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AA-96.01                        AUSCERT Advisory
                                 14 March 1996
                     Vulnerability in NCSA/Apache CGI example code

Last Revised:	24 September, 1996
		Updated patched version information
		Added probe detection notes

		A complete revision history is at the end of this file.
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The Australian Computer Emergency Response Team (AUSCERT) has received
information that example CGI code, as found in the NCSA 1.5a-export and APACHE
1.0.3 httpd (and possibly previous distributions of both servers), contains
a security vulnerability.  Programs using this code may be vulnerable to
attack.

The CGI program "phf", included with those distributions, is an example of
such a vulnerable program.  This program may have been installed as part of
the installation process for the httpd.

AUSCERT recommends that sites that have installed any CGI program incorporating
the vulnerable code (such as "phf") apply one of the workarounds as described 
in Section 3.

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1.  Description

    A security vulnerability has been reported in example CGI code, as
    provided with the NCSA httpd 1.5a-export and APACHE httpd 1.0.3 (and
    possibly previous distributions of both servers).  The example code
    contains a library function escape_shell_cmd() (in cgi-src/util.c).
    This function, which attempts to prevent exploitation of shell-based
    library calls, such as system() and popen(), contains a vulnerability.
    It fails to detect the newline shell meta-character.

    Any program which relies on escape_shell_cmd() to prevent exploitation
    of shell-based library calls may be vulnerable to attack.

    In particular, this includes the "phf" program which is also distributed
    with the example code.  Some sites may have installed phf by default,
    even though it is not required to run httpd successfully.

    Any vulnerable program which is installed as a CGI application may
    allow unauthorised activity on the HTTP server.  Any HTTP server (not
    limited to NCSA or Apache) which has installed CGI programs which rely
    on escape_shell_cmd() may be vulnerable to attack.

    To test whether your site has been probed using this vulnerability,
    search for the newline character in your access logs.  An example of
    how to do this is:

	grep -i '%0a' {WWW_HOME}/logs/access_log

    If this command returns anything, further investigation is necessary.

    If sites find any evidence showing that they have been probed using
    this method, they are encouraged to report the incident to AUSCERT.
    Reports of all attacks help AUSCERT gain a better overview of intruder
    activity within the constituency.

    Sites which have the source code to their CGI applications available can
    determine whether their applications may be vulnerable by examining the
    source for usage of the escape_shell_cmd() function which is defined in
    cgi-src/util.c.

    Sites which do not have the source code for their CGI applications
    should contact the distributors of the applications for more information.

    It is important to note that attacks similar to this may succeed
    against any CGI program which has not been written with due
    consideration for security.  Sites using HTTP servers, and in
    particular CGI applications, are encouraged to develop an understanding 
    of the security issues involved.  References in Section 4 provide some
    initial pointers in this area.

2.  Impact

    A remote user may retrieve any world readable files, execute arbitrary
    commands and create files on the server with the privileges of the httpd
    process which answers HTTP requests.  This may be used to compromise the
    http server and under certain configurations gain privileged access.

3.  Workarounds

    The use of certain C library calls (including system() and popen()) in
    security critical code (such as CGI programs) has been a notorious source
    of security vulnerabilities.  Good security coding practice usually
    dictates that easily exploitable system or library calls should not be
    used.  While secure CGI coding techniques are beyond the scope of this
    advisory many useful guidelines are available.  

    Sites planning to install or write their own CGI programs are encouraged
    to read the references in Section 4 first.

    3.1.  Remove CGI programs

    Any CGI program which uses the escape_shell_cmd() function and is not
    required should be disabled.  This may be accomplished by removing
    execute permissions from the program or removing the program itself.

    In particular, sites which have installed the "phf" program and do not
    require it should disable it.  The "phf" program is not required to
    run httpd successfully.  Sites requiring "phf" functionality should apply
    one of the workarounds given in sections 3.2 and 3.3.

