=========================================================================== A U S C E R T A L E R T AL-2004.031 -- AUSCERT ALERT Canonicalisation issue in ASP.NET 8 October 2004 =========================================================================== AusCERT Alert Summary --------------------- Product: Microsoft IIS web server with ASP.NET Operating System: Windows Impact: Access Confidential Data Inappropriate Access Access: Remote/Unauthenticated Comment: Microsoft have released an email alert on this issue with a subject of "Alert - ASP.NET Security Issue and Guidance", which, due to the appearance of some of its links and header information, may appear to be unauthentic. Microsoft have confirmed that this email is authentic, and that discrepancies in the email are due to their third-party email service. - --------------------------BEGIN INCLUDED TEXT-------------------- PROBLEM: An exploit has been recently disclosed for Microsoft IIS servers with ASP.NET enabled. By sending a specially crafted URL, application level authentication can be bypassed, potentially exposing sensitive information and programs. Web applications in ASP.NET may use a web.config file to control authentication mechanisms. If a website visitor uses a backslash character in a URL string in place of an expected forward slash, these authentication mechanisms are bypassed and access is granted to underlying components that should be secured. Please note that Internet Explorer automatically converts backslashes to forward slashes, but the hex-encoded value of a backslash can be substituted to successfully run this exploit. VERSIONS: All Windows servers running IIS with ASP.NET are potentially vulnerable. Anecdotal evidence suggests that Windows Server 2003 may not be vulnerable, but this has not been verified by AusCERT. IMPACT: ASP.NET application authentication mechanisms are bypassed and access may be granted to underlying components and data that should be secured. MITIGATION: An official mitigation method is yet to be announced, but the following techniques have been suggested by Microsoft and others. Microsoft will be updating infomation on their website [1] about mitigating this vulnerability as information becomes available. 1. Install the Microsoft HTTP module to check for canonicalization issues. Instructions and downloads are available from the Microsoft website [2]. 2. Install URLScan to block incoming URLs with blackslash characters. Note that URLScan configuration should be tested before deploying to a production environment; otherwise, unexpected filtering behavior may occur. URLScan can be downloaded from the Microsoft website [3]. Please note that AusCERT has not verified or tested either of these techniques - it is recommended that sites test their configurations to check for their exposure to this vulnerability before and after applying fixes. Due to the ease of execution of this exploit and potential consequences, it is recommended that IIS server operators undertake immediate preventative action against this flaw. AusCERT will continue to monitor the situation and will release updates as appropriate. REFERENCES: [1] http://www.microsoft.com/security/incident/aspnet.mspx [2] http://support.microsoft.com/?kbid=887289 [3] http://www.microsoft.com/technet/security/tools/urlscan.mspx - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. The mailing list you are subscribed to is maintained within your organisation, so if you do not wish to continue receiving these bulletins you should contact your local IT manager. If you do not know who that is, please send an email to auscert@auscert.org.au and we will forward your request to the appropriate person. NOTE: Third Party Rights This security bulletin is provided as a service to AusCERT's members. As AusCERT did not write the document quoted above, AusCERT has had no control over its content. The decision to follow or act on information or advice contained in this security bulletin is the responsibility of each user or organisation, and should be considered in accordance with your organisation's site policies and procedures. AusCERT takes no responsibility for consequences which may arise from following or acting on information or advice contained in this security bulletin. NOTE: This is only the original release of the security bulletin. It may not be updated when updates to the original are made. If downloading at a later date, it is recommended that the bulletin is retrieved directly from the author's website to ensure that the information is still current. Contact information for the authors of the original document is included in the Security Bulletin above. If you have any questions or need further information, please contact them directly. Previous advisories and external security bulletins can be retrieved from: http://www.auscert.org.au/render.html?cid=1980 If you believe that your computer system has been compromised or attacked in any way, we encourage you to let us know by completing the secure National IT Incident Reporting Form at: http://www.auscert.org.au/render.html?it=3192 =========================================================================== Australian Computer Emergency Response Team The University of Queensland Brisbane Qld 4072 Internet Email: auscert@auscert.org.au Facsimile: (07) 3365 7031 Telephone: (07) 3365 4417 (International: +61 7 3365 4417) AusCERT personnel answer during Queensland business hours which are GMT+10:00 (AEST). On call after hours for member emergencies only. =========================================================================== -----BEGIN PGP SIGNATURE-----