Corsaire Security Advisory - Port80 Software ServerMask inconsistencies

– 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 [[email protected]]
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
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 –

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 –
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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
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