-----BEGIN PGP SIGNED MESSAGE----- CERT Advisory CA-2001-12 Superfluous Decoding Vulnerability in IIS Original release date: May 15, 2001 Last revised: -- Source: CERT/CC A complete revision history is at the end of this file. Systems Affected * Systems running Microsoft IIS Overview A serious vulnerability in Microsoft IIS may allow remote intruders to execute commands on an IIS web server. This vulnerability closely resembles a previous vulnerability in IIS that was widely exploited. The CERT/CC urges IIS administrators to take action to correct this vulnerability. I. Description URIs may be encoded according to RFC 2396. Among other things, this RFC provides an encoding for arbitrary octets using the percent sign (%) and hexadecimal characters. Quoting from RFC 2396: An escaped octet is encoded as a character triplet, consisting of the percent character "%" followed by the two hexadecimal digits representing the octet code. For example, "%20" is the escaped encoding for the US-ASCII space character. escaped = "%" hex hex hex = digit | "A" | "B" | "C" | "D" | "E" | "F" Like all web servers, Microsoft IIS decodes input URIs to a canonical format. Thus, the following encoded string: A%20Filename%20With%20Spaces will get decoded to A Filename With Spaces Unfortunately, IIS decodes some of the input twice. The second decoding is superfluous. Security checks are applied to the results of the first decoding, but IIS utilizes the results of the second decoding. If the results of the first decoding pass the security checks and the results of the second decoding refer to a valid file, access will be granted to the file even if it should not be. More information is available at http://www.microsoft.com/technet/security/bulletin/MS01-026.asp http://www.nsfocus.com/english/homepage/sa01-02.htm http://www.kb.cert.org/vuls/id/789543 Note that this does not permit intruders to bypass ACLs enforced by the filesystem, only security checks performed by IIS. We encourage you to configure your web server according to the guidelines provided in http://www.microsoft.com/technet/security/iis5chk.asp http://www.microsoft.com/technet/security/iischk.asp http://www.microsoft.com/technet/security/tools.asp Theses guidelines can help you reduce your exposure to this problem, and possibly to problems that have not yet been discovered. This issue was discovered by NSFocus. The CVE Project has assigned the following identifier to this vulnerability: http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2001-0333 This vulnerability has many similarities to the Web Server Folder Directory Traversal Vulnerability, which has been widely exploited. For more information on that vulnerability, see http://www.kb.cert.org/vuls/id/111677 II. Impact Intruders can run arbitrary commands with the privileges of the IUSR_machinename account. III. Solutions Apply a patch from your vendor Information on patches from Microsoft is available at http://www.microsoft.com/technet/security/bulletin/MS01-026.asp Additional advice on securing IIS web servers is available from http://www.microsoft.com/technet/security/iis5chk.asp http://www.microsoft.com/technet/security/tools.asp Appendix A. Vendor Information Microsoft Corporation The following documents regarding this vulnerability are available from Microsoft: http://www.microsoft.com/technet/security/bulletin/MS01-026.asp Authors: Shawn Hernan. ______________________________________________________________________ This document is available from: http://www.cert.org/advisories/CA-2001-12.html ______________________________________________________________________ CERT/CC Contact Information Email: cert@cert.org Phone: +1 412-268-7090 (24-hour hotline) Fax: +1 412-268-6989 Postal address: CERT Coordination Center Software Engineering Institute Carnegie Mellon University Pittsburgh PA 15213-3890 U.S.A. CERT personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4) Monday through Friday; they are on call for emergencies during other hours, on U.S. holidays, and on weekends. Using encryption We strongly urge you to encrypt sensitive information sent by email. Our public PGP key is available from http://www.cert.org/CERT_PGP.key If you prefer to use DES, please call the CERT hotline for more information. Getting security information CERT publications and other security information are available from our web site http://www.cert.org/ To subscribe to the CERT mailing list for advisories and bulletins, send email to majordomo@cert.org. Please include in the body of your message subscribe cert-advisory * "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office. ______________________________________________________________________ NO WARRANTY Any material furnished by Carnegie Mellon University and the Software Engineering Institute is furnished on an "as is" basis. Carnegie Mellon University makes no warranties of any kind, either expressed or implied as to any matter including, but not limited to, warranty of fitness for a particular purpose or merchantability, exclusivity or results obtained from use of the material. Carnegie Mellon University does not make any warranty of any kind with respect to freedom from patent, trademark, or copyright infringement. _________________________________________________________________ Conditions for use, disclaimers, and sponsorship information Copyright 2001 Carnegie Mellon University. Revision History May 15, 2001: Initial Release -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 5.0i for non-commercial use Charset: noconv iQCVAwUBOwFD9wYcfu8gsZJZAQEc0AP6A7XLQiQ7to6uzTeOyFRb+vXUBI1zBmT1 TvVwLodq6wfeS0vG/+Ta0KC28CFthDs9vUrw6HTnVeeFilKRqUhPgR8Izgd56ePc SKalqxv41DRvkusTlvrygFw1IUzdCJ0/EzWUiRpqu1QV7ZWmNTTVG4ycoEM++cLh 67h5IqMR/iU= =z3yR -----END PGP SIGNATURE-----