Yale CAS Clients 2.0.2

Owned by Andrew Petro
Last updated: Feb 01, 2007 by ScottSVersion comment
2 min read

Legacy release

There are more recent releases which improve upon this release.

Getting the files

The release is attached to this Wiki page.

Release notes

Addresses a potential security issue whereby the HTTP "Host" header is used as the basis for determining the service URL in some high-level clients - particularly the Java filter and tag library.

The potential problem is best explained as follows. Clients that use a method based on the HTTP "Host" header to determine the current request's URL (e.g., via calls to ServletRequest.getRequestURL()) are vulnerable to an "illicit proxy" attack.

Like all HTTP headers, the HTTP "Host" header is under the control of the client. Suppose service A, running at http://A, is contacted by a user that supplies "Host: B" as a header. If the web server on which service A runs passes through requests for unknown "virtual hosts" to service A, then when service A calls getRequestURL(), the result will be "http://B".

This leads to a situation where service B can conduct an illegal proxy authentication to service A without A's knowledge as follows:

  1. User U visits service B B.
  2. B can acquire a service ticket from CAS coded to user U and service http://B by implementing the CAS protocol correctly
  3. B can then construct a client HTTP request to service A, passing "Host: B" as a header. In this request, it sends the CAS ticket it acquired.
  4. Service A will receive a request that includes a ticket, and it will attempt to validate the ticket with CAS by sending the ticket, along with http://B as its service, to CAS. CAS will approve the authentication.
  5. Service B can now access service A as user U.

All of the following are solutions to this problem:

  • Use the low-level JavaBean interface provided with the CAS distribution, which requires you to determine your service URL yourself. (Of course, this asssumes you can make this determination securely.)
  • Use the newly provided CAS filter or CAS tag library, which accept static configuration parameters that identify the server name. For instance, for the CAS filter, you should add the following initialization parameter:
      <init-param>
        <param-name>edu.yale.its.tp.cas.client.filter.serverName</param-name>
        <param-value>myserver:myport</param-value>
      </init-param>

(where ":myport" is optional).

  • Use the Perl module from version 2.0.2 or later, which requires that you pass in the service URL when making the call to the check_login() method.
  • Use the new Apache modules from version 2.0.2 or later.

This potential attack is not relevant for the PAM module, and the documentation for the ASP page has been adjusted to provide new information.