# Run-As Authentication Replacement

## Overview

The AbstractSecurityInterceptor is able to temporarily replace the Authentication object in the SecurityContext and SecurityContextHolder during the secure object callback phase. This only occurs if the original Authentication object was successfully processed by the AuthenticationManager and AccessDecisionManager. The `RunAsManager` will indicate the replacement Authentication object, if any, that should be used during the `SecurityInterceptorCallback`.

By temporarily replacing the Authentication object during the secure object callback phase, the secured invocation will be able to call other objects which require different authentication and authorization credentials. It will also be able to perform any internal security checks for specific GrantedAuthority objects. Because Spring Security provides a number of helper classes that automatically configure remoting protocols based on the contents of the SecurityContextHolder, these run-as replacements are particularly useful when calling remote web services

## Configuration

A `RunAsManager` interface is provided by Spring Security:

      Authentication buildRunAs(Authentication authentication, Object object, ConfigAttributeDefinition config);
      boolean supports(ConfigAttribute attribute);
      boolean supports(Class clazz);

The first method returns the Authentication object that should replace the existing Authentication object for the duration of the method invocation. If the method returns `null`, it indicates no replacement should be made. The second method is used by the AbstractSecurityInterceptor as part of its startup validation of configuration attributes. The `supports(Class)` method is called by a security interceptor implementation to ensure the configured `RunAsManager` supports the type of secure object that the security interceptor will present.

One concrete implementation of a `RunAsManager` is provided with Spring Security. The `RunAsManagerImpl` class returns a replacement `RunAsUserToken` if any `ConfigAttribute` starts with `RUN_AS_`. If any such `ConfigAttribute` is found, the replacement `RunAsUserToken` will contain the same principal, credentials and granted authorities as the original Authentication object, along with a new `GrantedAuthorityImpl` for each `RUN_AS_` `ConfigAttribute`. Each new `GrantedAuthorityImpl` will be prefixed with `ROLE_`, followed by the `RUN_AS` `ConfigAttribute`. For example, a `RUN_AS_SERVER` will result in the replacement `RunAsUserToken` containing a `ROLE_RUN_AS_SERVER` granted authority.

The replacement `RunAsUserToken` is just like any other Authentication object. It needs to be authenticated by the AuthenticationManager, probably via delegation to a suitable AuthenticationProvider. The `RunAsImplAuthenticationProvider` performs such authentication. It simply accepts as valid any `RunAsUserToken` presented.

To ensure malicious code does not create a `RunAsUserToken` and present it for guaranteed acceptance by the `RunAsImplAuthenticationProvider`, the hash of a key is stored in all generated tokens. The `RunAsManagerImpl` and `RunAsImplAuthenticationProvider` is created in the bean context with the same key:

    <bean id="runAsManager" class="org.springframework.security.runas.RunAsManagerImpl">
      <property name="key" value="my_run_as_password"/>
    </bean>

    <bean id="runAsAuthenticationProvider"
        class="org.springframework.security.runas.RunAsImplAuthenticationProvider">
      <property name="key" value="my_run_as_password"/>
    </bean>

By using the same key, each `RunAsUserToken` can be validated it was created by an approved `RunAsManagerImpl`. The `RunAsUserToken` is immutable after creation for security reasons