Solution Spring Backend OAuth2 Client for both web apps as for native (mobile) apps - spring

Question

For the past days I have been trying to figuring out how to make OAuth2 work on a native app with the OAuth2 client consisting of a separate frontend application with a Spring backend. Good news! I figured out a way to make it work both as web app (on a browser) as on a native (mobile) app. Here I would like to share my findings and ask for any suggestions on possible improvements.
Where Spring works out of the box
Spring Oauth2 works out of the box for web apps. We add the dependency <artifactId>spring-security-oauth2-autoconfigure</artifactId>. We add the annotation #EnableOAuth2Client. Furthermore, we add the configuration. For an in detail tutorial I would like to refer you to this tutorial.
Where challenges start to arise
Spring works with a session cookie (JSESSIONID) to establish a session which is send to the frontend using a Set-Cookie header. In a mobile application this Set-Cookie header is not send back to the backend on subsequent requests. This means that on a mobile application the backend sees each request as a new session. To solve this, I implement a session header rather than a cookie. This header can be read and therefore added to the subsequent requests.
#Bean
public HttpSessionIdResolver httpSessionIdResolver() {
return HeaderHttpSessionIdResolver.xAuthToken();
}
However, that solves only part of the problem. The frontend makes a request using window.location.href which makes it impossible to add custom headers (REST call cannot be used because it would make it impossible to redirect the caller to the authorization server login page, because the browser blocks this). The browser automatically adds cookies to calls made using window.location.href. That's why it works on browser, but not on a mobile application. Therefore, we need to modify Spring's OAuth2 process to be able to receive REST calls rather than a call using window.location.href.
The OAuth2 Client process in Spring
Following the Oauth2 process the frontend makes two calls to the backend:
Using window.location.href a call to be redirected to the Authorization server (e.g. Facebook, Google or your own authorization server).
Making a REST GET request with the code and state query parameter to retrieve an access token.
However, if Spring does not recognise the session (like on mobile phone) it creates a new OAuth2ClientContext class and therefore throws an error on the second call: InvalidRequestException("Possible CSRF detected - state parameter was required but no state could be found"); by the AuthorizationCodeAccessTokenProvider.class. The reason it throws this error is because the preservedState property is null on the request. This is nicely explained by this post's answer of #Nico de wit.
I created a visual of the Spring OAuth2 process which shows the box 'Context present in session?'. This is where it goes wrong as soon as you have retrieved the authorization code from logging into the authorization server. This is because further on in in the getParametersForToken box it checks the preservedState which is then null because it came from a new OAuth2ClientContext object (rather than the same object that was used when redirecting the first call to the page of the authorization server).
The solution
I solved this problem by extending OAuth2ClientContextFilter.class. This class is responsible for redirecting the user to the authorization server login page if no authorization code has been retrieved yet. Instead of redirecting, the custom class now sends back a 200 and the in the body an url to which the frontend needs to be redirected. Also the frontend can now make a REST call rather than using window.location.href to be redirected. That looks something like:
#Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain chain) throws IOException,
ServletException {
HttpServletRequest request = (HttpServletRequest)servletRequest;
HttpServletResponse response = (HttpServletResponse)servletResponse;
request.setAttribute(CURRENT_URI, this.calculateCurrentUri(request));
try {
chain.doFilter(servletRequest, servletResponse);
} catch (IOException var9) {
throw var9;
} catch (Exception var10) {
Throwable[] causeChain = this.throwableAnalyzer.determineCauseChain(var10);
UserRedirectRequiredException redirect = (UserRedirectRequiredException)this.throwableAnalyzer.getFirstThrowableOfType(UserRedirectRequiredException.class, causeChain);
if (redirect == null) {
if (var10 instanceof ServletException) {
throw (ServletException)var10;
}
if (var10 instanceof RuntimeException) {
throw (RuntimeException)var10;
}
throw new NestedServletException("Unhandled exception", var10);
}
// The original code redirects the caller to the authorization page
// this.redirectUser(redirect, request, response);
// Instead we create the redirect Url from the Exception and add it to the body
String redirectUrl = createRedirectUrl(redirect);
response.setStatus(200);
response.getWriter().write(redirectUrlToJson(redirectUrl));
}
}
The createRedirectUrl contains some logic building the Url:
private String createRedirectUrl(UserRedirectRequiredException e) {
String redirectUri = e.getRedirectUri();
UriComponentsBuilder builder = UriComponentsBuilder.fromHttpUrl(redirectUri);
Map<String, String> requestParams = e.getRequestParams();
Iterator it = requestParams.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, String> param = (Map.Entry)it.next();
builder.queryParam(param.getKey(), param.getValue());
}
if (e.getStateKey() != null) {
builder.queryParam("state", e.getStateKey());
}
return builder.build().encode().toUriString();
}
I hope it helps others in the future by implementing OAuth2 using Spring on web and mobile applications. Feel free to give feedback!
Regards,
Bart