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Port80 Software ServerMask Inconsistencies
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SUMMARY
The <http://www.port80software.com/products/servermask/> 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."
In practice, 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, anonymization, is therefore not fully
achieved as only a subset of identifying traits are obfuscated.
DETAILS
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 anonymizing the server, whilst adding an additional product
into the equation that must 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 used.
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
IIS.
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 Information:
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0105>
CAN-2003-0105
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 that 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.
ADDITIONAL INFORMATION
The information has been provided by <mailto:jane.frankland@corsaire.com>
Jane Frankland.
The original article can be found at:
<http://www.corsaire.com/advisories/c030224-001.txt>
http://www.corsaire.com/advisories/c030224-001.txt
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