How to propagate FlashMap attributes using Springs RedirectStrategy?

Question

I want to add some FlashMessages to the request/response cycle.

Since I add these attributes not within a Controller, but inside an AuthenticationFailureHandler, I cannot rely on the usual redirect attribute helper. Therefore I use RequestContextUtils to get the outputFlashMap and add the parameter I want. However, I can't access them in Thymeleaf, neither by using ${#request.getAttribute('message')} nor directly by using the key itself ${message}.

The handler class looks like this:

public class CustomAuthenticationFailureHandler implements AuthenticationFailureHandler {

    private final RedirectStrategy redirectStrategy = new DefaultRedirectStrategy();

    @Override
    public void onAuthenticationFailure(
            HttpServletRequest request,
            HttpServletResponse response,
            AuthenticationException exception) throws IOException {

        FlashMap outputFlashMap = RequestContextUtils.getOutputFlashMap(request);
        outputFlashMap.addTargetRequestParam("message", "Elvis says Aloha from Hawaii");

        redirectStrategy.sendRedirect(request, response, "/secrets");
    }
}

What am I missing here?