RESTful Authentication via Spring

Question

Problem:
We have a Spring MVC-based RESTful API which contains sensitive information. The API should be secured, however sending the user's credentials (user/pass combo) with each request is not desirable. Per REST guidelines (and internal business requirements), the server must remain stateless. The API will be consumed by another server in a mashup-style approach.

Requirements:

• Client makes a request to .../authenticate (unprotected URL) with credentials; server returns a secure token which contains enough information for the server to validate future requests and remain stateless. This would likely consist of the same information as Spring Security's Remember-Me Token.

• Client makes subsequent requests to various (protected) URLs, appending the previously obtained token as a query parameter (or, less desirably, an HTTP request header).

• Client cannot be expected to store cookies.

• Since we use Spring already, the solution should make use of Spring Security.

We've been banging our heads against the wall trying to make this work, so hopefully someone out there has already solved this problem.

Given the above scenario, how might you solve this particular need?

Answer 1

We managed to get this working exactly as described in the OP, and hopefully someone else can make use of the solution. Here's what we did:

Set up the security context like so:

<security:http realm="Protected API" use-expressions="true" auto-config="false" create-session="stateless" entry-point-ref="CustomAuthenticationEntryPoint">
    <security:custom-filter ref="authenticationTokenProcessingFilter" position="FORM_LOGIN_FILTER" />
    <security:intercept-url pattern="/authenticate" access="permitAll"/>
    <security:intercept-url pattern="/**" access="isAuthenticated()" />
</security:http>

<bean id="CustomAuthenticationEntryPoint"
    class="com.demo.api.support.spring.CustomAuthenticationEntryPoint" />

<bean id="authenticationTokenProcessingFilter"
    class="com.demo.api.support.spring.AuthenticationTokenProcessingFilter" >
    <constructor-arg ref="authenticationManager" />
</bean>

As you can see, we've created a custom AuthenticationEntryPoint, which basically just returns a 401 Unauthorized if the request wasn't authenticated in the filter chain by our AuthenticationTokenProcessingFilter.

CustomAuthenticationEntryPoint:

public class CustomAuthenticationEntryPoint implements AuthenticationEntryPoint {
    @Override
    public void commence(HttpServletRequest request, HttpServletResponse response,
            AuthenticationException authException) throws IOException, ServletException {
        response.sendError( HttpServletResponse.SC_UNAUTHORIZED, "Unauthorized: Authentication token was either missing or invalid." );
    }
}

AuthenticationTokenProcessingFilter:

public class AuthenticationTokenProcessingFilter extends GenericFilterBean {

    @Autowired UserService userService;
    @Autowired TokenUtils tokenUtils;
    AuthenticationManager authManager;

    public AuthenticationTokenProcessingFilter(AuthenticationManager authManager) {
        this.authManager = authManager;
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        @SuppressWarnings("unchecked")
        Map<String, String[]> parms = request.getParameterMap();

        if(parms.containsKey("token")) {
            String token = parms.get("token")[0]; // grab the first "token" parameter

            // validate the token
            if (tokenUtils.validate(token)) {
                // determine the user based on the (already validated) token
                UserDetails userDetails = tokenUtils.getUserFromToken(token);
                // build an Authentication object with the user's info
                UsernamePasswordAuthenticationToken authentication = 
                        new UsernamePasswordAuthenticationToken(userDetails.getUsername(), userDetails.getPassword());
                authentication.setDetails(new WebAuthenticationDetailsSource().buildDetails((HttpServletRequest) request));
                // set the authentication into the SecurityContext
                SecurityContextHolder.getContext().setAuthentication(authManager.authenticate(authentication));         
            }
        }
        // continue thru the filter chain
        chain.doFilter(request, response);
    }
}

Obviously, TokenUtils contains some privy (and very case-specific) code and can't be readily shared. Here's its interface:

public interface TokenUtils {
    String getToken(UserDetails userDetails);
    String getToken(UserDetails userDetails, Long expiration);
    boolean validate(String token);
    UserDetails getUserFromToken(String token);
}

That ought to get you off to a good start. Happy coding. :)