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CERT Vendor-Initiated Bulletin VB-95:08 - X Authentication Vul
CERT Vendor-Initiated Bulletin VB-95:08 November 2, 1995 Topic: X Authentication Vulnerability Source: X Consortium To aid in the wide distribution of essential security information, the CERT Coordination Center is forwarding the following information from the X Consortium. The X Consortium urges you to act on this information as soon as possible. X Consortium contact information is included in the forwarded text below; please contact them if you have any questions or need further information. ========================FORWARDED TEXT STARTS HERE============================ Two widely used X Window System authorization schemes have weaknesses in the sample implementation. These weaknesses could allow unauthorized remote users to connect to X displays and are present in X11 Release 6 and earlier releases of the X11 sample implementation. There are reports that systems have been broken into using at least one of these weaknesses and that there are now exploit programs available in the intruder community. MIT-MAGIC-COOKIE-1 Description: On systems on which xdm is built without the HasXdmAuth config option, the MIT-MAGIC-COOKIE-1 key generated by xdm may be guessable. If you use MIT-MAGIC-COOKIE-1 to authenticate X connections, and your keys are generated by xdm, and xdm does not also support XDM-AUTHORIZATION-1 authentication (that is, your X tree was not built with the HasXdmAuth config option), you may be at risk. On systems with poor pseudo-random number generators, the key may be guessable by remote users. On other systems, users with access to the file system where xdm stores its keys for use by local servers may be able to use information in the file system to guess the key. If your xdm program was built with HasXdmAuth set to YES (the compiler command line includes the -DHASXDMAUTH flag), MIT-MAGIC-COOKIE-1 keys generated by xdm are not vulnerable; the DES code is used to generate cryptographically secure keys. Impact Remote users anywhere on the Internet may be able to connect to your X display server. It is NOT necessary that they be able to snoop your key first. XDM-AUTHORIZATION-1 Description: The X server does not correctly check the XDM-AUTHORIZATION-1 data and can be fooled into accepting invalid data. A user who can snoop the encrypted authorization data of a valid connection can create fake auth data that the X server will accept. If you do not use XDM-AUTHORIZATION-1, you are not vulnerable. Determining whether your server is vulnerable: this problem is fixed in X servers from the X Consortium with a vendor release number of 6001 or higher. Impact Remote users may be able to connect to your X display server. SOLUTIONS A. Install a vendor supplied patch if available. B. If your site is using X11 built from X Consortium X11R6 sources, install public patch #13. This patch is available via anonymous FTP from ftp.x.org as the file /pub/R6/fixes/fix-13. It is also available from the many sites that mirror ftp.x.org. Apply all patches not already applied, up to and including fix-13. The file xc/bug-report shows what public patches have been already applied to your source tree. The MD5 checksum of fix-13 is as follows: MD5 (fix-13) = 0d81d843acf803a8bedf90d3a18b9ed6 C. If your site is using an earlier version of the X Consortium's X11, upgrade to X11R6. Install all patches up to and including fix-13. D. Work arounds. 1. Building with HasXdmAuth will eliminate the first vulnerability. The necessary DES code is available for FTP from both inside the US (for US sites only) and outside (for non-US sites). Read <ftp://ftp.x.org/pub/R6/xdm-auth/README> for details on obtaining this code. 2. If you cannot use DES, you can determine your exposure to remote attackers by testing the strength of your rand() function using the program rand-test; the source is available as <ftp://ftp.x.org/pub/DOCS/rand-test/rand-test.c>. 