Microsoft Active Server Pages Cookie Retrieval Issue
5 May 2004
Risk Level: Low
Summary: The Active Server Pages (ASP) engine does not properly handle special cookie values when they are retrieved. Because of this, an unhandled error is returned to the client. This behavior can be used maliciously to gather sensitive information from web applications.
Versions Affected: All Microsoft Internet Information Server (IIS) web applications using Active Server Pages (ASP).
Details: ASP is an extension to IIS which allows HTML pages to be dynamically generated on the server side. When the server receives a request for an ASP file, it processes server-side scripts contained in the file to build the page that is sent back to the browser. ASP files can also contain HTML, including related client-side scripts, as well as calls to COM components that perform a variety of tasks such as connecting to a database or processing business logic. ASP pages are supported on all Microsoft Web Servers including Personal Web Server and Internet Information Server.
ASP exposes many objects to enable easy development of web applications. These objects are used to allow browsers and web applications to easily exchange information over HTTP. When a special value, ("="), is sent in a Cookie header value and an ASP page tries to access this value, an unhandled error is returned by the ASP engine.
Example HTTP request:
GET /somepage.asp HTTP/1.0 Host: hostname Cookie: =
Source code snippet for somepage.asp:
value=request.cookies("cookiename") 'here the error is triggered
Value returned by the IIS server:
(0x80004005) Unespecified error /file.inc, line 2
In this example, the attacker was able to determine the name of the include file by setting the cookie to "=". Revealing information such as the include file name could be used to find other more sensitive information.
Other possible problems include being able to tell when a cookie is being read. If a cookie is set to "=" and the following return value is generated:
(0x80004005) Unespecified error /somepage.asp, line 19
The attacker has verified that the cookie was accessed on this page.
This vulnerability can be executed remotely and allows an attacker to map web application logic determining when cookies are read, etc... This allows an attacker to generate errors messages possibly exposing sensitive information that can be used in further attacks.
Fix: IIS Web Servers should be configured to return custom error pages which do not reveal details about the script which caused the error. Information about how to create custom error pages is available at http://support.microsoft.com/?id=834452.
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