In this guide we'll cover how to secure your Servlet application by validating incoming requests to your Twilio webhooks that are, in fact, from Twilio.
With a few lines of code, we'll write a custom filter for our Servlet app that uses the Twilio Java SDK's validator utility. This filter will then be invoked on the relevant paths that accept Twilio webhooks to confirm that incoming requests genuinely originated from Twilio.
Let's get started!
The Twilio Java SDK includes a RequestValidator
class we can use to validate incoming requests.
We could include our request validation code as part of our Servlet, but this is a perfect opportunity to write a Java filter. This way we can reuse our validation logic across all our Servlets which accept incoming requests from Twilio.
Confirm incoming requests to your Servlets are genuine with this filter.
1package guide;23import com.twilio.security.RequestValidator;45import javax.servlet.*;6import javax.servlet.http.HttpServletRequest;7import javax.servlet.http.HttpServletResponse;8import java.io.IOException;9import java.util.Arrays;10import java.util.Collections;11import java.util.List;12import java.util.Map;13import java.util.stream.Collectors;1415public class TwilioRequestValidatorFilter implements Filter {1617private RequestValidator requestValidator;1819@Override20public void init(FilterConfig filterConfig) throws ServletException {21requestValidator = new RequestValidator(System.getenv("TWILIO_AUTH_TOKEN"));22}2324@Override25public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)26throws IOException, ServletException {2728boolean isValidRequest = false;29if (request instanceof HttpServletRequest) {30HttpServletRequest httpRequest = (HttpServletRequest) request;3132// Concatenates the request URL with the query string33String pathAndQueryUrl = getRequestUrlAndQueryString(httpRequest);34// Extracts only the POST parameters and converts the parameters Map type35Map<String, String> postParams = extractPostParams(httpRequest);36String signatureHeader = httpRequest.getHeader("X-Twilio-Signature");3738isValidRequest = requestValidator.validate(39pathAndQueryUrl,40postParams,41signatureHeader);42}4344if(isValidRequest) {45chain.doFilter(request, response);46} else {47((HttpServletResponse)response).sendError(HttpServletResponse.SC_FORBIDDEN);48}49}5051@Override52public void destroy() {53// Nothing to do54}5556private Map<String, String> extractPostParams(HttpServletRequest request) {57String queryString = request.getQueryString();58Map<String, String[]> requestParams = request.getParameterMap();59List<String> queryStringKeys = getQueryStringKeys(queryString);6061return requestParams.entrySet().stream()62.filter(e -> !queryStringKeys.contains(e.getKey()))63.collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()[0]));64}6566private List<String> getQueryStringKeys(String queryString) {67if(queryString == null || queryString.length() == 0) {68return Collections.emptyList();69} else {70return Arrays.stream(queryString.split("&"))71.map(pair -> pair.split("=")[0])72.collect(Collectors.toList());73}74}7576private String getRequestUrlAndQueryString(HttpServletRequest request) {77String queryString = request.getQueryString();78String requestUrl = request.getRequestURL().toString();79if(queryString != null && !queryString.equals("")) {80return requestUrl + "?" + queryString;81}82return requestUrl;83}84}85
The doFilter
method will be executed before our Servlet, so it's here where we will validate that the request originated genuinely from Twilio, and prevent it from reaching our Servlet if it didn't.
First, we gather the relevant request metadata (URL, query string and X-TWILIO-SIGNATURE
header) and the POST
parameters. We then pass this data onto the validate
method of RequestValidator
, which will return whether the validation was successful or not.
If the validation turns out successful, we continue executing other filters and eventually our Servlet. If it is unsuccessful, we stop the request and send a 403 - Forbidden
response to the requester, in this case, Twilio.
Now we're ready to apply our filter to any path in our Servlet application that handles incoming requests from Twilio.
Apply a custom Twilio request validation filter to a set of Servlets used for Twilio webhooks.
1<?xml version="1.0" encoding="UTF-8"?>2<web-app version="3.0"3metadata-complete="true"4xmlns="http://java.sun.com/xml/ns/javaee"5xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"6xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">78<servlet>9<servlet-name>voiceHandler</servlet-name>10<servlet-class>guide.VoiceHandlerServlet</servlet-class>11</servlet>1213<servlet>14<servlet-name>messageHandler</servlet-name>15<servlet-class>guide.MessageHandlerServlet</servlet-class>16</servlet>1718<servlet-mapping>19<servlet-name>voiceHandler</servlet-name>20<url-pattern>/voice</url-pattern>21</servlet-mapping>2223<servlet-mapping>24<servlet-name>messageHandler</servlet-name>25<url-pattern>/message</url-pattern>26</servlet-mapping>2728<filter>29<filter-name>requestValidatorFilter</filter-name>30<filter-class>guide.TwilioRequestValidatorFilter</filter-class>31</filter>32<filter-mapping>33<filter-name>requestValidatorFilter</filter-name>34<url-pattern>/*</url-pattern>35</filter-mapping>36</web-app>37
To use the filter just add <filter>
and <filter-mapping>
sections to your web.xml
. No changes are needed in the actual Servlets.