    3.2.  Rewrite CGI programs

    The intent of the escape_shell_cmd() function is to prevent passing shell
    meta-characters to susceptible library calls.  A more secure approach is
    to avoid the use of these library calls entirely.

    AUSCERT recommends that sites which are currently using CGI programs
    which use shell-based library calls (such as system() and popen())
    consider rewriting these programs to remove direct calls to easily
    compromised library functions.

    Sites should note that this is only one aspect of secure programming
    practice.  More details on this approach and other guidelines for secure
    CGI programming may be found in the references in Section 4.

    3.3.  Recompile CGI programs with patched util.c

    The vulnerability described in this advisory was first patched in NCSA
    httpd 1.5.1 and Apache httpd 1.0.4.  Sites using any programs requiring
    the escape_shell_cmd() function should download the current version
    of the chosen httpd and recompile such programs.

    The current NCSA distribution can be found at:

	ftp://ftp.ncsa.uiuc.edu/Web/httpd/Unix/ncsa_httpd/current/

    The current Apache distribution can be found at:

	ftp://www.apache.org/apache/dist/


4.  Additional measures

    Sites should consider taking this opportunity to examine their httpd
    configuration.  In particular, all CGI programs that are not required
    should be removed, and all those remaining should be examined for possible
    security vulnerabilities.

    It is also important to ensure that all child processes of httpd are
    running as a non-privileged user.  This is often a configurable option.
    See the documentation for your httpd distribution for more details.

    Numerous resources relating to WWW security are available.  The following
    pages provide a useful starting point.  They include links describing 
    general WWW security, secure httpd setup and secure CGI programming.

	The World Wide Web Security FAQ:
		http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html

	NSCA's "Security Concerns on the Web" Page:
		http://hoohoo.ncsa.uiuc.edu/security/

    The following book contains useful information including sections on
    secure programming techniques.

	"Practical Unix & Internet Security", Simson Garfinkel and 
        Gene Spafford, 2nd edition, O'Reilly and Associates, 1996.

    Please note that the URLs referenced in this advisory are not under
    AUSCERT's control and therefore AUSCERT cannot be responsible for their
    availability or content.  Please contact the administrator of the site in
    question if you encounter any difficulties with the above sites.

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AUSCERT thanks Jeff Uphoff of NRAO, IBM-ERS, NASIRC and Wolfgang Ley of
DFN-CERT for their assistance.
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The AUSCERT team have made every effort to ensure that the information
contained in this document is accurate.  However, the decision to use the
information described is the responsibility of each user or organisation.
The appropriateness of this document for an organisation or individual system
should be considered before application in conjunction with local policies
and procedures.  AUSCERT takes no responsibility for the consequences of
applying the contents of this document.

If you believe that your system has been compromised, contact AUSCERT or your
representative in FIRST (Forum of Incident Response and Security Teams).

AUSCERT is located at The University of Queensland within the Prentice Centre.
AUSCERT is a full member of the Forum of Incident Response and Security Teams
(FIRST).

AUSCERT maintains an anonymous FTP service which is found on:
ftp://ftp.auscert.org.au.  This archive contains past SERT and AUSCERT
Advisories, and other computer security information.

AUSCERT also maintains a World Wide Web service which is found on:
http://www.auscert.org.au.

Internet Email: auscert@auscert.org.au
Facsimile:      (07) 3365 4477
Telephone:      (07) 3365 4417 (International: +61 7 3365 4417)
                AUSCERT personnel answer during Queensland business hours
                which are GMT+10:00 (AEST).
                On call after hours for emergencies.

Postal:
Australian Computer Emergency Response Team
c/- Prentice Centre
The University of Queensland
Brisbane
Qld.  4072.
AUSTRALIA


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Revision History

Sept 24, 1996	New version information added for both NCSA and Apache
		HTTP Daemons.

		Information on how to test if you have been probed using
		this vulnerability.

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