@PostMapping("/login")
    public String loginClient(@ModelAttribute("client") Client client) throws Exception {
        String page = "";
        String input = client.getPostUsername();
        System.out.println(input);
        Client existingClient = service.getClient(input);
        if((client.getPostUsername().equals(existingClient.getUsername())&
                (client.getPostPassword().equals(existingClient.getPassword())))){
            page = "Auth";
            SpotifyAPI newApi = new SpotifyAPI();
            newApi.startServer(8081);
        }

<form method="post" th:action="@{/login}" th:object="${client}" class="login">
    <p>
        <label th:for="postUsername">Username: </label>
        <input type="text" th:name="postUsername" id="postUsername"/>
    </p>

    <p>
        <label th:for="postPassword">Password: </label>
        <input type="text" th:name="postPassword" id="postPassword"/>
    </p>

Forwarding to error page from request [/news/169947] due to exception [The request was rejected because the URL contained a potentially malicious String "//"]

org.springframework.security.web.firewall.RequestRejectedException: The request was rejected because the URL contained a potentially malicious String "//"