Corsaire Security Advisory 2003-02-24.1

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-- 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 --  
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   
http://www.penetration-testing.com   
  
  
Copyright 2003 Corsaire Limited. All rights reserved.   
  
  
  
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