The Hacker's Encyclopedia 1998

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─ ALT.2600.MODERATED (1:340/26) ─────────────────────────── ALT.2600.MODERATED ─ Msg : 35 of 55 From : Jack Burroughs 1:340/13 21 Mar 96 14:07:32 To : All 22 Mar 96 10:13:28 Subj : CIAC Bulletin G-17: Vulnerabilities in Sample HTTPD CGIs ──────────────────────────────────────────────────────────────────────────────── .RFC-Path: news.spydernet.com!nntp.pinc.com!news.bctel.net!imci2!news.internetMCI.com!newsf eed.internetmci.com!howland.reston.ans.net!ix.netcom.com!ix-dc7-13 From: got@popd.ix.netcom.com (Jack Burroughs) .RFC-Approved: GOT .RFC-Message-ID: <4itts0$dfp@cloner2.ix.netcom.com> .RFC-NNTP-Posting-Host: ix-dc7-13.ix.netcom.com .RFC-X-NETCOM-Date: Fri Mar 22 2:02:40 AM PST 1996 .RFC-X-Newsreader: News Xpress Version 1.0 Beta #4 ---BEGIN PGP SIGNED MESSAGE----- __________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Vulnerabilities in Sample HTTPD CGIs March 12, 1996 21:00 GMT Number G-17 ______________________________________________________________________________ PROBLEM: C-language based Common Gateway Interface (CGI) programs that call a shell to execute other programs. PLATFORM: Any CGI program built using the sample code distributed with NCSA HTTPD Version 1.5A-Export and earlier or Apache HTTPD version 1.0.3 and earlier that allows input from user and passes that input as arguments to a shell command. DAMAGE: Unauthorized access to the server host may allow an intruder to read, modify, or destroy files. SOLUTION: Check for vulnerability and install the proper patch. ______________________________________________________________________________ VULNERABILITY Knowledge of how to exploit this vulnerability is becoming ASSESSMENT: widely known. This is not an HTTPD server program vulnerability. ______________________________________________________________________________ CIAC has obtained information from IBM-ERS pertaining to vulnerabilities with software distributed with NCSA HTTPD and Apache HTTPD. CIAC recommends that you install the proper patch and/or follow the solutions describe below. [ Start of IBM Bulletin ] ======= ============ ====== ====== ======= ============== ======= ======= === === ==== ====== ====== === =========== ======= ======= === =========== === ======= === === === ==== === ===== === ======= ============== ===== === ===== ======= ============ ===== = ===== EMERGENCY RESPONSE SERVICE SECURITY VULNERABILITY ALERT 26 February 1996 14:00 GMT Number: ERS-SVA-E01-1996:002.1 =============================================================================== VULNERABILITY SUMMARY VULNERABILITY: Some C-language based Common Gateway Interface programs that call a shell to execute other programs can be tricked into executing any arbitrary command. PLATFORMS: Any CGI program built using the sample code distributed with NCSA HTTPD Version 1.5A-Export and earlier or Apache HTTPD Version 1.0.3 and earlier that accepts input from the user and passes that input as arguments to a shell command. SOLUTION: If using the sample code, apply the patch described below. If using other code, check for the vulnerability as described in the following text. THREAT: Many World Wide Web sites have been built using the programs in question. Therefore, the potential security exposure may be widespread. =============================================================================== NOTE The vulnerability described in this alert is contained in ancillary, example software distributed with NCSA HTTPD and Apache HTTPD. THIS IS NOT AN HTTPD SERVER PROGRAM VULNERABILITY. Commercial HTTPD server products developed and sold by IBM do not include this sample code on their distribution media. Commercial HTTPD server products developed by Netscape Communications Corporation and sold by Netscape and IBM do not include this sample code on their distribution media. If you have purchased one of these products and have not installed any programs other than those on the distribution media, you are not vulnerable to this problem. Note, however, that if you previously had NCSA HTTPD or Apache HTTPD installed, you may have remnants of the installation left over which may be vulnerable. If you have purchased another commercial HTTPD server product, you may wish to contact your vendor to see if any action is