Fix NPE for missing Content Type header in OIDC Authenticator (#126191)

* Fix NPE for missing Content Type header in OIDC Authenticator

* Update docs/changelog/126191.yaml

Author: richard-dennehy

docs/changelog/126191.yaml

@@ -0,0 +1,5 @@
+pr: 126191
+summary: Fix NPE for missing Content Type header in OIDC Authenticator
+area: Authentication
+type: bug
+issues: []

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectAuthenticator.java

@@ -629,18 +629,20 @@ public void cancelled() {
     /**
      * Handle the Token Response from the OpenID Connect Provider. If successful, extract the (yet not validated) Id Token
      * and access token and call the provided listener.
+     * (Package private for testing purposes)
      */
-    private static void handleTokenResponse(HttpResponse httpResponse, ActionListener<Tuple<AccessToken, JWT>> tokensListener) {
+    static void handleTokenResponse(HttpResponse httpResponse, ActionListener<Tuple<AccessToken, JWT>> tokensListener) {
         try {
             final HttpEntity entity = httpResponse.getEntity();
             final Header encodingHeader = entity.getContentEncoding();
             final Header contentHeader = entity.getContentType();
-            if (ContentType.parse(contentHeader.getValue()).getMimeType().equals("application/json") == false) {
+            final String contentHeaderValue = contentHeader == null ? null : ContentType.parse(contentHeader.getValue()).getMimeType();
+            if (contentHeaderValue == null || contentHeaderValue.equals("application/json") == false) {
                 tokensListener.onFailure(
                     new IllegalStateException(
                         "Unable to parse Token Response. Content type was expected to be "
                             + "[application/json] but was ["
-                            + contentHeader.getValue()
+                            + contentHeaderValue
                             + "]"
                     )
                 );
@@ -688,7 +690,7 @@ private static void handleTokenResponse(HttpResponse httpResponse, ActionListene
         } catch (Exception e) {
             tokensListener.onFailure(
                 new ElasticsearchSecurityException(
-                    "Failed to exchange code for Id Token using the Token Endpoint. " + "Unable to parse Token Response",
+                    "Failed to exchange code for Id Token using the Token Endpoint. Unable to parse Token Response",
                     e
                 )
             );

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectAuthenticatorTests.java

@@ -968,6 +968,23 @@ public void testHandleUserinfoResponseFailure() throws Exception {
         );
     }
 
+    public void testHandleTokenResponseNullContentType() {
+        final HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, RestStatus.OK.getStatus(), "");
+        final StringEntity entity = new StringEntity("", (ContentType) null);
+        response.setEntity(entity);
+
+        final PlainActionFuture<Tuple<AccessToken, JWT>> future = new PlainActionFuture<>();
+        OpenIdConnectAuthenticator.handleTokenResponse(response, future);
+        final IllegalStateException exception = expectThrows(IllegalStateException.class, future::actionGet);
+
+        assertThat(
+            exception,
+            TestMatchers.throwableWithMessage(
+                "Unable to parse Token Response. Content type was expected to be [application/json] but was [null]"
+            )
+        );
+    }
+
     public void testLogIdTokenAndNonce() throws URISyntaxException, BadJOSEException, JOSEException, IllegalAccessException {
         final Logger logger = LogManager.getLogger(OpenIdConnectAuthenticator.class);
         Loggers.setLevel(logger, Level.DEBUG);