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Related information

  Дырка в IIS (double decoding directory traversal)

  ISS Alert: IIS URL Decoding Vulnerability

  Advisory CA-2001-12

  NSFOCUS SA2001-02 : Microsoft IIS CGI Filename Decode Error Vulnerability

From:MICROSOFT <secure_(at)_microsoft.com>
Date:15.05.2001
Subject:Security Bulletin MS01-026

- ----------------------------------------------------------------------
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the patches superseded by this patch is provided in the web-hosted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.

- ---------------------------------------------------------------------

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