AIX REXD Daemon Vulnerability

Original issue date: March 5, 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 with the rexd daemon in versions 3.1 and 3.2 of AIX for IBM RS/6000 machines.

IBM is aware of the problem and it will be fixed in future updates to AIX 3.1 and 3.2.  Sites may call IBM Support (800-237-5511) and ask for the patch for apar ix21353.  Patches may be obtained outside the U.S. by contacting your local IBM representative.

The fix is also provided below.

I. Description

In certain configurations, particularly if NFS is installed, the rexd (RPC remote program execution) daemon is enabled.

Note: Installing NFS with the current versions of "mknfs" will re-enable rexd even if it was previously disabled.

II. Impact

If a system allows rexd connections, anyone on the Internet can gain access to the system as a user other than root.

III. Solution

CERT/CC and IBM recommend that sites take the following actions immediately.  These steps should also be taken whenever "mknfs" is run.
  1. Be sure the rexd line in /etc/inetd.conf is commented out by having a '#' at the beginning of the line:
    #rexd   sunrpc_tcp tcp  wait  root  /usr/etc/rpc.rexd rexd 100017 1
  2. Refresh inetd by running the following command as root:
    refresh -s inetd

The CERT/CC wishes to thank Darren Reed of the Australian National University for bringing this vulnerability to our attention and IBM for their response to the 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.

Any material furnished by Carnegie Mellon University and the Software Engineering Institute is furnished on an "as is" basis. Carnegie Mellon University makes no warranties of any kind, either expressed or implied as to any matter including, but not limited to, warranty of fitness for a particular purpose or merchantability, exclusivity or results obtained from use of the material. Carnegie Mellon University does not make any warranty of any kind with respect to freedom from patent, trademark, or copyright infringement.

Conditions for use, disclaimers, and sponsorship information

Copyright 1992 Carnegie Mellon University.

Revision History
September 19,1997  Attached copyright statement