necessary. =============================================================================== DETAILED INFORMATION I. Description A. Background Information The Common Gateway Interface (CGI) specifies a protocol by which a World Wide Web server can interface with external programs run on the server host. These external programs perform tasks on behalf of the user such as sending mail, accessing databases, and so forth. Many of these programs accept input from the user via an interactive form containing dialog boxes, menus, buttons, etc. Upon completion of the task, the programs may return the results of the operation to the user, usually in the form of a Hypertext Markup Language (HTML) document. Programs which implement the Common Gateway Interface are usually called "CGI scripts," although they are also referred to as "CGI-BIN programs," after the name of the directory in which they are commonly stored on UNIX systems. CGI programs, because they often accept input from the user and then pass that input to other programs as command-line arguments, must be written with great care. Because the languages used to implement these programs (Perl, Bourne shell, Korn shell, C shell, etc.) attribute special meaning to a large variety of characters ('>', ';', '&', '$', etc.), the program must check its input and remove these characters before passing them on to other commands. If this is not done, or is done incorrectly, an attacker can provide "bogus" input to these programs that causes them to behave in unexpected ways, often resulting in allowing the attacker to gain unauthorized access to the server host, or to execute unauthorized commands on the host. Because these problems are so difficult to avoid when using these powerful scripting languages, many experts now recommend that CGI programs be written in the C programming language instead. Because C does not attempt to evaluate character strings, a program written in C is less vulnerable to this type of attack. However, if a CGI program written in C uses the input it receives to execute another command using the system() or popen() library functions, the benefits of using C are negated, and the program must once again carefully check its inputs before passing them as command line arguments to other programs. B. Vulnerability Details NCSA HTTPD and Apache HTTPD, two of the most popular freely-available World Wide Web server implementations, come with several CGI programs that are compiled by the default build procedure. These programs, because they are meant to serve as examples, are frequently installed at a site even if they are not being used. Furthermore, because the source code for these programs is meant to be an example of how to implement CGI programs, it is frequently copied into other CGI programs programs as well. One of the utility functions offered by the CGI example source code is called escape_shell_cmd(). It is intended to help programmers avoid the vulnerability described above. This function, when given an input string received from the user, scans the string for characters that have special meaning to the UNIX shell, and inserts escapes in front of these characters to remove their special meaning. However, the list of special characters used by escape_shell_cmd(): &;`'"|*?~<>^()[]{}$\ is incomplete: it is missing the newline ('\n', octal 012, hex 0x0a) character. The CGI example source code also includes a program called "phf," which implements a form-based interface to a local CCSO Name Server. (The CCSO Name Server is a white pages service used for looking up name and address information about people.) The "phf" demonstration program uses the escape_shell_cmd() function to check its inputs, and is thus vulnerable to attack as described above. Any World Wide Web server host that has been configured to allow access to and execution of Common Gateway Interface programs and 1. Has installed the "phf" program from the NCSA HTTPD distribution or the Apache HTTPD distribution in a CGI-BIN directory (even if the program is not being used) and/or 2. Has installed programs obtained from the Internet or elsewhere that are using the escape_shell_cmd() function contained in the sample CGI-BIN source code distributed with NCSA HTTPD or Apache HTTPD and/or 3. Has installed programs developed internally using the escape_shell_cmd() function