the << operator.

I. Description

When performing the "<<" redirection, /bin/sh creates a temporary file in /tmp with a name based on the process id, writes subsequent input out to that file, and then closes the file before re-opening it as the standard input of the command to be executed. At no stage are the results of the creat(), write(), or open() calls checked for an error status.

II. Impact

It is possible for another user to alter what is read from this file.

If the sticky bit is not set on /tmp, the file can be simply removed, and a new file created in its place
If the sticky bit is set, then it is possible to guess what the file will be called and create it before /bin/sh does (the creat() call performed by the shell does not result in an open() call with O_EXCL set) and hence it is possible to maintain a handle on the underlying file.
If a fifo is created in place of the temporary file it is particularly easy to insert an extra command into the input transparently, and without having to worry about ensuring the bug is exploited during the narrow window of time in which it occurs.

Even without reading, creating this file may block the execution of commands using the << operator.
It may also be possible to create a symbolic link named as the temporary file and pointed to any other file on the system writable by the user of the shell, which may lead to corruption of the file to which the link is pointed.

III. Solution

Apply vendor patches; see the Systems Affected section below.

Avoid the use of << operator in cron jobs and similar administration scripts.

Systems Affected

Vendor	Status	Date Updated
Apple Computer, Inc.	Vulnerable	25-Sep-2001
Berkeley Software Design, Inc.	Not Vulnerable	15-May-2001
Compaq Computer Corporation	Vulnerable	13-Jun-2003
Data General	Unknown	11-Jun-2001
Debian Linux	Unknown	11-Jun-2001
DEC	Vulnerable	30-Jan-2002
FreeBSD, Inc.	Vulnerable	15-May-2001
Fujitsu	Unknown	11-Jun-2001
Hewlett-Packard Company	Vulnerable	13-Jun-2003
IBM Corporation	Vulnerable	13-Jun-2001
Mandriva, Inc.	Vulnerable	16-Jul-2001
NEC Corporation	Unknown	11-Jun-2001
NetBSD	Unknown	11-Jun-2001
NeXT	Unknown	11-Jun-2001
OpenBSD	Not Vulnerable	5-Jul-2001
Red Hat, Inc.	Unknown	11-Jun-2001
Sequent Computer Systems, Inc.	Unknown	11-Jun-2001
SGI	Vulnerable	29-Jan-2002
Siemens Nixdorf	Unknown	11-Jun-2001
Sony Corporation	Unknown	11-Jun-2001
Sun Microsystems, Inc.	Vulnerable	17-May-2001
The SCO Group (SCO Linux)	Vulnerable	19-Jun-2001
The SCO Group (SCO Unix)	Vulnerable	29-Jan-2002
Unisys	Unknown	11-Jun-2001

References

http://www.securityfocus.com/bid/3996
http://www.securityfocus.com/bid/2006
ftp://patches.sgi.com/support/free/security/advisories/20011103-01-I

Credit

The original discoverer of this vul was Gordon Irlam of the Univeristy of Adelaide, Australia.

This document was written by James T. Ellis, modified by Tim Shimeall to reflect 2001 reporting

Other Information

Date Public	07/17/91
Date First Published	07/02/2001 05:04:05 PM
Date Last Updated	04/24/2007
CERT Advisory
CVE Name	CVE-2000-1134
US-CERT Technical Alerts
Metric	1.73
Document Revision	19

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