Add permission boundaries to the stack

When I deploy a CDK stack - it creates several roles, both explicitly e.g. via iam.Role construct, and implicitly e.g. when roles are created internally by Level 2 constructs.

Is there a way to attach an existing permission boundary to all the roles being created by the stack - both explicit and implicit?

Yes, through aspects.

You can add the following in bin/app.ts (the file might be named differently):

class ConfigurePermissionBoundary implements IAspect {
  visit(node: IConstruct): void {
    if (node instanceof CfnRole) {
      const stack1 = Stack.of(node);
      let policy = stack1.node.tryFindChild('GlobalPermissionBoundaryPolicy') as ManagedPolicy | undefined

      if (!policy) {
        policy = new ManagedPolicy(stack1, 'GlobalPermissionBoundaryPolicy', {
          statements: [new PolicyStatement({
            effect: Effect.DENY,
            actions: ['sts:*'],
            resources: ['*']
          })]
        })
      }

      node.permissionsBoundary = policy.managedPolicyArn
    }
  }
}

Aspects.of(app).add(new ConfigurePermissionBoundary())

Please note that we're adding a managed policy GlobalPermissionBoundaryPolicy that defines the permission boundary once. Also, the aspect handles both new iam.Role() as well as any new CfnRole() defined by your or library code.