contained in the sample CGI-BIN source code distributed with NCSA HTTPD or Apache HTTPD and/or 4. Has installed CGI programs that do not adequately check their inputs for the special characters listed above (including newline) before passing these inputs to a shell command is vulnerable to attack in this manner. II. Impact An attacker who knows how to exercise this vulnerability may have the ability to: 1. Execute arbitrary commands on the server host using the same user-id as the user running the "httpd" server. If "httpd" is being run as "root," the attacker's commands are also run as "root." 2. Access any file on the system that is accessible to the user-id that is running the "httpd" server. If the "httpd" server user-id has read access to the file, the attacker can also read the file. If the "httpd" server user-id has write access to the file, the attacker can change or destroy the contents of the file. If the "httpd" server is being run as "root," the attacker can read, modify, or destroy any file on the server host. 3. Given an X11-based terminal emulator ("xterm" or equivalent) installed on the "httpd" server host, gain full interactive access to the server host just as if he were logging in locally. III. Solutions A. Suggested actions The IBM Emergency Response Service (IBM-ERS) team recommends that you consider taking the following actions (subject to any licensing restrictions that may apply to your copies of the programs): 1. If you have installed the "phf" program from the NCSA or Apache HTTPD binary or source distributions and are not using it, remove it. 2. If you have installed the "phf" program from the NCSA or Apache HTTPD source distributions and are using it, apply one of the patches below, rebuild the "phf" program, and install the new version. 3. If you have installed the "phf" program from the NCSA or Apache HTTPD binary distributions and are using it, obtain the NCSA or Apache source distributions: NCSA: ftp://ftp.ncsa.uiuc.edu/Web/httpd/Unix/ncsa_httpd/httpd_1.5/ httpd_1.5a-export_source.tar.Z Apache: ftp://ftp.apache.org/apache/dist/apache_1.0.3.tar.gz Then apply one of the patches below, rebuild the "phf" program, and install the new version. 4. If you have installed other C-language CGI programs obtained from the Internet or elsewhere and have the source code for these programs, examine them closely and make sure that they are properly checking their inputs before passing them to a shell. If these programs are using the escape_shell_cmd() function from the NCSA or Apache distributions, the patches below can be adapted to these programs. Once you have identified any problem areas and developed fixes for them, rebuild the programs and install the new versions. 5. If you have developed your own C-language CGI programs using the sample code contained in the NCSA or Apache distributions, adapt the patches below to your code, rebuild these programs, and install the new versions. 6. If you have purchased CGI programs from third-party vendors, contact your vendor to determine whether or not these programs contain this vulnerability. 7. If you are running your "httpd" server as "root," consider running it as an unprivileged user instead. B. Patch for escape_shell_cmd() The escape_shell_cmd() function is contained in the "util.c" file in the "cgi-src" directory of the NCSA HTTPD and Apache HTTPD source distributions. The patch below can be applied to the NCSA version of "util.c" to fix this vulnerability: - - ---------------------------------- cut here ---------------------------------- *** httpd_1.5a-export/cgi-src/util.c.old Tue Nov 14 11:38:40 1995 - - --- httpd_1.5a-export/cgi-src/util.c Thu Feb 22 20:37:07 1996 *************** *** 139,145 **** l=strlen(cmd); for(x=0;cmd[x];x++) { ! if(ind("&;`'\"|*?~<>^()[]{}$\\",cmd[x]) != -1){ for(y=l+1;y>x;y--) cmd[y] = cmd[y-1]; l++; /* length has been increased */ - - --- 139,145 ---- l=strlen(cmd); for(x=0;cmd[x];x++) { ! if(ind("&;`'\"|*?~<>^()[]{}$\\\n",cmd[x]) != -1){ for(y=l+1;y>x;y--) cmd[y] = cmd[y-1]; l++; /* length has been increased */ - - ---------------------------------- cut here ---------------------------------- The patch below can be applied to the Apache version of "util.c" to fix this vulnerability: - - ---------------------------------- cut here ---------------------------------- *** apache_1.0.3/cgi-src/util.c.old Sat Feb 17 03:32:14 1996 - - --- apache_1.0.3/cgi-src/util.c Fri Feb 23 10:48:43 1996 *************** *** 135,141 **** l=strlen(cmd); for(x=0;cmd[x];x++) { ! if(ind("&;`'\"|*?