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CERT Advisory 111
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=============================================================================
CERT(sm) Advisory CA-96.06
Original issue date: March 20, 1996
Last revised: June 4, 1997
Updates section - added information about other cgi programs
being exploited.
A complete revision history is at the end of this file.
Topic: Vulnerability in NCSA/Apache CGI example code
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The text of this advisory was originally released on March 14, 1996,
as AUSCERT Advisory AA-96.01, developed by the Australian Computer
Emergency Response Team. Because of the seriousness of the problem, we
are reprinting the AUSCERT advisory here with their permission. Only
the contact information at the end has changed: AUSCERT contact
information has been replaced with CERT/CC contact information.
We will update this advisory as we receive additional information.
Please check advisory files regularly for updates that relate to your site.
Note: The vulnerability described in this advisory is being actively
exploited.
=============================================================================
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.
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.
Please note that this vulnerability is not in httpd itself, but in CGI
programs which rely on the supplied escape_shell_cmd() function. 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.
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
For sites that still wish to use programs using the escape_shell_cmd()
function, a patched version of cgi-src/util.c has been made available by
NCSA which addresses this particular vulnerability. The patched version
of util.c is available as part of the http1.5.1b3-export distribution.
This is available from:
http://hoohoo.ncsa.uiuc.edu/beta-1.5
Please note that this is a beta-release of the NCSA httpd and is not a
stable version of the httpd. The patched version of cgi-src/util.c may be
used independently.
CGI programs which are required and use the escape_shell_cmd() should be
recompiled with the new version of cgi-src/util.c and then reinstalled.
Apache have reported that they intend to fix this vulnerability in a
future release. Until then the patched version of util.c as supplied
in the http1.5.1b3-export release should be compatible.
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.
==============================================================================
CERT Contact Information
- ------------------------
If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident
Response and Security Teams (FIRST).
We strongly urge you to encrypt any sensitive information you send by email.
The CERT Coordination Center can support a shared DES key and PGP. Contact the
CERT staff for more information.
Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key
Email cert@cert.org
Phone +1 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30-5:00 p.m. EST
(GMT-5)/EDT(GMT-4), and are on call for
emergencies during other hours.
Fax +1 412-268-6989
Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
USA
To be added to our mailing list for CERT advisories and bulletins, send your
email address to
cert-advisory-request@cert.org
CERT publications, information about FIRST representatives, and other
security-related information are available for anonymous FTP from
ftp://info.cert.org/pub/
CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce
CERT is a service mark of Carnegie Mellon University.
============================================================================
UPDATES
Similar attacks may succeed against other cgi scripts if the scripts
are written without appropriate care regarding security issues. We
encourage sites to evaluate all programs in their cgi-bin directory
and remove any scripts that are not in active use.
We would like to point out that along with "phf" we have received
reports that "php" programs are also being exploited.
CERT/CC received the following update from NASIRC concerning the
vulnerability described in this advisory:
NEW INFORMATION
The routine "escape_shell_cmd()" also occurs in the file
"src/util.c". Note that the files "cgi-src/util.c" and
"src/util.c" are not identical, however they both contain an
identical copy of the routine "escape_shell_cmd()", which has the
vulnerability. The file "src/util.c" is used to build the HTTP
daemon, therefore the "newline" hole exists within the server.
PATCH
The patch recommended by NCSA modifies the routine
"escape_shell_cmd()" to expand the list of characters that it
will escape. In the routine "escape_shell_cmd()", the line:
if(ind("&;`'\"|*?~<>^()[]{}$\\",cmd[x]) != -1){
Must be changed to:
if(ind("&;`'\"|*?~<>^()[]{}$\\\n",cmd[x]) != -1){
NCSA HTTPD 1.5.1
Instead of patching the source, the most up-to-date version of
NCSA HTTPd source may be downloaded from:
ftp://ftp.ncsa.uiuc.edu/Web/httpd/Unix/ncsa_httpd/current/httpd_1.5.1-export_source.tar.Z
MD5 (httpd_1.5.1-export_source.tar.Z) = bcf1fd410b5839c51dc75816a155fbb8
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Revision history
June 4, 1997 Updates section - added information about other cgi programs
being exploited.
Aug. 30, 1996 Information previously in the README was inserted into the
advisory.
Apr. 17, 1996 Updates section - added new information provided by the
NASA Automated Systems Incident Response Capability (NASIRC).
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