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Related information

  Multiple bugs in Microsoft Internet Information Server

  [SNS Advisory No.58] Microsoft IIS Local Cross-site Scripting Vulnerability

  [A3SC] MS IIS out of process privilege elevation vulnerability(A3C
R@K-Vul-2002-06-
002)

  Microsoft Internet Information Server 5/5.1 Denial of Service (#NISR31102002)

  Microsoft Security Bulletin MS02-062: Cumulative Patch for Internet Information Service (Q327696)

From:MICROSOFT <secure_(at)_microsoft.com>
Date:10.04.2002
Subject: Security Bulletin MS02-018 : Cumulative Patch for Internet Information Services (Q319733)

Title:      Cumulative Patch for Internet Information Services
           (Q319733)
Date:       10 April 2002
Software:   Microsoft Internet Information Server 4.0,
           Microsoft Internet Information Services 5.0,
           Microsoft Internet Information Services 5.1
Impact:     Ten new vulnerabilities, the most serious of which
           could enable code of an attacker's choice to be run
           on a server.
Max Risk:   High
Bulletin:   MS02-018

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-018.asp.
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Issue:
======
This patch is a cumulative patch that includes the functionality of
all security patches released for IIS 4.0 since Windows NT 4.0
Service Pack 6a, and all security patches released to date for IIS
5.0 and 5.1. A complete listing of the patches superseded by this
patch is provided below, 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.

In addition to including previously released security patches,
this patch also includes fixes for the following newly
discovered security vulnerabilities affecting IIS 4.0, 5.0 and/or
5.1:

- A buffer overrun vulnerability involving the operation of
  the chunked encoding transfer mechanism via Active Server
  Pages in IIS 4.0 and 5.0. An attacker who exploited this
  vulnerability could overrun heap memory on the system, with
  the result of either causing the IIS service to fail or
  allowing code to be run on the server.
- A Microsoft-discovered vulnerability that is related to the
  preceding one, but which lies elsewhere within the ASP data
  transfer mechanism. It could be exploited in a similar manner
  as the preceding vulnerability, and would havethe same scope.
  However, it affects IIS 4.0, 5.0, and 5.1.
- A buffer overrun involving how IIS 4.0, 5.0 and 5.1 process
  HTTP header information in certain cases. IIS performs a
  safety check prior to parsing the fields in HTTP headers, to
  ensure that expected delimiter fields are present and in
  reasonable places. However, it is possible to spoof the check,
  and convince IIS that the delimiters are present even when they
  are not. This flaw could enable an attacker to create an URL
  whose HTTP header field values would overrun a buffer used to
  process them.
- A Microsoft-discovered buffer overrun vulnerability in IIS 4.0,
  5.0 and 5.1 that results from an error in safety check that
  is performed during server-side includes. In some cases, a user
  request for a web page is properly processed by including the
  file into an ASP script and processing it. Prior to processing
  the include request, IIS performs an operation on the user-
  specified file name, designed to ensure that the file name is
  valid and sized appropriately to fit in a static buffer. However,
  in some cases it could be possible to provide a bogus, extremely
  long file name in a way that would pass the safety check, thereby
  resulting in a buffer overrun.
- A buffer overrun affecting the HTR ISAPI extension in IIS 4.0
  and 5.0.  By sending a series of specially malformed HTR
  requests, it could be possible to either cause the IIS service to
  fail or, under a very difficult operational scenario, to cause
  code to run on the server.
- A denial of service vulnerability involving the way IIS 4.0,
  5.0, and 5.1 handle an error condition from ISAPI filters.
  At least one ISAPI filter (which ships as part of FrontPage
  Server Extensions and ASP.NET), and possibly others, generate
  an error when a request is received containing an URL that
  exceeds the maximum length set by the filter. In processing
  this error, the filter replaces the URL with a null value. A
  flaw results because IIS attempts to process the URL in the course
  of sending the error message back to the requester, resulting in
  an access violation that causes the IIS service to fail.
- A denial of service vulnerability involving the way the FTP
  service in IIS 4.0, 5.0 and 5.1 handles a request for the status
  of the current FTP session. If an attacker were able to establish
  an FTP session with an affected server,and levied a status
  request that created a particular error condition, a flaw in the
  FTP code would prevent it from correctly reporting the error.
  Other code within the FTP service would then attempt to use
  uninitialized data, with an access violation as the result. This
  would result in the disruption of not only FTP services, but also
  of web services.
- A trio of Cross-Site Scripting (CSS) vulnerabilities affecting
  IIS 4.0, 5.0 and 5.1: one involving the results page that's
  returned when searching the IIS Help Files, one involving HTTP
  error pages; and one involving the error message that's returned
  to advise that a requested URL has been redirected. All of these
  vulnerabilities have the same scope and effect: an attacker who
  was able to lure a user into clicking a link on his web site
  could relay a request containing script to a third-party web
  site running IIS, thereby causing the third-party site's response
  (still including the script) to be sent to the user. The script
  would then render using the security settings of the third-party
  site rather than the attacker's.


Mitigating Factors:
====================
Buffer overrun in Chunked Encoding transfer:
- On default installations of IIS 5.0 and 5.1, exploiting the
  vulnerability to run code would grant the attacker the privileges
  of the IWAM_computername account, which has only the privileges
  commensurate with those of an interactively logged-on
  unprivileged user.
- The vulnerability requires that Active Server Pages (ASP) be
  enabled on the system in order to be exploited. Version 1.0 of
  the IIS Lockdown Tool removes ASP by default, and the current
  version (version 2.1) removes it by default if Static Web Server
  has been selected.
- The URLScan tool can be configured to prevent chunked encoding
  requests. If this has been done, the vulnerability could not be
  exploited.