~<>^()[]{}$\\",cmd[x]) != -1){ for(y=l+1;y>x;y--) cmd[y] = cmd[y-1]; l++; /* length has been increased */ - - --- 135,141 ---- l=strlen(cmd); for(x=0;cmd[x];x++) { ! if(ind("&;`'\"|*?~<>^()[]{}$\\\n",cmd[x]) != -1){ for(y=l+1;y>x;y--) cmd[y] = cmd[y-1]; l++; /* length has been increased */ - - ---------------------------------- cut here ---------------------------------- To apply the patch, save the text between the two "-- cut here --" lines in a file, change directories to your source tree (the directory that contains "httpd_1.5a-export" or "apache_1.0.3" as a subdirectory) and issue the command: patch < filename If you do not have the "patch" program, you can obtain it from ftp://prep.ai.mit.edu/pub/gnu/patch-2.1.tar.gz or you can apply the patch by hand. This fix is provied "AS IS" without warranty of any kind, including, without limitation, any implied warranties of merchantibility or fitness for a particular purpose. This advisory does not create or imply any support obligations or any other liability on the part of IBM or its subsidiaries. C. Pending fix from NCSA NCSA states that they have fixed this problem in HTTPD Version 1.5.1, which is currently in the beta-testing stage. You can obtain NCSA HTTPD 1.5.1 Beta 3 from http://hoohoo.ncsa.uiuc.edu/beta-1.5/ if you wish to experiment with it. However, NCSA warns that this is NOT a stable version of the server. D. Pending fix from Apache The Apache Project is aware of this problem, and states that they plan to provide a fix for it in Apache HTTPD Version 1.1, which is scheduled for release in early March. For more information about the pending release of Apache HTTPD Version 1.1, see the Apache Project home page at http://www.apache.org/ E. A second potential vulnerability When examining your CGI programs that make use of the escape_shell_cmd() function, note that escape_shell_cmd() does not perform any check on the length of the buffer it is passed. Because each character in the buffer has the potential to be escaped with a backslash, the resulting string can be up to twice as long as the original. Any buffer that is passed into this function should be at least (2n+1) bytes in size, where n is the length of the unescaped string. Programs that do not adhere to this requirement are vulnerable to buffer overrun attacks, much like those used successfully by the Internet worm against the finger daemon, as well as in several more recent attacks against other programs. F. Additional information If you have enabled the Common Gateway Interface in your server, even if you are not yet actively using it, IBM-ERS recommends that you learn as much as possible about the security issues involved. We have provided URLs for some good sources of information on this topic below: CGI Security ------------ http://hoohoo.ncsa.uiuc.edu/cgi/security.html http://www.cerf.net/~paulp/cgi-security/ WWW Security (Including CGI) ---------------------------- http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html Please note that the URLs referenced in this advisory are not under IBM's control and therefore IBM cannot be responsible for their availability or content. If you have any questions regarding any of these sites, please contact that site's administrator or webmaster. IV. Acknowledgements IBM-ERS would like to thank the IBM Global I/T Security Consulting Group and the IBM Global Security Analysis Laboratory at the IBM T. J. Watson Research Center, for bringing this exposure to our attention, providing the patches for it, and their assistance in the development of this alert. [ End of IBM Bulletin ] _______________________________________________________________________________ CIAC wishes to acknowledge the contributions of IBM Emergency Response Service for the information contained in this bulletin. _______________________________________________________________________________ CIAC, the Computer Incident Advisory Capability, is the computer security incident response team for the U.S. Department of Energy (DOE) and the National Institute of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. CIAC services are available to DOE, DOE