CERT(sm) Advisory CA-96.06 March 20, 1996 Topic: Vulnerability in NCSA/Apache CGI example code 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. As usual, we will place updated information in a README file (ftp://info.cert.org/pub/cert_advisories/CA-96.06.README). 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. ----------------------------------------------------------------------------- 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. ----------------------------------------------------------------------------- AUSCERT thanks Jeff Uphoff of NRAO, IBM-ERS, NASIRC and Wolfgang Ley of DFN-CERT for their assistance. ----------------------------------------------------------------------------- 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.