Microsoft-discovered variant of Chunked Encoding buffer overrun:
- This vulnerability is subject to exactly the same mitigating
  factors as the buffer overrun in the Chunked Encoding transfer,
  with one exception. The URLScan tool could not be used to protect
  against the vulnerability.

Buffer Overrun in HTTP header handling:
- On default installations of IIS 5.0 and 5.1, exploiting the
  vulnerability to run code would grant the attacker the
  privileges of the IWAM_computername account, which has only
  the privileges commensurate with those of an interactively
  logged-on unprivileged user.
- The vulnerability requires that Active Server Pages (ASP) be
  enabled on the systemin order to be exploited. Version 1.0
  of the IIS Lockdown Tool removes ASP by default, and the
  current version (version 2.1) removes it by default if
  Static Web Server has been selected.
- The URLScan tool's default ruleset would likely limit the
  attacker to using this vulnerability for denial of service
  attacks only.

Buffer Overrun in ASP Server-Side Include Function:
- On default installations of IIS 5.0 and 5.1, exploiting the
  vulnerability to run code would grant the attacker the privileges
  of the IWAM_computername account, which has only the privileges
  commensurate with those of an interactively logged-on user.
- The vulnerability requires that Active Server Pages (ASP) be
  enabled on the system in order to be exploited. Version 1.0
  of the IIS Lockdown Tool removes ASP by default, and the current
  version (version 2.1) removes it by default if Static Web Server
  has been selected.
- The URLScan tool's default ruleset would likely limit the
  attacker to using this vulnerability for denial of service
  attacks only.

Buffer overrun in HTR ISAPI extension:
- Microsoft has long recommended disabling the HTR ISAPI extension.
  Systems on which this has been done would be at no risk from the
  vulnerability. (All versions of the IIS Lockdown Tool disable HTR
  support by default).
- The URLScan tool, if using its default ruleset, would prevent
  this vulnerability from being exploited to run code on the server
  even if HTR support was enabled.
- The vulnerability could only be used to run code on the server if
  the attacker knew the locations of certain information in memory.
  In practice,  the most likely such situation would occur if the
  web server had never served any web content since being rebooted.
  In all other cases, it would only be possible to use the
  vulnerability for denial of service attacks.
- On default installations of IIS 5.0 and 5.1, exploiting the
  vulnerability to run code would grant the attacker the privileges
  of the IWAM_computername account, which has only the privileges
  commensurate with those of an interactively logged-on user.
- If the vulnerability were used in a denial of service attack,
  normal operation could be restored on an IIS 4.0 server by
  restarting the IIS service; on IIS 5.0 and higher, the service
  would automatically restart itself.

Access violation in URL error handling:
- An IIS 4.0 server could be put back into normal operation by
  restarting the service. An IIS 5.0 or 5.1 server would
  automatically restart the service.
- The vulnerability could only be used for denial of service
  attacks. There is no capability to use the vulnerability to gain
  privileges on the system.
- The sole ISAPI filter known to generate the error that results in
  the access violation ships only as part of FrontPage Server
  Extensions and ASP.NET. ASP.NET is not installed by default, and
  FPSE can be uninstalled if desired.

Denial of service via FTP Status request:
- The IIS Lockdown Tool disables FTP support by default.
- An IIS 4.0 server could be put back into normal operation by
  restarting the service. An IIS 5.0 or 5.1 server would
  automatically restart the service.
- The vulnerability could only be used for denial of service
  attacks. There is no capability to use the vulnerability to gain
  privileges on the system.

Cross-site Scripting in IIS Help File search facility, HTTP Error
Page, and Redirect Response message:
- The vulnerabilities could only be exploited if the attacker could
  entice another user into visiting a web page and clicking a link
  on it, or opening an HTML mail.
- The Redirect Response vulnerability could only be exploited if
  the user was running a browser other than Internet Explorer. IE
  does not actually render the text in the Redirect Response, but
  instead recognizes it by its response header and processes the
  redirect without displaying any text.


Risk Rating:
============
- Internet systems: Critical
- Intranet systems: Critical
- Client systems: Critical

Patch Availability:
===================
- A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-018.asp
  for information on obtaining this patch.

Acknowledgment:
===============
- eEye Digital Security (http://www.eeye.com) for reporting the
  buffer overrun in the ASP chunked encoding implementation.
- Entrust Technologies (http://www.entrust.com) for reporting the
  buffer overrun affecting the HTTP header handling.
- Chris Wysopal of @Stake (http://www.atstake.com) and Peter
  Grundl of KPMG for reporting the buffer overrun in the HTR
  ISAPI extension and the access violation in URL error handling.
- Joe Smith ([email protected]) and zenomorph
  ([email protected]) of http:// www.cgisecurity.com for
  reporting the cross-site scripting vulnerability in the IIS
  Help File search facility.
- Keigo Yamazaki of the LAC SNS Team
  (http://www.lac.co.jp/security/) for reporting the
  cross-site scripting vulnerability affecting redirect
  response messages.
- Thor Larholm of Jubii A/S for reporting the cross-site scripting
  vulnerability affecting HTTP error pages.

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