For scripting systems open to untrusted user-land scripts, it is always best to limit the amount of resources used by a script so that it does not consume more resources that it is allowed to.
The most important resources to watch out for are:
CPU: A malicious script may run an infinite tight loop that consumes all CPU cycles.
Time: A malicious script may run indefinitely, thereby blocking the calling system which is waiting for a result.
Stack: A malicious script may attempt an infinite recursive call that exhausts the call stack.
Alternatively, it may create a degenerated deep expression with so many levels that the parser exhausts the call stack when parsing the expression; or even deeply-nested statement blocks, if nested deep enough.
Another way to cause a stack overflow is to load a self-referencing module.
Overflows: A malicious script may deliberately cause numeric over-flows and/or under-flows, divide by zero, and/or create bad floating-point representations, in order to crash the system.
Files: A malicious script may continuously
importan external module within an infinite loop, thereby putting heavy load on the file-system (or even the network if the file is not local).
Even when modules are not created from files, they still typically consume a lot of resources to load.
Data: A malicious script may attempt to read from and/or write to data that it does not own. If this happens, it is a severe security breach and may put the entire system at risk.
Rhai is designed to not bring down the host system, regardless of what a script may do to it. This is a central design goal – Rhai provides a Don’t Panic guarantee.
When using Rhai, any panic outside of API’s with explicitly documented panic conditions is considered a bug in Rhai and should be reported as such.
All the above safe-guards can be turned off via the
unchecked feature, which disables all safety
checks (even fatal ones such as stack overflow, arithmetic overflow and division-by-zero).
This increases script evaluation performance somewhat, but at the expense of breaking the no-panic guarantee.
unchecked, it is very possible for a malicious script to panic and bring down the host system.