Multiple Vendor Lynx Command Injection Vulnerability

iDefense Security Advisory 11.11.05
www.idefense.com/application/poi/display?id=338&type=vulnerabilities
November 11, 2005

I. BACKGROUND

Lynx is a fully-featured WWW client for users running cursor-
addressable, character-cell display devices such as vt100 terminals and
terminal emulators. Lynx support a number of protocols including HTTP,
HTTPS, gopher, FTP, WAIS, NNTP, finger or cso/ph/qi servers, and
services accessible via logons to telnet, tn3270 or rlogin accounts.

II. DESCRIPTION

Remote exploitation of a command injection vulnerability in various
vendors' implementations of Lynx could allow attackers to execute
arbitrary commands with the privileges of the underlying user.

The problem specifically exists within the feature to execute local
cgi-bin programs via the "lynxcgi:" URI handler. The handler is
generally intended to be restricted to a specific directory or
program(s). However, due to a configuration error on multiple platforms,
the default settings allow for arbitrary websites to specify commands to
run as the user running Lynx.

III. ANALYSIS

Successful exploitation of the described vulnerability allows remote
attackers to execute arbitrary commands with the privileges of the
underlying user. Exploitation requires that an attacker convince a
target user to follow a malicious link from within a vulnerable version
of Lynx. The "lynxexec" and "lynxprog" URI handlers can also be used to
trigger the issue. However, they are rarely compiled into the Lynx
binary.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in the latest
stable release of Lynx, version 2.8.5. It is suspected that earlier
versions are also affected. The following vendors include susceptible
Lynx packages within their respective distributions:

    * Red Hat Inc.
    * Gentoo Foundation Inc.
    * Mandriva SA

Other vendors are suspected as also being vulnerable. The following
vendors include Lynx packages that are not susceptible to exploitation
as the "lynxcgi" feature is not compiled into Lynx by default:

    * The FreeBSD Project
    * OpenBSD

V. WORKAROUND

Disable "lynxcgi" links by specifying the following directive in
lynx.cfg:

    TRUSTED_LYNXCGI:none

VI. VENDOR RESPONSE

Development version 2.8.6dev.15 has been released to address this issue
and is available from the following URLs:

  http://lynx.isc.org/current/lynx2.8.6dev.15.tar.Z
  http://lynx.isc.org/current/lynx2.8.6dev.15.tar.bz2
  http://lynx.isc.org/current/lynx2.8.6dev.15.tar.gz
  http://lynx.isc.org/current/lynx2.8.6dev.15.zip

Alternately, an incremental patch is available at:

  http://lynx.isc.org/current/2.8.6dev.15.patch.gz

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2005-2929 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

10/27/2005  Initial vendor notification
10/28/2005  Initial vendor response
11/11/2005  Public disclosure

IX. CREDIT

vade79 (http://fakehalo.us) is credited with this discovery.

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X. LEGAL NOTICES

Copyright © 2005 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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