-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 @stake, Inc. www.atstake.com Security Advisory Advisory Name: Named Pipe Filename Local Privilege Escalation Release Date: 07/08/2003 Application: Microsoft SQL Server Platform: Windows NT/2000/XP Severity: Local privilege escalation Author: Andreas Junestam (andreas@atstake.com) Vendor Status: Fix available in Windows 2000 SP4 CVE Candidate: CAN-2003-0496 Named Pipe Filename Local Privilege Escalation Reference: www.atstake.com/research/advisories/2003/a070803-1.txt Overview: By specifying the name of a named pipe instead of a file, as an argument to SQL Server's xp_fileexist extended stored procedure, one can impersonate the user account Microsoft SQL Server is running under. This is due to the behavior of the CreateFile system call and Windows named pipe impersonation. This is not limited to Microsoft SQL Server, but a system wide problem. Detailed Description: The API call CreateFile is used to open and/or create files, named pipes, mail slots and much more. Today, there is no mechanism in this API call to limit what kind of resource one want to open. This is due to the fact that most resources are implemented as part of the filesystem. Most services in WIN32 are running under the local system account and handling files in one way or another. If there exists a way to specify which file a service should open, it is possible to impersonate the account this service is running under. Additionally, if UNC paths are used, there is no need to do a read operation on the named pipe before it is possible to impersonate the client end of the pipe. This behaviour is easy to exploit in Microsoft SQL Server since there are a large number of procedures where we can specify which file to use. As an example, we will use xp_fileexist, an extended stored proceudre that public can execute. By creating a named pipe server with an arbitrary name and execute xp_fileexist with the UNC name of the named pipe as an argument, one can impersonate the user account SQL Server is running under. Note that this is a system wide behaviour and not limited to Microsoft SQL Server. See the example section for an easy to follow example, which describes the scenario. Example: Here follows a session which is cut-and-pasted from two command shells. Mssqlpipe.exe is a program that creates a named pipe, waits for a client to connect, and then impersonates the client. It then executes the program specified on the command line as the impersonated user. - - From command shell #1: C:\>mssqlpipe.exe cmd.exe Creating pipe: \\.\Pipe\atstake Pipe created, waiting for connectection Connect to the database (with isql for example) and execute: xp_fileexist '\\SERVERNAME\pipe\atsstake' Then in command shell #2: C:\>isql -U andreas Password: 1> xp_fileexist '\\TEMP123\pipe\atstake' 2> go File Exists File is a Directory Parent Directory Exists ----------- ------------------- ----------------------- 1 0 1 Then, back in command shell #1: Impersonate user successful, we are running as user: SYSTEM Vendor Response Vendor first contacted on 06/21/2002 Vendor responded that they were working on fix: 07/08/2002 Vendor responded that fix would be in SP4: 10/02/2002 Vendor has fix in Windows 2000 SP4 available at: http://www.microsoft.com/Windows2000/downloads/servicepacks/sp4/ The fix introduced a new user right in Windows 2000, "Impersonate a Client AfterAuthentication". This permission is only granted to Administrators and service accounts by default. More information is available in the Microsoft Knowledge Base: http://support.microsoft.com/default.aspx?scid=kb;[LN];821546 @stake Recommendation If you are running Windows 2000 you should install SP4. SQL Server 2000 can run as a less privileged account than SYSTEM which helps mitigate against this problem. Always configure your servers to run as the least privileged user account possible. Credits: Blake Watts (bwatts@securityinternals.com) for his Named Pipes whitepaper "Discovering and Exploiting Named Pipe Security Flaws for Fun and Profit". Common Vulnerabilities and Exposures (CVE) Information: The Common Vulnerabilities and Exposures (CVE) project has assigned the following names to these issues. These are candidates for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems. CAN-2003-0496 Named Pipe Filename Local Privilege Escalation @stake Vulnerability Reporting Policy: http://www.atstake.com/research/policy/ @stake Advisory Archive: http://www.atstake.com/research/advisories/ PGP Key: http://www.atstake.com/research/pgp_key.asc @stake is currently seeking application security experts to fill several consulting positions. Applicants should have strong application development skills and be able to perform application security design reviews, code reviews, and application penetration testing. Please send resumes to jobs@atstake.com. Copyright 2003 @stake, Inc. All rights reserved. -----BEGIN PGP SIGNATURE----- Version: PGP 8.0 iQA/AwUBPwquuUe9kNIfAm4yEQLPpACcDtshMFUb1AxNKppOa0xF9zGl99UAoMiS X1mqnuKGNLGP+fRGGStgYfzp =EeIn -----END PGP SIGNATURE-----