# Safety and Protection Against DoS Attacks

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:

• Memory: A malicious script may continuously grow a string, an array or object map until all memory is consumed.

It may also create a large array or object map literal that exhausts all memory during parsing.

• 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 import an 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.

## unchecked

All these safe-guards can be turned off via the unchecked feature, which disables all safety checks (even fatal errors such as arithmetic overflows and division-by-zero).

This will increase script evaluation performance, at the expense of having an erroneous script able to panic the entire system.