3. Limiting use of X connections using XDM-AUTHORIZATION-1 to trusted networks will prevent unauthorized parties from snooping X protocol traffic, thus preventing exploitation of the second vulnerability. Acknowledgements: The X Consortium would like to thank Chris Hall of the University of Colorado for analyzing these problems and bringing them to our attention. ----------------------------------------------------------------- Vendor Status The following information was supplied by vendors for this bulletin. The X Consortium and CERT have not verified this information. Cray Research UNICOS 8.0 and 9.0 are not vulnerable. These systems have robust pseudo-random number generators, making them not vulnerable to the first problem, and do not support an X server, making them not vulnerable to the second problem. GSSC (formerly Solbourne) Has concluded they are not vulnerable. Hewlett-Packard All versions of X on HP-UX 9.x and 10.x (based on X11R5) do not have the first vulnerability. X Consortium (Sample implementation of X.) You can patch X11R6 by applying all public patches up to and including fix-13. Patches are available via FTP from ftp.x.org in /pub/R6/fixes/ and from mirroring sites. You can check that the X server has fix-13 installed by verifying that the server has a vendor release number of 6001 or higher. General questions about the X Window System can be asked on the xpert mailing list hosted at x.org. Send a "subscribe" message to [email protected] to subscribe. This list is gatewayed with the comp.windows.x newsgroup. The FAQ for this newsgroup is available from <ftp://ftp.x.org/contrib/faqs/FAQ.Z> and other locations. <http://www.x.org/consortium/news_and_mail.html> describes other newsgroups and mailing lists for the discussion of issues related to the X Window System. Bugs encounted in X Consortium code can be reported to [email protected] using the format in xc/bug-report. Please see the X11R6 Release Notes for additional details. XFree86 Project The XFree86 Project, Inc has patched binaries for XFree86 version 3.1.2 running on FreeBSD 1.1.5, FreeBSD 2.0.5, ISC, NetBSD and SVR4. They are available from ftp://ftp.xfree86.org/pub/XFree86/3.1.2/binaries/. The files are: FreeBSD-1.1.5/X312Sxdm.tgz FreeBSD-2.0.5/X312Sxdm.tgz ISC/X312Sxdm.tgz NetBSD/X312Sxdm.tgz SVR4/X312Sxdm.tgz The MD5 checksums are: MD5 (FreeBSD-1.1.5/X312Sxdm.tgz) = 43166109c88fcd623d27de1fa90e8f5b MD5 (FreeBSD-2.0.5/X312Sxdm.tgz) = 3314a623b2c31a9130445e9237ff65f9 MD5 (ISC/X312Sxdm.tgz) = e4e16fc5f4d06ad455e572a2e1eb0eb5 MD5 (NetBSD/X312Sxdm.tgz) = 0bc74cbee0214366ac15658bf5436853 MD5 (SVR4/X312Sxdm.tgz) = bf5dfea2a86cdf92621421e3f68af203 Installation instructions (assuming X312xdm.tgz is in /tmp): Kill any xdm processes that are running, then: For FreeBSD 1.1.5 and FreeBSD 2.0.5: cd /usr mv X11R6/bin/xdm X11R6/bin/xdm-3.1.2 chmod 0500 X11R6/bin/xdm-3.1.2 gzip -d < /tmp/X312xdm.tgz | tar vxf - For NetBSD: mv /usr/X11R6/bin/xdm /usr/X11R6/bin/xdm-3.1.2 chmod 0500 /usr/X11R6/bin/xdm-3.1.2 pkg_add /tmp/X312Sxdm.tgz For ISC and SVR4: cd /usr/X11R6 mv bin/xdm bin/xdm-3.1.2 chmod 0500 bin/xdm-3.1.2 gzip -d < /tmp/X312xdm.tgz | tar vxf - =========================FORWARDED TEXT ENDS HERE============================= CERT publications, information about FIRST representatives, and other information related to computer security are available for anonymous FTP from info.cert.org. CERT advisories and bulletins are also posted on the USENET newsgroup comp.security.announce. If you would like to have future advisories and bulletins mailed to you or to a mail exploder at your site, please send mail to [email protected] If you wish to send sensitive incident or vulnerability information to CERT staff by electronic mail, we strongly advise that the e-mail be encrypted. The CERT Coordination Center can support a shared DES key, PGP (public key available via anonymous FTP on info.cert.org), or PEM (contact CERT staff for details). Internet email: [email protected] Telephone: +1 412-268-7090 (24-hour hotline) CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4), and are on call for emergencies during other hours. Fax: +1 412-268-6989 CERT Coordination Center Software Engineering Institute Carnegie Mellon University Pittsburgh, PA 15213-3890 USA CERT is a service mark of Carnegie Mellon University.