[VulnWatch] Corsaire Security Advisory - Port80 Software ServerMask inconsistencies

From: advisories (advisories_at_corsaire.com)
Date: 08/10/04

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    Date: Tue, 10 Aug 2004 09:10:34 +0100

    -- Corsaire Security Advisory --

    Title: Port80 Software ServerMask inconsistencies
    Date: 24.02.03
    Application: Port80 Software ServerMask 2.2 and prior
    Environment: IIS 4 / IIS 5 / IIS 5.1
    Author: Martin O'Neal [martin.oneal@corsaire.com]
    Audience: General distribution
    Reference: c030224-001

    -- Scope --

    The aim of this document is to clearly define some issues related to the
    ServerMask product, as supplied by Port80 Software [1]

    -- History --

    Discovered: 20.02.03 (Martin O'Neal)
    Vendor notified: 24.02.03
    Document released: 10.08.04

    The release process on this advisory has been drawn-out due to repeated
    requests from Port80, on the basis that a revised product which resolved
    the issues would be available shortly. However, after eighteen months of
    waiting (during which time Port80 has continued to actively sell the
    product) it has become clear that no such fix is imminent.

    This advisory has been publicly released without a vendor fix being
    immediately available because the issues identified are not critical,
    and do not allow the host to be remotely compromised.

    -- Overview --

    The ServerMask product is marketed as a solution for improving the
    security of Microsoft IIS servers by obfuscating header fields within
    HTTP responses:

    "ServerMask 2.0 removes or modifies unnecessary response data. The
    software provides control over what Server header data, if any, is
    visible in HTTP responses." [2]

    In practise, ServerMask changes only a subset of the HTTP header fields,
    leaving a number of responses unmodified. These remaining headers still
    provide reliable clues to the server being Microsoft IIS.

    The stated goal of the product, anonymisation, is therefore not fully
    achieved as only a subset of identifying traits are obfuscated.

    -- Analysis --

    The ServerMask product is provided as an ISAPI filter for Microsoft IIS,
    and works by intercepting requests to the server and rewriting the HTTP
    header fields in responses.

    The product rewrites some server headers, removes some unnecessary ones,
    and reorders the remaining headers.
    However, it leaves several obvious header fields unchanged that can be
    used to identify the server as Microsoft IIS, including:

         - ETag:
         - HTTP Status Message
         - Allow: header in response to OPTIONS request

    As it stands, the ServerMask product provides at best only an incomplete
    solution to anonymising the server, whilst adding an additional product
    into the equation that must itself be maintained (and could potentially
    contain exploitable flaws).

    -- Proof of concept --

    To reproduce these issues, all that is required is access to a telnet
    client (or similar client providing equivalent functionality) and a
    suitable web server using the ServerMask product. For the purposes of
    this example Port80 Software's home site (www.port80software.com) is

    Issue#1 - Standard IIS format ETag header

    From a command prompt or shell, telnet, netcat or other similar client
    should be used to connect to the web server on TCP port 80, e.g.

         telnet www.port80software.com 80

    The following extract should then be pasted into the session:

         GET /images/H_horline.gif HTTP/1.1
         Accept: */*
         Connection: Keep-Alive
         Host: www.port80software.com

    The response received back should include:

         HTTP/1.1 200 OK
         Date: Mon, 24 Feb 2003 12:37:38 GMT
         Server: Yes - We Use ServerMask
         Last-Modified: Thu, 26 Sep 2002 00:07:29 GMT
         ETag: "8e9dc0b3f064c21:9b0"
         Accept-Ranges: bytes
         Content-Length: 59
         Content-Type: image/gif

    The ETag header is the standard format returned by Microsoft IIS, and
    can be considered unique to that product.

    Issue#2 - 404 Status Message Format

    A session should again be initiated to the web port on the target, e.g.

         telnet www.port80software.com 80

    The following request should then be used to attempt to retrieve a non-
    existent file from the server:

         GET /not.there HTTP/1.1
         Accept: */*
         Connection: Keep-Alive
         Host: www.port80software.com

    The response should include headers similar to the following:

         HTTP/1.1 404 Object Not Found
         Date: Mon, 24 Feb 2003 12:49:54 GMT
         Server: Yes - We Use ServerMask
         Content-Length: 15383
         Connection: close
         Content-Type: text/html

    The HTTP status message on the first line ("404 Object Not Found") is
    the standard format returned by Microsoft IIS, and differs from most
    other vendors.

    Issue#3 - Standard IIS Format Allow header

    A new session should be initiated to the web-server:

         telnet www.port80software.com 80

    An OPTIONS request should then be sent to solicit a server response:

         OPTIONS /images/H_horline.gif HTTP/1.1
         Accept: */*
         Connection: Keep-Alive
         Host: www.port80software.com

    The headers should include the Allow response to the OPTIONS request:

         HTTP/1.1 200 OK
         Date: Mon, 24 Feb 2003 13:05:07 GMT
         Server: Yes - We Use ServerMask
         Content-Length: 0
         Allow: OPTIONS, TRACE, GET, HEAD
    The Allow header is the standard format, content and order returned by

    -- Recommendations --

    The ServerMask product should be revised and improved to provide full
    control over modifying the values of all header fields, to prevent such
    analysis revealing the nature of the underlying web-server.

    -- CVE --

    The Common Vulnerabilities and Exposures (CVE) project has assigned
    the name CAN-2003-0105 to this issue. This is a candidate for
    inclusion in the CVE list (http://cve.mitre.org), which standardizes
    names for security problems.

    -- References --

    [1] http://www.port80software.com/products/servermask/
    [2] http://www.port80software.com/products/servermask/faq.asp

    -- Revision --

    a. Initial release.
    b. Minor revisions.
    c. Minor revisions.
    d. Revised to include CVE reference.
    e. Minor revisions.

    -- Distribution --

    This security advisory may be freely distributed, provided that it
    remains unaltered and in its original form.

    -- Disclaimer --

    The information contained within this advisory is supplied "as-is" with
    no warranties or guarantees of fitness of use or otherwise. Corsaire
    accepts no responsibility for any damage caused by the use or misuse of
    this information.

    -- About Corsaire --
    Corsaire are a leading information security consultancy, founded in 1997
    and based in Surrey, UK. Corsaire bring innovation, integrity and
    analytical rigour to every job, which means fast and dramatic security
    performance improvements. Our services centre on the delivery of
    information security planning, assessment, implementation, management
    and vulnerability research.

    A free guide to selecting a security assessment supplier is available at

    Copyright 2003 Corsaire Limited. All rights reserved.

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