Flexible Sandboxed Evaluation

JSON is a data exchange format, and a the same time is valid javascript. You don’t need a parser to read them, and you can use the almighty eval to interpret the data. This is however a security breach. Malicious JSON data could be provide and would be evaluted without further notice. Parsing is therefore still necessary to validate the data. There’s nothing really new there: reflection and security are known to be orthogonal.

It doesn’t need necessary to be the case, though. Reflection could be limited so as to allow what is necessary and deny malicious actions. In the case of JSON evaluation, one should prevent the access to global data, and the creation of new functions. That is possible, e.g. with a custom first-class interpreter. The data might still not be valid JSON data from point of view of the data format, and might contain expressions. More advanced “security” policies could specify execution protocols that would be forbidden, so as to prevent the executions of certain expression, or sequences of expressions.

But then comes the question of error handling. Isn’t it “too late” to catch errors at run-time? An error–even if cought in time–might leave the system in a invalid transient state from which it is hard to recover. Validating the data statically prior any execution seems therefore an easier approach. This is for instance the approach taking by the Java platform: mobile code is verified when loaded.

Checking for potential errors statically is however more restrictive than checking for errors at run-time. Similarly to type systems, valid programs or data might be rejected because of the conservative nature of static checking.

Going back to a dynamic perspective on security, we need a way to (1) catch errors at run-time and (2) recover from errors at run-time. To provide the same level of security as its static counterpart, such an approach should make sure that nothing can happen at run-time that can not be undone. Recovery blocks, transactional memory, and first-class worlds are all techniques to enable undo easily.

What can be recovered must match what is allowed. Undoing changes to memory will not be enough if one can read from I/O streams ; either I/O operations should be denied, or the recovery mechanism should be strong enough to safely undo I/O read and write if needed. Spawning threads might be allowed, if we are able to detect and escape from infinite loop and reliably cancel execution of untrusted code.

Let’s take for instance that case of Java Enterprise Edition (JEE). Applications deployed on JEE application server must obey such specific restrictions. Most constraints arise from the transactional nature of EJBs, and the fact that clustring should be transparent. JEE applications run in a managed environement and should not spawn threads, access static data, performing blocking I/O, etc. they should be side-effect free. The sandboxing facilities of the JVM do not suffice to enforce these constraints, and many of them are conventions. The run-time itself does not prevent messing with the infrastructure.

We need a configurable runtime with adequate programming language abstractions to define and enforce security policies at various level of granularities. Such a runtime should be flexible enough to enforce the safe evaluation of JSON data, and the safe execution of JEE applications.

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