50.4 Securing HTTP Resources

When a request URI is matched by multiple constrained URL patterns, the constraints that apply to the request are those that are associated with the best matching URL pattern. The servlet matching rules defined in Chapter 12, "Mapping Requests To Servlets," in the Java Servlet 3.1 Specification, are used to determine the best matching URL pattern to the request URI. No protection requirements apply to a request URI that is not matched by a constrained URL pattern. The HTTP method of the request plays no role in selecting the best matching URL pattern for a request.

When HTTP methods are listed within a constraint definition, the protections defined by the constraint are applied to the listed methods only.

When HTTP methods are not listed within a constraint definition, the protections defined by the constraint apply to the complete set of HTTP methods, including HTTP extension methods.

When constraints with different protection requirements apply to the same combination of URL patterns and HTTP methods, the rules for combining the protection requirements are as defined in Section 13.8.1, "Combining Constraints," in the Java Servlet 3.1 Specification.

Follow these guidelines to properly secure a web application.

• Do not list HTTP methods within constraint definitions. This is the simplest way to ensure that you are not leaving HTTP methods unprotected. For example:

  <!-- SECURITY CONSTRAINT #1 -->
  <security-constraint>
      <display-name>Do not enumerate Http Methods</display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
      </web-resource-collection>
      <auth-constraint>
          <role-name>sales</role-name>
      </auth-constraint>
  </security-constraint>
  

  If you list methods in a constraint, all non-listed methods of the effectively infinite set of possible HTTP methods, including extension methods, will be unprotected. Use such a constraint only if you are certain that this is the protection scheme you intend to define. The following example shows a constraint that lists the GET method and thus defines no protection on any of the other possible HTTP methods:

  <!-- SECURITY CONSTRAINT #2 -->
  <security-constraint>
      <display-name>
          Protect GET only, leave all other methods unprotected
      </display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
          <http-method>GET</http-method>
      </web-resource-collection>
      <auth-constraint>
          <role-name>sales</role-name>
      </auth-constraint>
  </security-constraint>
  

• If you need to apply specific types of protection to specific HTTP methods, make sure that you define constraints to cover every method that you want to permit, with or without constraint, at the corresponding URL patterns. If there are any methods that you do not want to permit, you must also create a constraint that denies access to those methods at the same patterns; for an example, see security constraint #5 in the next bullet.

  For example, to permit GET and POST, where POST requires authentication and GET is permitted without constraint, you could define the following constraints:

  <!-- SECURITY CONSTRAINT #3 -->
  <security-constraint>
      <display-name>Allow unprotected GET</display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
          <http-method>GET</http-method>
      </web-resource-collection>
  </security-constraint>
  
  <!-- SECURITY CONSTRAINT #4 -->
  <security-constraint>
      <display-name>Require authentication for POST</display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
          <http-method>POST</http-method>
      </web-resource-collection>
      <auth-constraint>
          <role-name>sales</role-name>
      </auth-constraint>
  </security-constraint>
  

• The simplest way to ensure that you deny all HTTP methods except those that you want to be permitted is to use http-method-omission elements to omit those HTTP methods from the security constraint, and also to define an auth-constraint that names no roles. The security constraint will apply to all methods except those that were named in the omissions, and the constraint will apply only to the resources matched by the patterns in the constraint.

  For example, the following constraint excludes access to all methods except GET and POST at the resources matched by the pattern /company/*:

  <!-- SECURITY CONSTRAINT #5 -->
  <security-constraint>
      <display-name>Deny all HTTP methods except GET and POST</display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
          <http-method-omission>GET</http-method-omission>
          <http-method-omission>POST</http-method-omission>
      </web-resource-collection>
      <auth-constraint/>
  </security-constraint>
  

  If you want to extend these exclusions to the unconstrained parts of your application, also include the URL pattern / (forward slash):

  <!-- SECURITY CONSTRAINT #6 -->
  <security-constraint>
      <display-name>Deny all HTTP methods except GET and POST</display-name>
      <web-resource-collection>
          <url-pattern>/company/*</url-pattern>
          <url-pattern>/</url-pattern>
          <http-method-omission>GET</http-method-omission>
          <http-method-omission>POST</http-method-omission>
      </web-resource-collection>
      <auth-constraint/>
  </security-constraint>
  

• If, for your web application, you do not want any resource to be accessible unless you explicitly define a constraint that permits access to it, you can define an auth-constraint that names no roles and associate it with the URL pattern /. The URL pattern / is the weakest matching pattern. Do not list any HTTP methods in this constraint:

  <!-- SECURITY CONSTRAINT #7 -->
  <security-constraint>
      <display-name>
          Switch from Constraint to Permission model
          (where everything is denied by default)
      </display-name>
      <web-resource-collection>
          <url-pattern>/</url-pattern>
      </web-resource-collection>
      <auth-constraint/>
  </security-constraint>