-----BEGIN PGP SIGNED MESSAGE----- __________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Center ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Microsoft CGI Filename Decode Error Vulnerability in IIS [Microsoft Security Advisory MS01-026] May 16, 2001 16:00 GMT Number L-083 ______________________________________________________________________ ________ PROBLEM: The IIS CGI filename decoder will decode the filename twice in error. This presents a vulnerability that could allow an intruder several means of attack. PLATFORM: Microsoft IIS 4.0 Microsoft IIS 5.0 DAMAGE: An intruder could get around security checks or be able to run arbitrary system commands. This could result in code execution or unauthorized file disclosure. SOLUTION: Apply the patch described below. ______________________________________________________________________ ________ VULNERABILITY The risk is MEDIUM to HIGH; attacker would gain access at IUSR ASSESSMENT: privilege level and be able to execute programs. IIS is currently one of the most targeted applications for hackers. ______________________________________________________________________ ________ [****** Start Microsoft Advisory ******] - ---------------------------------------------------------------------- - -- Title: Superfluous Decoding Operation Could Allow Command Execution via IIS Date: May 14, 2001 Software: IIS 4.0 and 5.0 Impact: Three vulnerabilities: Code execution; denial of service, information disclosure. Bulletin: MS01-026 Microsoft encourages customers to review the Security Bulletin at: http://www.microsoft.com/technet/security/bulletin/MS01-026.asp. - ---------------------------------------------------------------------- - -- Issue: ====== This patch is a cumulative patch that includes the functionality of all security patches released to date for IIS 5.0, and all patches released for IIS 4.0 since Windows NT(r) 4.0 Service Pack 5. A complete listing of the patches superseded by this patch is provided in the web-hosted security bulletin, in the section titled "Additional information about this patch". Before applying the patch, system administrators should take note of the caveats discussed in the same section. The patch also eliminates three newly discovered vulnerabilities: - A vulnerability that could enable an attacker to run operating system commands on an affected server. When IIS receives a user request to run a script or other server-side program, it performs a decoding pass to render the request in a canonical form, then performs security checks on the decoded request. A vulnerability results because a second, superfluous decoding pass is performed after the security checks are completed. If an attacker submitted a specially constructed request, it could be possible for the request to pass the security checks, but then be mapped via the second decoding pass into one that should have been blocked -- specifically, it could enable the request to execute operating system commands or programs outside the virtual folder structure. These would be executed in the security context of the IUSR_machinename account which, by virtue of its membership in the Everyone group, would grant the attacker capabilities similar to those of a non-administrative user interactively logged on at the console. - A vulnerability that could enable denial of service attacks against the FTP service. A function that processes wildcard sequences in FTP commands doesn't always allocate sufficient memory when performing pattern matching. Under unusual circumstances, it could be possible for an attacker to levy an FTP command containing a wildcard sequence that, when expanded, would overrun the allocated memory and cause an access violation. This would cause the IIS service (which provides both the web and FTP functionality) to fail. As a result, all web or FTP sessions in progress at the time would be severed, and no new sessions could be established until the IIS service was restarted. In IIS 5.0, the service would restart automatically. In IIS 4.0, operator intervention would be required to restart the service. - A vulnerability that could make it easier for an attacker to find Guest accounts that had been inadvertently exposed via FTP. By design, if a user wishes to log onto an FTP server using a domain user account, rather than a local one, he should be required to precede it with the name of the domain. However, if an attacker preceded an account name with a particular set of characters, the FTP service would search the domain, and all trusted domains, for the user account. The account would need to be enabled, and the attacker would still need to know the correct password in order to log into the account. For all practical purposes, this would limit the attacker to attacking the Guest account, as it is the only account with both a well-known account name and a well-known default password. The patch also corrects errors in three previous patches: - The patch originally provided in Microsoft Security Bulletin MS00-060 successfully eliminated the vulnerability at issue there, but created an opportunity to cause the server to expend an inordinate amount of time processing a particular type of invalid request. - The patches originally provided in Microsoft Security Bulletins MS01-014 and MS01-016 (which superseded MS01-014) successfully eliminated the vulnerabilities at issue there, but created a potential denial of service condition via a memory leak. Mitigating Factors: ==================== IIS vulnerability: - The vulnerability does not provide a way for the attacker to learn the folder structure on the server. As a result, if the operating system were installed on a separate drive from the web root or in non-standard folders, it could prevent an attacker from locating programs of interest. - The vulnerability does not provide administrative access to the server. If the recommendations in the IIS 4.0 and IIS 5.0 security checklists have been followed, sensitive programs will have been moved to folders that can only be accessed by the Administrator, and non-administrative access to server resources will be have been severely restricted. FTP denial of service vulnerability: - The attacker would require the ability to start an FTP session in order to exploit the vulnerability. FTP user account vulnerability: - The vulnerability could only be exploited if the FTP server was a domain member. However, this is usually not appropriate for Internet-connected FTP servers. - The vulnerability could only be exploited if the Guest account on the local machine was disabled, but the Guest account on a trusted domain was enabled. By default, the Guest account is disabled in both Windows NT 4.0 and Windows 2000. Patch Availability: =================== - A patch is available to fix this vulnerability. Please read the Security Bulletin http://www.microsoft.com/technet/security/bulletin/ms01-026.asp for information on obtaining this patch. Acknowledgment: =============== - NSfocus (http://www.nsfocus.com) for reporting the vulnerability affecting IIS. - Lukasz Luzar of Developers.of.PL and Aiden ORawe for reporting the FTP denial of service. - Kevin Kotas of eSecurityOnline (http://www.esecurityonline.com) for reporting the problem in the fixes that were provided in MS00-060, MS01-014 and MS01-016. - - - --------------------------------------------------------------------- THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. [****** End Microsoft Advisory ******] ______________________________________________________________________ _________ CIAC wishes to acknowledge the contributions of Microsoft for the information contained in this bulletin. ______________________________________________________________________ _________ CIAC, the Computer Incident Advisory Center, 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. 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: +1 925-422-8193 (7x24) FAX: +1 925-423-8002 STU-III: +1 925-423-2604 E-mail: ciac@ciac.org Previous CIAC notices, anti-virus software, and other information are available from the CIAC Computer Security Archive. World Wide Web: http://www.ciac.org/ Anonymous FTP: ftp.ciac.org 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 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. 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) L-073: Microsoft ISA Web Proxy Service Denial of Service L-074: Microsoft WebDAV Runs Scripts As User L-075: FreeBSD IPFilter May Incorrectly Pass Packets L-076: Red Hat Ptrace and Exec Race Conditions L-077: The Glacier Backdoor L-078: Microsoft Unchecked Buffer in ISAPI Extension L-079: Microsoft Domain Controller Core Memory Leak Vulnerability L-080: SGI IRIX rpc.espd Buffer Overflow L-081: Microsoft Index Server Search Function Buffer Overflow L-082: Cisco IOS BGP Attribute Corruption Vulnerability -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.8 for non-commercial use iQCVAwUBOwWlzLnzJzdsy3QZAQHeewP/WJW4GQTiwY9KgfQ0g36QMOXxEfCRx67+ D0uX343bXKwX6VbWEmGzQmomdDoY5VTGxtP06JsCHwHH8X5vPydji3P7XtnW9EM0 2jEb0NEu+zhGY7M5mfi2WmXxT7aXpyRetCnRXG5fobwlxwpbh5MQ8WmYjJmBHFrx WmhElWtHBmk= =6MYB -----END PGP SIGNATURE-----