Because public_send does not restrict the target method to those explicitly defined by the Service, a guest script can invoke ambient reflection methods such as send, __send__, or public_send. By pivoting through these methods, an attacker can access sensitive Kernel methods, specifically eval, to execute arbitrary Ruby code within the host process. This bypasses the primary security guarantee of the Kobako library. The issue was resolved in version 0.9.1 by implementing a blocklist that prevents the invocation of methods belonging to core Ruby modules like Kernel, Object, and Module.

Impact

Successful exploitation results in a full sandbox escape, granting an attacker the ability to execute arbitrary Ruby code within the context of the host process. This can lead to unauthorized access to host memory, modification of host state, or the execution of arbitrary system commands, depending on the privileges of the Ruby process. All deployments running untrusted mruby scripts using Kobako versions 0.9.0 and earlier are affected.

Recommendation

• Upgrade to Kobako version 0.9.1 immediately to implement the required method invocation filtering.
• If immediate patching is not feasible, unbind all host "Service" objects from sandboxes processing untrusted scripts until the upgrade is performed.
• Audit existing mruby script execution environments for signs of unusual method invocation patterns if the host process is exposed to untrusted input.