When should I use eval()?

October 07, 2013


That’s got that off my chest.


is possibly the most dangerous thing ever. It’s basically a way to execute arbitrary code from a string or variable.

Here’s a few reasons why it’s dangerous.

It leaves you open to injection attacks.

In Javascript, eval() forces the engine to drop into Interpreter mode, which slows down your application, and it will remain slow, as there’s no opportunity for optimisation-level caching to take place.

It’s a bugger to debug, because there’s no line numbers.

In Javascript (client-side), eval() is dangerous because it exposes you to cross site scripting attacks.

In server-side code, eval() is downright lethal, because it exposes the entire server to anything that the user wants to run.

Python has a “safer” eval, called literal_eval in the ast module, which allows for parsing of user-provided data, without having to write a parser to sanitise it yourself. I’d still avoid it like the plague, given a choice.

This is all fairly fresh in my mind, because I discovered a snippet of code somewhere (not disclosing where, as I’m doing the responsible thing and doing the disclosure properly), that was along the lines of

var jsonData = eval ("(" + string + ")");

Apparently JSON.parse() isn’t good enough for them.


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Written by Tom O'Connor, an AWS Technical Specialist, with background in DevOps and scalability. You should follow them on Twitter