AT&T System V Release 4 /bin/login Vulnerability

Original issue date: May 23, 1991
Last revised: September 18, 1997
Attached copyright statement

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

The Computer Emergency Response Team/Coordination Center (CERT/CC) has received information concerning a security vulnerability in AT&T's UNIX(r) System V Release 4 operating system.  AT&T is providing a software upgrade for Release 4 operating system vendors and a patch for AT&T Computer Systems customers.  AT&T has also provided a suggested fix for all Release 4 based systems.

I. Description

A security vulnerability exists in /bin/login in AT&T's System V Release 4 operating system.

II. Impact

System users can gain unauthorized privileges.

III. Solution

A.  AT&T Computer Systems customers

Log into the root account.  Change the execution permission on the file /bin/login.
chmod 500 /bin/login
Contact AT&T Computer Systems at 800-922-0354 to obtain a fix. The numbers associated with the fix are 156 (3.5" media) and 157 (5.25" media).

International customers should contact their local AT&T Computer Systems representative.

B.  All other System V Release 4 based systems

Log into the root account.  Change the execution permission on the file /bin/login.
chmod 500 /bin/login
Release 4 customers should contact their operating system supplier for details on the availability of the software update.

The CERT/CC would like to thank AT&T for their timely response to our report of this vulnerability.

This document is available from:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4) Monday through Friday; they are on call for emergencies during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.  Our public PGP key is available from

If you prefer to use DES, please call the CERT hotline for more information.

Getting security information

CERT publications and other security information are available from our web site

* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.

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Copyright 1991 Carnegie Mellon University.

Revision History
September 18,1997  Attached copyright statement