Pakistan Computer Emergency Response Team

The Security Standard


Copyright | Disclaimer




CA-2001-33 Multiple Vulnerabilities in WU-FTPD


CERT Advisory CA-2001-33 Multiple Vulnerabilities in WU-FTPD

Original release date: November 29, 2001
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

* Systems running WU-FTPD and its derivatives


WU-FTPD is a widely deployed software package used to provide File
Transport Protocol (FTP) services on UNIX and Linux systems. There are
two vulnerabilities in WU-FTPD that expose a system to potential
remote root compromise by anyone with access to the FTP service. These
vulnerabilities have recently received increased scrutiny.

I. Description

There are two remote code execution vulnerabilities in the Washington
University FTP daemon (WU-FTPD). Both of these vulnerabilities have
been discussed in public forums and have received widespread exposure.

VU#886083: WU-FTPD does not properly handle glob command

WU-FTPD features globbing capabilities that allow a user to specify
multiple file names and locations using typical shell notation. See
CERT Advisory CA-2001-07 for a more complete explanation of globbing.

WU-FTPD implements its own globbing code instead of using libraries in
the underlying operating system. When the globbing code is called, it
allocates memory on the heap to store a list of file names that match
the expanded glob expression. The globbing code is designed to
recognize invalid syntax and return an error condition to the calling
function. However, when it encounters a specific string, the globbing
code fails to properly return the error condition. Therefore, the
calling function proceeds as if the glob syntax were correct and later
frees unallocated memory that can contain user-supplied data.
If intruders can place addresses and shellcode in the right locations
on the heap using FTP commands, they may be able to cause WU-FTPD to
execute arbitrary code by later issuing a command that is mishandled
by the globbing code.

This vulnerability is potentially exploitable by any user who is able
to log in to a vulnerable server, including users with anonymous
access. If the exploit is successful, an attacker may be able to
execute arbitrary code with the privileges of WU-FTPD, typically root.
If the exploit is unsuccessful, the thread servicing the request will
fail, but the WU-FTPD process will continue to run.

This vulnerability has been assigned the identifier CAN-2001-0550 by
the Common Vulnerabilities and Exposures (CVE) group:

CORE Security Technologies has published a Vulnerability Report on
this issue:

VU#639760: WU-FTPD configured to use RFC 931 authentication running in
debug mode contains format string vulnerability

WU-FTPD can perform RFC 931 authentication when accepting inbound
connections from clients. RFC 931 defines the Authentication Server
Protocol, and is obsoleted by RFC 1413 which defines the Identity
Protocol. RFC 931 is commonly known as "auth" or "authd", and RFC 1413
is commonly known "ident" or "identd". Both are named after the daemon
that commonly provides the service.

When using RFC 931 authentication, WU-FTPD will request ident
information before authorizing a connection request from a client. The
auth or ident service running on the client returns user-specific
information, allowing WU-FTPD to make authentication decisions based
on data in the ident response.

WU-FTPD can also be run in debugging mode, which provides detailed
information about its operation.

When WU-FTPD is configured to perform RFC 931 authentication and is
run in debug mode, it logs connection information using syslog(3)
function calls. The logging code does not include format string
specifiers in some syslog(3) calls, nor does the code perform adequate
input validation on the contents of the identd response received from
a client. As a result, a crafted identd response containing
user-supplied format string specifiers is interpreted by syslog(3),
possibly overwriting arbitrary locations in memory. By carefully
designing such a request, an attacker may execute arbitrary code with
the privileges of WU-FTPD.

This vulnerability is potentially exploitable by any user who is able
to log in to a vulnerable server, including users with anonymous
access. The intruder must also be able to control their response to
the ident request. If successful, an attacker may be able to execute
arbitrary code with the privileges of WU-FTPD, typically root.

Note that this vulnerability does not manifest unless WU-FTPD is
configured to use RFC 931 authentication and is run in debug mode.

This vulnerability has been assigned the identifier CAN-2001-0187 by
the Common Vulnerabilities and Exposures (CVE) group:

II. Impact

Both of these vulnerabilities can be exploited remotely by any user
with access to the FTP service, including anonymous access. Both
vulnerabilities allow an intruder to execute arbitrary code with the
privileges of WU-FTPD, typically root. An exploit attempt that does
not succeed in executing code may crash WU-FTPD or end the connection
used by the intruder.

For additional information about the impacts of each of these
vulnerabilities, please consult the CERT Vulnerability Notes Database

III. Solution

Apply patches from your vendor

Appendix A contains information for this advisory provided by vendors.
As they report new information to the CERT/CC, we will update this
section and note the changes in our revision history. If a particular
vendor is not listed below, we have not received their comments.
Please contact your vendor directly.

Restrict access to WU-FTPD

As a general practice, the CERT/CC recommends disabling services and
access that are not explicitly required. You may wish to disable
WU-FTPD until you are able to apply a patch.

If you cannot disable the service, you can limit your exposure to
these vulnerabilities by blocking or restricting access to the control
channel (by default, port 21/tcp) used by WU-FTPD. In the case of the
format string vulnerability (VU#639760), an exploit would be
transmitted from port 113/tcp on the attacking host to the WU-FTPD
server that made the identd request. Note that blocking access from
untrusted networks such as the Internet does not protect your systems
against attacks from within your network.

Disable anonymous FTP access

Although disabling anonymous FTP access does not prevent attacks from
occurring, it does prevent unauthenticated users from attempting to
exploit the globbing vulnerability (VU#886083).

Appendix A. Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their
comments. Note that this advisory discusses two distinct
vulnerabilities, and vendor statements may address one or both.


Caldera has released Security Advisory CSSA-2001-041.0:


Cray, Inc. is not vulnerable since the ftp supplied with UNICOS and
UNICOS/mk is not based on the Washington University version. Cray did
check their ftp code and does not see this exploit.


Debian addressed VU#639760 with Debian Security Advisory DSA-016 in
January 2001:

Hewlett-Packard Company

HP's HP-UX is immune to this issue. It was fixed in conjunction with
the last "globbing" issue announced in CERT Advisory CA-2001-07,
released April 10, 2001. The lab did a complete check/scan of the
globbing software, and fixed this issue then as well. Customers should
apply the patches listed in HP Security Bulletin #162 released July

HPSBUX0107-162 Security Vulnerability in ftpd and ftp

Hewlett-Packard Security Bulletins are available at the IT Resource
Center web site (registration required):

IBM Corporation

IBM's AIX operating system does not use WU-FTPD, hence is not
vulnerable to the exploit described by CORE ST.


Immunix has released Security Advisory IMNX-2001-70-036-01:


OpenBSD does not use WU-FTPD.

RedHat Inc.

RedHat has released Errata Advisory RHSA-2001-147:


SGI does not ship IRIX with wu-ftpd, so IRIX is not vulnerable to
these issues.


SuSE has released SuSE Security Announcement SuSE-SA:2001:043.


The WU-FTPD Development Group has provided source code patches that
address both of these issues.
* VU#886083:
* VU#639760:

The CERT Coordination Center thanks CORE Security Technologies and the
WU-FTPD Development Group for their help

Author: Art Manion


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

Getting security information

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

To subscribe to the CERT mailing list for advisories and bulletins,
send email to Please include in the body of your

subscribe cert-advisory

* "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 2001 Carnegie Mellon University.

Revision History
November 29, 2001: Initial release

Version: PGP 6.5.8


All rights reserved. Copyright© PakCERT 2000-2018