-----BEGIN PGP SIGNED MESSAGE----- [ For Public Release ] __________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Lynx Temporary Files & LYDownload.c Vulnerabilities July 16, 1997 16:00 GMT Number H-82 ______________________________________________________________________________ PROBLEM: Two vulnerabilities exist for Lynx: 1) temporary files, and 2) LYDownload.c. PLATFORM: All Unix or Unix-like systems running Lynx up to and including version 2.7.1 DAMAGE: 1) May allow local users to gain root privileges. 2) This vulnerability may be exploited by anyone who can provide Lynx a carefully crafted URL. SOLUTION: Apply patches or workarounds listed below. ______________________________________________________________________________ VULNERABILITY Exploit details involving these vulnerabilities have been made ASSESSMENT: publicly available. ______________________________________________________________________________ [ Start CERT Advisories ] 1) Temporary Files ============================================================================= CERT* Vendor-Initiated Bulletin VB-97.05 July 15, 1997 Topic: Vulnerability in Lynx Temporary Files Source: Jim Spath To aid in the wide distribution of essential security information, the CERT Coordination Center is forwarding the following information from Jim Spath, who coordinated this bulletin with several members of the lynx-dev mailing list. They urge you to act on this information as soon as possible. information is included in the forwarded text below; please contact them if you have any questions or need further information. Questions about the bulletin only can be sent to Jim Spath ; questions about Lynx can be sent to . =======================FORWARDED TEXT STARTS HERE============================ I. Description Lynx typically stores persistent temporary files in /tmp on Un*x systems. The filenames Lynx chooses can be predicted, and another user on the system may be able to exploit a race condition to replace the temporary file with a symbolic link or with another file. Installed versions of Lynx where a directory writeable by other users (such as /tmp on a machine to which multiple users have access) is used to store files during download are vulnerable. This vulnerability can only be exploited by a user with access to an account on the machine running Lynx. II. Impact A malicious user with access to the same machine as other Lynx users may be able to cause another user's Lynx process to overwrite another file. It may also be possible to replace the contents of a downloaded file with a file other than the one the user downloaded, or to cause the user to print a file other than the one selected for printing. III. Workarounds A workaround for Lynx 2.7.1 is described in the "solutions" section below. IV. Solutions There are several ways to solve this problem. A. The best solution to the problem is to apply the FOTEMODS patch set and to ensure that /tmp/ on your system is a "sticky directory." If you cannot apply this patch set, if your system does not support sticky directories, or if you cannot make /tmp/ a sticky directory, you must use one of the other solutions below. B. The other solution to this problem is to change the setting of TEMP_SPACE from the default ("/tmp/") to non-world-writeable directories. To do this with unpatched Lynx version 2.7.1: 1. Lynx can be rebuilt with the "#define TEMP_SPACE" in lynx2-7-1/userdefs.h changed from "/tmp" to point to a directory only writeable by the user executing Lynx. 2. The LYNX_TEMP_SPACE environment variable may be set before shell startup files (.profile, .cshrc, or equivalent) or into the system profile (/etc/profile or equivalent). As an aid to allowing Lynx to find user-specific temp. directories, Lynx 2.7.1 will replace "~" in the temp. space allocation with the path to the user's home directory. Individual users may also set the LYNX_TEMP_SPACE environment variable to point to another place known to be unwriteable by other users (for instance a subdirectory of the users' home directory, or a mode 0700 directory of a "sticky" /tmp). To do this with Lynx 2.7.1 with the FOTEMODS patch set applied: You may use any of the methods listed for "vanilla" Lynx 2.7.1. You may also use "$USER" in TEMP_SPACE (or $LYNX_TEMP_SPACE) to specify user-specific temp. directories such as /tmp/$USER/. The FOTEMODS patch set includes the changes described above as well as other fixes and feature enhancements. It can be found at: http://www.slcc.edu/lynx/fote/patches/ The FOTEMODS patches avoid any pre-existing filenames for new temporary files, thus skipping any symbolic link which may have been created with an upcoming temporary filename. These patches also allow the administrator or user to define TEMP_SPACE (or the LYNX_TEMP_SPACE environment variable) as "/tmp/$USER" (for example) for pre-existing directories that correspond to accounts' usernames and have protections/ACLs set for access only by the appropriate users. This patch set also does chmod(600) for temporary files which Lynx creates, but the account should be set up with an equivalent umask before invoking Lynx. C. One other solution (a source code patch) for this problem, by Klaus Weide, can be found at: http://www.slcc.edu/lynx/klaus/temp/ However, this patch should be considered "alpha" quality code, and its author is not supporting it at this time. The next release of Lynx will eliminate this vulnerability. Interested parties should subscribe to and read the LYNX-DEV mailing list (send mail to majordomo@sig.net with "subscribe lynx-dev" as the body) for information about this release. V. Contact information