Understanding Permissions in Apache Shiro

Wildcard Permissions

The above examples of permissions, "Open a file", "View the 'user/list' web page", etc are all valid permission statements. However, it would be very difficult computationally to interpret those natural language strings and determine if a user is allowed to perform that behavior or not.

So to enable easy-to-process yet still readable permission statements, Shiro provides powerful and intuitive permission syntax we refer to as the WildcardPermission.

Simple Usage

Let's you want to protect access to your company's printers such that some people can print to particular printers, while others can query what jobs are currently in the queue.

An extremely simple approach would be to use grant the user a "queryPrinter" permission. Then you could check to see if the user has the queryPrinter permission by calling:

subject.isPermitted("queryPrinter")

This is (mostly) equivalent to

subject.isPermitted( new WildcardPermission("queryPrinter") )

but more on that later.

The simple permission string may work for simple applications, but it requires you to have permissions like "printPrinter", "queryPrinter", "managePrinter", etc. You can also grant a user "*" permissions using the wildcard character (giving this permission construct its name), which means they have all permissions across the entire application.

But using this approach there's no way to just say a user has "all printer permissions". For this reason, Wildcard Permissions supports multiple levels of permissioning.

Multiple Parts

Wildcard Permissions support the concept of multiple levels or parts. For example, you could restructure the previous simple example by granting a user the permission

printer:query

The colon in this example is a special character used to delimit the next part in the permission string.

In this example, the first part is the domain that is being operated on (printer) and the second part is the action (query) being performed. The other above examples would be changed to:

printer:print
printer:manage

There is no limit to the number of parts that can be used, so it is up to your imagination in terms of ways that this could be used in your application.

Multiple Values

Each part can contain multiple values. So instead of granting the user both the "printer:print" and "printer:query" permissions, you could simply grant them one:

printer:print,query

which gives them the ability to print and query printers. And since they are granted both those actions, you could check to see if the user has the ability to query printers by calling:

subject.isPermitted("printer:query")

which would return true

All Values

What if you wanted to grant a user all values in a particular part? It would be more convenient to do this than to have to manually list every value. Again, based on the wildcard character, we can do this. If the printer domain had 3 possible actions (query, print, and manage), this:

printer:query,print,manage

simply becomes this:

printer:*

Then, any permission check for "printer:XXX" will return true. Using the wildcard in this way scales better than explicitly listing actions since, if you added a new action to the application later, you don't need to update the permissions that use the wildcard character in that part.

Finally, it is also possible to use the wildcard token in any part of a wildcard permission string. For example, if you wanted to grant a user the "view" action across all domains (not just printers), you could grant this:

*:view

Then any permission check for "foo:view" would return true

Instance-Level Access Control

Another common usage of wildcard permissions is to model instance-level Access Control Lists. In this scenario you use three parts - the first is the domain, the second is the action(s), and the third is the instance(s) being acted upon.

So for example you could have

printer:query:lp7200
printer:print:epsoncolor

The first defines the behavior to query the printer with the ID lp7200. The second permission defines the behavior to print to the printer with ID epsoncolor. If you grant these permissions to users, then they can perform specific behavior on specific instances. Then you can do a check in code:

if ( SecurityUtils.getSubject().isPermitted("printer:query:lp7200") {
    // Return the current jobs on printer lp7200
}

This is an extremely powerful way to express permissions. But again, having to define multiple instance IDs for all printers does not scale well, particularly when new printers are added to the system. You can instead use a wildcard:

printer:print:*

This does scale, because it covers any new printers as well. You could even allow access to all actions on all printers:

printer:*:*

or all actions on a single printer:

printer:*:lp7200

or even specific actions:

printer:query,print:lp7200

The '*' wildcard and ',' sub-part separator can be used in any part of the permission.

Missing Parts

One final thing to note about permission assignments: missing parts imply that the user has access to all values corresponding to that part. In other words,

printer:print

is equivalent to

printer:print:*

and

printer

is equivalent to

printer:*:*

However, you can only leave off parts from the end of the string, so this:

printer:lp7200

is not equivalent to

printer:*:lp7200

Checking Permissions

While permission assignments use the wildcard construct quite a bit ("printer:print:*" = print to any printer) for convenience and scalability, permission checks at runtime should always be based on the most specific permission string possible.

For example, if the user had a UI and they wanted to print a document to the lp7200 printer, you should check if the user is permitted to do so by executing this code:

if ( SecurityUtils.getSubject().isPermitted("printer:print:lp7200") ) {
    //print the document to the lp7200 printer
}

That check is very specific and explicitly reflects what the user is attempting to do at that moment in time.

The following however is much less ideal for a runtime check:

if ( SecurityUtils.getSubject().isPermitted("printer:print") ) {
    //print the document
}

Why? Because the second example says "You must be able to print to any printer for the following code block to execute". But remember that "printer:print" is equivalent to "printer:print:*"!

Therefore, this is an incorrect check. What if the current user does not have the ability to print to any printer, but they do have the ability to print to say, the lp7200 and epsoncolor printers. Then the 2nd example above would never allow them to print to the lp7200 printer even though they have been granted that ability!

So the rule of thumb is to use the most specific permission string possible when performing permission checks. Of course, the 2nd block above might be a valid check somewhere else in the application if you really did only want to execute the code block if the user was allowed to print to any printer (suspect, but possible). Your application will determine what checks make sense, but in general, the more specific, the better.

Implication, not Equality

Why is it that runtime permission checks should be as specific as possible, but permission assignments can be a little more generic? It is because the permission checks are evaluated by implication logic - not equality checks.

That is, if a user is assigned the user:* permission, this implies that the user can perform the user:view action. The string "user:*" is clearly not equal to "user:view", but the former implies the latter. "user:*" describes a superset of functionality of that defined by "user:view".

To support implication rules, all permissions are translated in to object instances that implement the org.apache.shiro.authz.Permission interface. This is so that implication logic can be executed at runtime and that implication logic is often more complex than a simple string equality check. All of the wildcard behavior described in this document is actually made possible by the org.apache.shiro.authz.permission.WildcardPermission class implementation. Here are some more wildcard permission strings that show access by implication:

user:*

implies the ability to also delete a user:

user:delete

Similarly,

user:*:12345

implies the ability to also update user account with ID 12345:

user:update:12345

and

printer

implies the ability to print to any printer

printer:print

Performance Considerations

Permission checks are more complex than a simple equals comparison, so runtime implication logic must execute for each assigned Permission. When using permission strings like the ones shown above, you're implicitly using Shiro's default WildcardPermission which executes the necessary implication logic.

Shiro's default behavior for Realm implementations is that, for every permission check (for example, a call to subject.isPermitted ), all of the permissions assigned to that user (in their Groups, Roles, or directly assigned to them) need to be checked individually for implication. Shiro 'short circuits' this process by returning immediately after the first successful check occurs to increase performance, but it is not a silver bullet.

This is usually extremely fast when users, roles and permissions are cached in memory when using a proper CacheManager, which Shiro does support for Realm implementations. Just know that with this default behavior, as the number of permissions assigned to a user or their roles or groups increase, the time to perform the check will necessarily increase.

If a Realm implementor has a more efficient way of checking permissions and performing this implication logic, especially if based on the applicaton's data model, they should implement that as part of their Realm isPermitted* method implementations. The default Realm/WildcardPermission support exists to cover 80-90% of most use cases, but it might not be the best solution for applications that have massive amounts of permissions to store and/or check at runtime.

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