AT&T /usr/etc/rexecd Vulnerability

Original issue date: February 25, 1992
Last revised: September 19, 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 vulnerability in AT&T TCP/IP Release 4.0 running on SVR4 systems for both the 386/486 and 3B2 RISC platforms.

The existing error, in the remote execution server /usr/etc/rexecd, has been corrected, and a new executable for rexecd is available from AT&T by calling 800-543-9935.  Patches may be obtained outside the U.S. by calling your local technical support.  The numbers associated with the fix are 5127 (3.5" media) and 5128 (5.25" media).

The problem does not exist in TCP/IP release 3.2 for SVR3, or any earlier versions of the TCP/IP product running on either the 3B2 or 386 platforms.

The version of TCP/IP distributed with SVR4 by UNIX(r) System Laboratories, Inc. (a subsidiary of AT&T) does not contain this vulnerability.

UNIX(r) is a registered trademark of UNIX System Laboratories, Inc.

I. Description

    A vulnerability has been identified where root privileges may be accessed through the use of /usr/etc/rexecd.

II. Impact

A user on a remote machine may be able to run commands as root on the target host (the host running the affected /usr/etc/rexecd).

III. Solution

  1. Administrators of affected systems should execute, as root, the following command to immediately turn off access to rexecd until the new binary can be obtained.
    # chmod 400 /usr/etc/rexecd
  2. Obtain and install the new patch.  The fix will be supplied as one diskette, and it comes with one page of instructions documenting the procedure to replace the existing /usr/etc/rexecd binary.

The CERT/CC wishes to thank Bradley E. Smith, Network & Technical Services, Bradley University, for bringing this vulnerability to our attention and for providing a corresponding solution.  We would also like to thank AT&T for their very quick response to this problem.

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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Conditions for use, disclaimers, and sponsorship information

Copyright 1992 Carnegie Mellon University.

Revision History
September 19,1997  Attached Copyright Statement