In the <filter>
section we give a name to be used within your web.xml
. In this case requestValidatorFilter
. We also point to the filter class using its fully qualified name.
In the <filter-mapping>
section, we configure what paths in our container will use TwilioRequestFilter
when receiving a request. It uses URL patterns to select those paths, and you can have multiple <url-pattern>
elements in this section. Since we want to apply the filter to both Servlets, we use their common root path.
Note: If your Twilio webhook URLs start with https://
instead of http://
, your request validator may fail locally when you use Ngrok or in production if your stack terminates SSL connections upstream from your app. This is because the request URL that your Servlet application sees does not match the URL Twilio used to reach your application.
To fix this for local development with Ngrok, use http://
for your webhook instead of https://
. To fix this in your production app, your filter will need to reconstruct the request's original URL using request headers like X-Original-Host
and X-Forwarded-Proto
, if available.
If you write tests for your Servlets those tests may fail where you use your Twilio request validation filter. Any requests your test suite sends to those Servlets will fail the filter's validation check.
To fix this problem we recommend adding an extra check in your filter, like so, telling it to only reject incoming requests if your app is running in production.
Use this version of the custom filter if you test your Servlets.
1package guide;23import com.twilio.security.RequestValidator;45import javax.servlet.*;6import javax.servlet.http.HttpServletRequest;7import javax.servlet.http.HttpServletResponse;8import java.io.IOException;9import java.util.Arrays;10import java.util.Collections;11import java.util.List;12import java.util.Map;13import java.util.stream.Collectors;1415public class TwilioRequestValidatorFilter implements Filter {1617private final String currentEnvironment = System.getenv("ENVIRONMENT");1819private RequestValidator requestValidator;2021@Override22public void init(FilterConfig filterConfig) throws ServletException {23requestValidator = new RequestValidator(System.getenv("TWILIO_AUTH_TOKEN"));24}2526@Override27public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)28throws IOException, ServletException {2930boolean isValidRequest = false;31if (request instanceof HttpServletRequest) {32HttpServletRequest httpRequest = (HttpServletRequest) request;3334// Concatenates the request URL with the query string35String pathAndQueryUrl = getRequestUrlAndQueryString(httpRequest);36// Extracts only the POST parameters and converts the parameters Map type37Map<String, String> postParams = extractPostParams(httpRequest);38String signatureHeader = httpRequest.getHeader("X-Twilio-Signature");3940isValidRequest = requestValidator.validate(41pathAndQueryUrl,42postParams,43signatureHeader);44}4546if(isValidRequest || environmentIsTest()) {47chain.doFilter(request, response);48} else {49((HttpServletResponse)response).sendError(HttpServletResponse.SC_FORBIDDEN);50}51}5253@Override54public void destroy() {55// Nothing to do56}5758private boolean environmentIsTest() {59return "test".equals(currentEnvironment);60}6162private Map<String, String> extractPostParams(HttpServletRequest request) {63String queryString = request.getQueryString();64Map<String, String[]> requestParams = request.getParameterMap();65List<String> queryStringKeys = getQueryStringKeys(queryString);6667return requestParams.entrySet().stream()68.filter(e -> !queryStringKeys.contains(e.getKey()))69.collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()[0]));70}7172private List<String> getQueryStringKeys(String queryString) {73if(queryString == null || queryString.length() == 0) {74return Collections.emptyList();75} else {76return Arrays.stream(queryString.split("&"))77.map(pair -> pair.split("=")[0])78.collect(Collectors.toList());79}80}8182private String getRequestUrlAndQueryString(HttpServletRequest request) {83String queryString = request.getQueryString();84String requestUrl = request.getRequestURL().toString();85if(queryString != null && queryString != "") {86return requestUrl + "?" + queryString;87}88return requestUrl;89}90}91
Validating requests to your Twilio webhooks is a great first step for securing your Twilio application. We recommend reading over our full security documentation for more advice on protecting your app, and the Anti-Fraud Developer's Guide in particular.
To learn more about securing your Servlet application in general, check out the security considerations page in the official Oracle docs.