WS-Trust 1.4 introduced a new feature called as “ActAs” for addressing common scenarios where an application needs to call a service on behalf of the logged user or a service needs to call another service on behalf of the original caller. These are typical examples of what is usually resolved with the “Trusted Subsystem” pattern.
“ActAs” is not more than a new element in the RST message for including additional information about the original caller when a token is negotiated with the STS for consuming the final service in the Relying Party (that means that a trust relationship already exists between the STS and the final service). That element usually takes the form of a token with identity claims, which are secured (encrypted/signed) and included together with the credentials of the client application that is negotiating the token with the STS.
As this element is part of the WS-Trust specification, it only makes sense in scenarios where the client authentication is delegated to an STS. I already discussed this scenario several times in the past. The following message shows how this element is included in a RST message,
<saml:Assertion MajorVersion="1" MinorVersion="1" AssertionID="_cf5f224e-4bae-4f2d-9800-248023ac0e4d" Issuer="PassiveSTS" IssueInstant="2010-01-04T15:20:14.506Z" xmlns:saml="urn:oasis:names:tc:SAML:1.0:assertion">
<saml:Conditions NotBefore="2010-01-04T15:20:14.470Z" NotOnOrAfter="2010-01-04T16:20:14.470Z"></saml:Conditions>
<saml:Attribute AttributeName="role" AttributeNamespace="http://microsoft">
<saml:AuthenticationStatement AuthenticationMethod="http://microsoft/geneva" AuthenticationInstant="2010-01-04T15:20:14.481Z">
If you still want to support an scenario like this when an STS is not involved for client authentication, the closer thing you can find is the client authentication through supporting tokens method. I described this scenario in this post, and Dominick did a better job giving a concrete implementation in WCF that uses an Username Token as supporting token, a also a SAML token, which represents something very useful.
The images illustrate two scenarios where this new feature makes a lot of sense. Let’s discuss both of them more in detail.
Scenario 1: A Service A calling a Service B
In this scenario, we have a Service A that needs to make a call to an operation in the Service B. Both services are expecting an token issued by an STS that they trust (The example assumes that they both trust the same STS). This is how “ActAs” works in this scenario.
1. The client negotiates a SAML token with the STS for consuming the service A
2. The client invokes an operation in the Service A using the SAML token it got from the STS as client credentials.
3. Now, the Service A needs to invoke an operation in the Service B so it has to negotiate a new SAML token from the STS first. In order to do that, it includes the SAML token sent by the client as part of the ActAs element, and secures the message using any of the traditional WS-Security profiles. For example, Mutual Certificate, a client certificate for authenticating the client (Service A) and a service certificate for authenticating the service and protecting the messages. The STS issues a new token for consuming the service B using all the information received in the RST message.
4. The service A invokes an operation in the Service B using the SAML token it just got from the STS as client credentials.
Scenario 2: A Web Application calling a Service
In this scenario, the Web Application authenticates all its user through an STS that implements the WS-Trust passive profile. This means that the web application is a claim-aware application that receives the user identity as claims from a token sent by the STS (The user first authenticates in the STS, step 1). As the web application already has a token with the user claims, which is the token it received from the passive STS, it can include it as the ActAs element when it negotiates a SAML token from the Active STS (step 2). Once the application gets a SAML token from the Active STS, it can actually use it for consuming the service (step 3)
Fortunately, WIF already includes support in the client API for negotiating a token with the ActAs element, and also support for parsing and getting access to that information in the STS implementation. What’s more, the implementation of this new feature in WIF is totally interoperable with other Service stacks as well. This is something that have been proved in the Apache Stonehenge project by proving an example application that shows different interoperability scenarios with claim-based security with other service stacks like Sun Metro or WOS2.
On the client side, WIF provides a couple of extensions methods in the WCF channel for including a token in the ActAs element.
public static T CreateChannelActingAs<T>(this ChannelFactory<T> factory, SecurityToken actAs);
public static T CreateChannelActingAs<T>(this ChannelFactory<T> factory, EndpointAddress address, SecurityToken actAs);
public static T CreateChannelActingAs<T>(this ChannelFactory<T> factory, EndpointAddress address, Uri via, SecurityToken actAs);
In case you are using a SAML token, you need to negotiate it out of band with the WSTrustChannel class or use the one that is available in the web application when passive authentication is used with an STS (and the setting saveBootstapTokens is enabled in the microsoft.IdentityModel section). The following code illustrates a sample that uses the SAML token available as part of the Bootstrap tokens in the IClaimsPrincipal instance attached to the current thread.
SecurityToken callerToken = null;
IClaimsPrincipal claimsPrincipal = Thread.CurrentPrincipal as IClaimsPrincipal;
if (claimsPrincipal != null)
foreach (IClaimsIdentity claimsIdentity in claimsPrincipal.Identities)
if (claimsIdentity.BootstrapToken is SamlSecurityToken)
callerToken = claimsIdentity.BootstrapToken;
var channel = channelFactory.CreateChannelActingAs(callerToken);
The STS implementation can get the ActAs element information from the RequestSecurityToken.ActAs property,
if (request.ActAs != null)
IClaimsIdentity actAsIdentity = new ClaimsIdentity();
// Find the last delegate in the actAs identity
IClaimsIdentity lastActingVia = actAsIdentity;
while (lastActingVia.Actor != null)
lastActingVia = lastActingVia.Actor;
// Put the caller's identity as the last delegate to the ActAs identity
lastActingVia.Actor = outputIdentity;
// Return the actAsIdentity instead of the caller's identity in this case
outputIdentity = actAsIdentity;
The apache stonehenge application or the Fabrikam shipping sample in codeplex are using this feature, so you might be interested in taking a look there.