Servlet Security

As well as support for the standard servlet spec required authentication mechanisms, Undertow also allows you to create your own mechanisms, and provides an easy way to make the accessible to an end user.

Securing a Servlet Deployment

Undertow provides full support for all security constructs specified in the Servlet specification. These are configured via the DeploymentInfo structure, and in general closely mirror the corresponding structures as defined by annotations or web.xml. These structures are not detailed fully here, but are covered by the relevant javadoc.

If you are using Wildfly then then it is possible to configure multiple mechanisms using web.xml, by listing the mechanism names separated by commas. It is also possible to set mechanism properties using a query string like syntax.

For example:


The mechanisms will be tried in the order that they are listed. In the example silent basic auth will be tried first, which is basic auth that only takes effect if an Authorization header is present. If no such header is present then form auth will be used instead. This will allow programatic clients to use basic auth, while users connecting via a browser can use form based auth.

The built it list of mechanisms and the properties they take are as follows:

Mechanism Name Options Notes


silent (true/false), charset, user-agent-charsets

silent mode means that a challenge will only be issued if an Authorization header with invalid credentials is present. If this header is not present this auth method will never be used. charset specifies the default charset to use for decoding usernames and passwords. user-agent-charsets is a comma seperated list of the form pattern,charset,pattern,charset,…​ that allows you to change the charset used for decoding based on the browser user agent string. If the regex matches the requests user agent then the specified charset will be used instead.





Used when authentication is being done by a front end such as httpd

Selecting an Authentication Mechanism

The authentication mechanism is specified via the io.undertow.servlet.api.LoginConfig object that can be added using the method DeploymentInfo.setLoginConfig(LoginConfig config). This object contains an ordered list of mechanism names. The Servlet specification only allows you to specify a single mechanism name, while Undertow allows as many as you want (if you are using Wildfly you can make use of this by using a comma separated list of names in web.xml, and pass properties using a query string like syntax, for example BASIC?silent=true,FORM).

The mechanisms are standard Undertow AuthenticationMechanism implementations, and it should be noted that not all mechanisms are compatible. For example trying to combine FORM and BASIC does not work, just because they both require a different response code. Combining FORM and silent BASIC will work just fine however (silent basic auth means that if the user agent provides an Authorization: header then BASIC auth will be used, however if this header is not present then no action will be taken. This allows scripts to use basic auth, while browsers can use form).

When adding the mechanism name to the LoginConfig structure it is also possible to specify a property map. Custom authentication mechanisms may use these properties however they wish. The only built in mechanism that makes use of this mechanism is basic auth, which if passed Collections.singletonMap("silent", "true") will enable silent mode as described above.

The built in mechanisms are FORM, DIGEST, CLIENT_CERT and BASIC.

Adding a Custom Authentication Mechanism

Custom authentication mechanisms are added using the Undertow ServletExtension mechanism. This provides a way to hook into the Undertow deployment process, and add any additional mechanisms.

These extensions are discovered via the standard META-INF/services discovery mechanism, so if you have a jar that provides a custom authentication mechanism all that should be required is to add this jar to your deployment and then specify the mechanism name in web.xml.

For more info see the Servlet extensions guide.