Avoid validation with privilege return

Suppose the following function checks if a user can access a resource. What can go wrong?

function checkAccess(User user, Resource resource) boolean {
  if (IsAccessAllowed(user, resource) == ACCESS_DENIED) {
    return false
  } 
  return true
}

At first glance, it looks fine but what would happen if isAccessAllowed fails?

For example, system goes out of memory or it throws an exception that result into a different response. Any response aside from ACCESS_DENIED could make checkAccess to return true.

In other word, any other error authorises the user to access the resource.

The design of the above function has a security weakness and it allows for privilege return. It means by default it let the action to be performed therefore, any unforeseen error could result into a security breach (e.g. unauthorised access to a resource).

I have commonly seen this insecure design with validation functions, where it checks for a series of rules and throws an error if any rule doesn’t match.

A better alternative is to return to a low privilege by default. This means our function should disallow by default unless permitted by a previous state.

function checkAccess(User user, Resource resource) boolean {
  if (IsAccessAllowed(user, resource)) {
    return true
  } 
  return false <1>
}
  1. Unprivileged return by default

Therefore, if the validation function fails due to an unexpected error, the caller does not allow access. This simple change in the design could prevent a security breach.

The idea of this principle is similar to Deny by Default firewall rule that blocks inbound or outbound network traffic that has not been allowed by previous rules.

BTW, did you know it is always better not to validate but parse untrusted data?