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Custom Access Decision Voter

You might have a need for a custom access decision voter when security decisions are made based on who is accessing what domain object. Luckily Spring Security has quite a few options to implement such access control list (ACL) constraints.

Implementation

We’ll explore a system where users share spreadsheets, with access per spreadsheet stored separately. We’ve explicitly modeled the permission storage as simple as can be; imagine it’s calling out to a system of record elsewhere. Note that in this simplified implementation, access decisions are binary: you either have access or you don’t. There’s no distinction between READ/WRITE access in this implementation.

Secured annotation

Opening SpreadsheetService reveals a single method annotated with @Secured.

@Secured("com.jdriven.model.Spreadsheet")
public void read(Spreadsheet spreadsheet) {
  log.info("Reading {}", spreadsheet);
}

The @Secured annotation argument is the fully qualified class name of the domain object access we are aiming to restrict. The method has an argument of the same type, which is the specific instance we’ll secure.

Conveniently the @Secured annotation need not reference the argument by name; it can be retrieved by type alone in our AccessDecisionVoter.

AccessDecisionVoter

Access decisions are made by an AccessDecisionManager, which delegates out to a list of configured AccessDecisionVoters. Voters can choose to GRANT or DENY a particular method invocation, based on their application logic. If a voter can not decide on a particular method invocation, it can choose to ABSTAIN to leave the decision up to further voters. By default you’ll get the AffirmativeBased access decision manager, which allows a method invocation if just one of the voters votes to GRANT access, regardless of any votes to DENY access.

For our purposes we want a custom voter that validates user access to a spreadsheet against the stored access records. We do so by extending AbstractAclVoter, which for a configured processed domain object class and method invocation can look up the argument domain object instance. We’ll implement the vote method, which is passed the authenticated user, the secured MethodInvocation and a collection of ConfigAttributes.

@Override
public int vote(Authentication authentication, MethodInvocation methodInvocation, Collection<ConfigAttribute> attributes) {
  for (ConfigAttribute configAttribute : attributes) {
    if (supports(configAttribute)) {
      User principal = (User) authentication.getPrincipal();
      Spreadsheet domainObjectInstance = (Spreadsheet) getDomainObjectInstance(methodInvocation);
      return hasSpreadsheetAccess(principal, domainObjectInstance) ? ACCESS_GRANTED : ACCESS_DENIED;
    }
  }
  return ACCESS_ABSTAIN;
}

Our voter is passed one or more ConfigAttributes, as passed to the @Secured annotation itself, which we validate by calling boolean supports(ConfigAttribute):

@Override
public boolean supports(ConfigAttribute attribute) {
  return getProcessDomainObjectClass().getName().equals(attribute.getAttribute());
}

Given these implementations, the voter will ABSTAIN only when the ConfigAttribute does not match the configured ProcessDomainObjectClass. In all other cases the voter will vote to GRANT or DENY access, according to the access records stored.

Configuration

We need to configure two parts of our application for our custom access decision voter logic to trigger.

Firstly, we need to activate the @Secured annotations through @EnableGlobalMethodSecurity(securedEnabled = true), which is done in AccessDecisionConfiguration.

Secondly, we need to add our SpreadsheetAccessDecisionVoter to the list of decision voters considered by the AccessDecisionManager. For this we extend GlobalMethodSecurityConfiguration to override AccessDecisionManager accessDecisionManager(). We call super.accessDecisionManager() to get the default AffirmativeBased access decision manager, and merely add our own voter at the end.

Should you have a need to secure more than one domain object type, it’s easy enough to add more voters to the list.

Tests

Our SpreadsheetAccessDecisionVoterIT tests use mock users Alice and Bob, with a malicious third user Eve, all trying to access a single spreadsheet.

You can see access to the spreadsheet abides by the rules stored at the start of each test:

  • Alice is able to access the spreadsheet.

  • Bob is also able to access the spreadsheet.

  • Eve is unable to access the spreadsheet, as she gets an AccessDeniedException.

Alternatives

For highly customizable access storage you might adopt Spring Security’s ACL services, as shipped in spring-security-acl-xxx.jar. This provides a very powerful method of encoding, storing and retrieving access permissions.