Vulnerability Note VU#639760

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

Original Release date: 29 Nov 2001 | Last revised: 17 Dec 2001


WU-FTPD contains a format string vulnerability that manifests when WU-FTPD is configured to use RFC 931 authentication and is run in debug mode. A crafted identd response could be used to execute arbitrary code on a vulnerable server.


A format string vulnerability exists in the Washington University FTP daemon, WU-FTPD. WU-FTPD is a widely deployed FTP daemon that runs on UNIX and Linux systems and is included in a number of distributions. WU-FTPD can be compiled to use RFC 931 authentication using the '--enable-rfc931' configuration option, in which the server requests user information from the ident daemon running on an FTP client host. Note that RFC 1413 (Identification Protocol) obsoletes RFC 931 (Authentication Server). WU-FTPD can also be run in debugging mode using the '-d' option. Under these conditions, WU-FTPD logs connection information using syslog(3) calls without providing format string specifiers or adequately validating client identd responses. 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 the response of the auth or ident daemon. 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 style authentication and is run in debug mode.


A remote attacker may be able to execute arbitrary code with the privileges of WU-FTPD, typically root.


Apply Patch
Apply the appropriate patch supplied by your vendor. Alternatively, apply the patch provided by WU-FTPD:

Disable RFC 931 Authentication Do not run WU-FTPD configured with '--enable-RFC931'. Disable Debug Mode Do not run WU-FTPD with the '-d' option. Block or Restrict Access Block or restrict access to the port used by WU-FTPD, typically 21/tcp. It may be possible to use TCP Wrapper or a similar technology to provide improved access control and logging. Additionally, an application-level firewall may be able to filter requests made to WU-FTPD. Note that potential exploit strings would be transmitted from 113/tcp on clients to the WU-FTPD server that requested RFC 931 authentication. Disable Vulnerable Service Disable WU-FTPD until a patch can be applied.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
ConectivaAffected-04 Dec 2001
DebianAffected27 Nov 200104 Dec 2001
WU-FTPD Development GroupAffected22 Nov 200130 Nov 2001
FujitsuNot Affected27 Nov 200130 Nov 2001
NcFTP SoftwareNot Affected27 Nov 200130 Nov 2001
SGINot Affected27 Nov 200127 Nov 2001
SunNot Affected27 Nov 200130 Nov 2001
CalderaUnknown27 Nov 200104 Dec 2001
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



The CERT Coordination thanks INTEXXIA for bringing this matter to our attention.

This document was written by Art Manion.

Other Information

  • CVE IDs: CVE-2001-0187
  • Date Public: 23 Jan 2001
  • Date First Published: 29 Nov 2001
  • Date Last Updated: 17 Dec 2001
  • Severity Metric: 14.59
  • Document Revision: 19


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