If you believe you have found a security problem with the current version of Lynx, we urge you to forward it to the LYNX-DEV mailing list at . The LYNX-DEV mailing list (with further information about this vulnerability) is archived at: http://www.flora.org/lynx-dev/ Lynx security information is available at: http://www.crl.com/~subir/lynx/security.html General information about Lynx is available at: http://lynx.browser.org/ On-line help and documentation about Lynx is available using the (h)elp command. More help is available in the source distribution. Should your questions not be answered by these means, further questions may be directed to . Please don't contact Lynx developers personally about Lynx-related issues; please use either the mailing list or the "help" addresses given above. ========================FORWARDED TEXT ENDS HERE============================= 2) LYDownload.c ============================================================================= CERT* Vendor-Initiated Bulletin VB-97.06 July 15, 1997 Topic: Vulnerability in Lynx Downloading Source: Jim Spath To aid in the wide distribution of essential security information, the CERT Coordination Center is forwarding the following information from Jim Spath, who coordinated this bulletin with several members of the lynx-dev mailing list. They urge you to act on this information as soon as possible. Contact information is included in the forwarded text below; please contact them if you have any questions or need further information. Questions about the bulletin only can be sent to Jim Spath ; questions about Lynx can be sent to . =======================FORWARDED TEXT STARTS HERE============================ I. Description Lynx, on Un*x systems, may be coerced to read or execute arbitrary files on the local system regardless of restrictions set by the system administrator. Installed versions of Lynx up to and including version 2.7.1 on Unix or Unix-like operating systems are vulnerable. II. Impact A. Captive Lynx installations Users of Lynx in a captive situation (where the Lynx user does not normally have access to a shell prompt, or to a menu system that allows the user to run arbitrary commands) can get access to a shell prompt. This includes public Lynxes as well as any setup where the user is restricted as to which programs can be run. B. All Lynx installations This vulnerability could also conceivably allow malicious webmasters to add these carefully crafted URLs to their pages to cause unsuspecting Lynx users (in captive accounts or otherwise) to execute arbitrary commands. This vulnerability can be exploited by anyone who can provide Lynx a carefully crafted URL. III. Workaround If administrators of captive Lynxes cannot apply the code patches or obtain updated binaries as described below, they are advised to disable (g)oto on Lynx. There is currently no workaround for impact "B" above. The code patches below must be applied (or updated binaries obtained) to eliminate this impact. IV. Solution Current developmental releases of Lynx have fixed this problem since 1997-06-26. Patches you may find from before that date may not entirely eliminate the vulnerability. The most recent stable version of Lynx (version 2.7.1) can be patched to fix this problem by replacing the file "lynx2-7-1/src/LYDownload.c" with a replacement file. The replacement file to eliminate this vulnerability in version 2.7.1 is available (courtesy of Foteos Macrides) at: http://www.slcc.edu/lynx/fote/patches/lynx2-7-1/src/LYDownload.c All systems running Lynx versions 2.7.1 or earlier should be updated to fix this problem. Two development branches of the Lynx source code are available at: http://www.slcc.edu/lynx/fote/patches/ http://www.slcc.edu/lynx/current/ Binary distributions of Lynx may be found at: http://www.crl.com/~subir/lynx/binaries.html Note that producing binaries is a volunteer job and the latest (or any) version may not be available for a specific platform. V. Contact information If you believe you have found a security problem with the current version of Lynx, we urge you to forward it to the LYNX-DEV mailing list at . The LYNX-DEV mailing list (with further information about this vulnerability) is archived at: http://www.flora.org/lynx-dev/ Lynx security information is available at: http://www.crl.com/~subir/lynx/security.html General information about Lynx is available at: http://lynx.browser.org/ On-line help and documentation about Lynx is available using the (h)elp command. More help is available in the source distribution. Should your questions not be answered by these means, further questions may be directed to . Please don't contact Lynx developers personally about Lynx-related issues; please use either the mailing list or the "help" addresses given above. ========================FORWARDED TEXT ENDS HERE============================= [ End CERT Advisories ] ______________________________________________________________________________ CIAC wishes to acknowledge the contributions of CERT and Jim Spath 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 emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. 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World Wide Web: http://ciac.llnl.gov/ Anonymous FTP: ciac.llnl.gov (198.128.39.53) 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 Majordomo, which ignores E-mail header subject lines. 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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 via WWW at http://www.first.org/. 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. 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