contractors, and the NIH. CIAC can be contacted at: Voice: +1 510-422-8193 FAX: +1 510-423-8002 STU-III: +1 510-423-2604 E-mail: ciac@llnl.gov For emergencies and off-hour assistance, DOE, DOE contractor sites, and the NIH may contact CIAC 24-hours a day. During off hours (5PM - 8AM PST), call the CIAC voice number 510-422-8193 and leave a message, or call 800-759-7243 (800-SKY-PAGE) to send a Sky Page. CIAC has two Sky Page PIN numbers, the primary PIN number, 8550070, is for the CIAC duty person, and the secondary PIN number, 8550074 is for the CIAC Project Leader. Previous CIAC notices, anti-virus software, and other information are available from the CIAC Computer Security Archive. World Wide Web: http://ciac.llnl.gov/ Anonymous FTP: ciac.llnl.gov (128.115.19.53) Modem access: +1 (510) 423-4753 (28.8K baud) +1 (510) 423-3331 (28.8K baud) CIAC has several self-subscribing mailing lists for electronic publications: 1. CIAC-BULLETIN for Advisories, highest priority - time critical information and Bulletins, important computer security information; 2. CIAC-NOTES for Notes, a collection of computer security articles; 3. SPI-ANNOUNCE for official news about Security Profile Inspector (SPI) software updates, new features, distribution and availability; 4. SPI-NOTES, for discussion of problems and solutions regarding the use of SPI products. Our mailing lists are managed by a public domain software package called ListProcessor, which ignores E-mail header subject lines. To subscribe (add yourself) to one of our mailing lists, send the following request as the E-mail message body, substituting CIAC-BULLETIN, CIAC-NOTES, SPI-ANNOUNCE or SPI-NOTES for list-name and valid information for LastName FirstName and PhoneNumber when sending E-mail to ciac-listproc@llnl.gov: subscribe list-name LastName, FirstName PhoneNumber e.g., subscribe ciac-notes OHara, Scarlett W. 404-555-1212 x36 You will receive an acknowledgment containing address, initial PIN, and information on how to change either of them, cancel your subscription, or get help. PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing communities receive CIAC bulletins. If you are not part of these communities, please contact your agency's response team to report incidents. Your agency's team will coordinate with CIAC. The Forum of Incident Response and Security Teams (FIRST) is a world-wide organization. A list of FIRST member organizations and their constituencies can be obtained by sending email to docserver@first.org with an empty subject line and a message body containingt the line: send first-contacts. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. LAST 10 CIAC BULLETINS ISSUED (Previous bulletins available from CIAC) (G-7) SGI Object Server Vulnerability (G-8) splitvt(1) Vulnerability (G-9b) Unix sendmail Vulnerability (G-10a) Winword Macro Viruses (G-11) HP Syslog Vulnerability (G-12) SGI ATT Packaging Utility Security Vulnerability (G-13) Kerberos Version 4 Key Server Vulnerability (G-14) Domain Name Service Vulnerabilities (G-15) Sunsoft Demo CD Vulnerability (G-16) SGI rpc.statd Program Security Vulnerabilities RECENT CIAC NOTES ISSUED (Previous Notes available from CIAC) Notes 07 - 3/29/95 A comprehensive review of SATAN Notes 08 - 4/4/95 A Courtney update Notes 09 - 4/24/95 More on the "Good Times" virus urban legend Notes 10 - 6/16/95 PKZ300B Trojan, Logdaemon/FreeBSD, vulnerability in S/Key, EBOLA Virus Hoax, and Caibua Virus Notes 11 - 7/31/95 Virus Update, Hats Off to Administrators, America On-Line Virus Scare, SPI 3.2.2 Released, The Die_Hard Virus Notes 12 - 9/12/95 Securely configuring Public Telnet Services, X Windows, beta release of Merlin, Microsoft Word Macro Viruses, Allegations of Inappropriate Data Collection in Win95 -----BEGIN PGP SIGNATURE----- Version: 2.6.1 Comment: Processed by Mailcrypt 3.3, an Emacs/PGP interface iQCVAwUBMUXhb7nzJzdsy3QZAQFrvAQAutaWz1VhajATJywqK5zLysKF5O2fgsSX EV7GtgDYv0SIyq1PMXc6oeMsZbDEMIsC6PmMtcrhuG6I78BOi7SlvtixuIyHB26L Y7dsUhvL0PYQ1yTk3pyMi460qg+uX/9mXy/1N3rO50IA/O6+O97/bcaGoGQW5d7v Tfn+s0hSoDs= =vi8O -----END PGP SIGNATURE----- Frank R. Swift Computer Security Lawrence Livermore National Laboratory Voice (510) 422-1463 Fax (510) 423-0913 uncl@llnl.gov PGP Key fingerprint = 1A 14 02 5A 76 B2 BD 47 C0 3E ED 9A C5 3B 81 2D --- ifmail v.2.8.lwz * Origin: MDE (1:340/13@fidonet)