Javascript safe eval. js API, but can be given access using a conext object. Is my use of unsafe-eval attribute incorrect or is it impossible to allow unsafe-eval only for subset of 'self'? The intent is to allow unsafe-eval only for same origin path prefix /:static/math, strict CSP JS code execution for ' self ' and no JS code for any other method. js in html script with requirejs Those aren't sandboxes, just new environments. Introduced in the early days of the language, it allows you to execute arbitrary code stored as a string. A project fork is not without risks, and this time it's the safe-eval-2 npm package that is vulnerable to code injection attacks. js - Safe eval runtime The following tests demonstrate the capabilities of SandboxJS. mozilla. json, npm install or include lib/expr. Apr 19, 2014 · 5 As long as you control the input into eval, it's safe to use it. I wonder if it is possible to run code provided by users in a webpage in a safe way. I've written an application which can send JavaScript snippets from the server and execute them on the client. The parent window can be easily accessed via parent or top and you can use the eval method of the object. safe-eval has access to all the standard JavaScript API of the underlying engine. safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval(). In Javascript, eval () is one of the most interesting yet most dangerous functions in the language. js and Socket. Eval () takes a string and attempts to run it as Javascript code. Sandbox. To reduce all security concerns to a minimum safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval(). If you run eval() o 文章浏览阅读776次，点赞20次，收藏21次。探索安全的JavaScript执行环境：wasm-jseval在现代Web开发中，JavaScript的动态执行能力是一项强大的功能，但同时也带来了安全风险。为了在保证安全性的前提下，仍然能够灵活地执行JavaScript代码，wasm-jseval项目应运而生。本文将深入介绍这一开源项目，分析其 Mozilla docs on CSP, lists " eval() and similar methods" as possible offenders: […] 'unsafe-eval' Allows the use of eval() and similar methods for creating code from strings. For this we need to use eval to evaluate the code. Be careful about the objects you are passing to the context API, because they can completely defeat the purpose of safe-eval. Would filtering the string before going into eval be safe? 6 There is a safe and sandboxed way to execute JavaScript code without using eval, is to have a JavaScript interpreter written in JavaScript itself. An HTML string with the JavaScript code string embedded in a script tag of type module is sent to an ephemeral iframe through the src attribute. safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval (). The eval () function in JavaScript is a built-in function that evaluates JavaScript code represented as a string. There is an implementation by Neil Fraser at Google called JS-Interpreter, I am using it and it is a good and simple solution, this is the repo in Github. Code injection - eval () potentially runs a string of code under elevated privileges. To support the dev community, I’m excited to share the steps I followed to accomplish this. js, I figured that since JavaScript has math built in, this might be a good use for eval(). From security vulnerabilities to performance bottlenecks Jun 27, 2025 · JavaScript’s eval function is a powerful tool that allows developers to execute arbitrary strings as code at runtime. There is 1 other project in the npm registry using safe-evaluate-expression. It can execute any JavaScript code passed as a string, making it a prime target for security vulnerabilities like code injection and The 'wasm-unsafe-eval' source expression is more specific than 'unsafe-eval' which permits both compilation (and instantiation) of WebAssembly and, for example, the use of the eval operation in JavaScript. This library provides much safer function! When I wrote the JavaScript Function Plotter, I wanted a better alternative to using JavaScript’s eval function. 好在 safer-eval 这个 npm 包提供了一种更加安全的方式来执行 JavaScript 代码。 本文将介绍 safer-eval 的使用方法以及一些示例代码。 safer-eval 介绍 safer-eval 是一个支持限制执行环境的 JavaScript 执行器。 Because most people don't bother to know what it actually does. The concern comes in when you're using it to process input that you don't control. I'm using Node. Eval is evil. Discord Bot Studio is a visual programming tool, so I felt it was important to offer a familiar variable syntax. Don't use eval needlessly! eval () is a dangerous function, which executes the code it's passed with the privileges of the caller. Nothing else will be present on the web page except for this calculator. eval being called on something else, you should use context. So, can I just ban eval() from submissions and be done with it? Or are there other ways to evaluate code or cause mass panic in javascript? There are other things to disallow, but my main concern is that unless I can prevent strings being executed, whatever other filters I put in for specific methods can be circumvented. Jan 21, 2026 · The eval() function evaluates JavaScript code represented as a string and returns its completion value. Is it safe to use "eval" in this scenario? Or put another way, does the use of eval in this case cause additional security risk? To me, it doesn't seem like the user can do anything nefarious with this page they can't do by simply opening the browsers development tools. eval_vm - Evaluate JavaScript-like expressions safely Probably you know, that regular eval() is not safe, because things like Content-Security-Policy: script-src can break it. Discover best practices for handling JSON data and constructing dynamic strings in your JavaScript applications. It is implemented using node's vm module. (I do see issues with eval ()'ing code on the server, but client-side eval () seems to be safe. This Safer version of eval() for nodejs and browser. Do NOT use eval () Executing JavaScript from a string is an BIG security risk. safeEval (code, [context], [options]) code is the JavaScript code you want to execute. If the 'unsafe-eval' source keyword is used, then this overrides any occurrence of 'wasm-unsafe-eval' in the CSP policy. With eval (), malicious code can run inside your application without permission. With eval (), third-party code can see the scope of your application, which can lead to possible attacks. In JavaScript, code evaluation functions like eval() and new Function() might seem useful for executing dynamic code, but they pose significant security risks and performance concerns. CSP safe usage of 'unsafe-inline' 'unsafe-eval' Asked 5 years, 6 months ago Modified 5 years, 6 months ago Viewed 8k times. The source is parsed as a script. 在前端开发中，有时需要动态执行一些 JavaScript 代码，例如解析模板字符串，进行动态计算等等。然而，动态执行 JavaScript 代码存在一定的安全风险，可能会导致应用程序被攻击。为了避免这种情况，我们可以使用 npm 包 safe-eval 来安全地执行 JavaScript 代码。 Your attempt at using a "more relevant regex" is doomed to fail. safe-eval What is this? safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval (). SandboxJS - Safe eval runtime This is a javascript sandboxing library. What is this? safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval(). If you use eval() with user input, you’re essentially letting anyone execute arbitrary JavaScript in your application. The iframe's sandbox attribute is set to allow-scripts and the style attribute is set to display:none. Hope it helps. 44 Because eval is literally unsafe. ? An alternative to eval () supporting javascript syntax for expressiveness and safe execution Just add exprjs to your package. I ended up having to use the eval () function which worked fine for a basic calculator app but on further research I found out the eval () function is a security risk in most casese, I've used a regex expression to cleanse the result but don't know if this is safe enough. Regarding the function, no your code does not do that. eval = function(str) { return eval(str) }. bind(context); return context;. To me this is 100% safe since nothing will get executed unless it is simply the name of one of my known components, or is there something about the eval() command that could be exploited in this code sample, e. They already have access to everything. JavaScript's eval() function allows developers to evaluate or execute a string of JavaScript code dynamically. safe-eval has access to all the standard APIs of the V8 JavaScript Engine. Instead of building a function to handle the math, or using a library like math. There’s no security risk currently, because you can only run code in your own browser, but it’s not as convenient for math (Math. JSON was based on a subset of the JavaScript scripting language (specifically, Standard ECMA -262 3rd Edition—December 1999 [12]) and is commonly used with JavaScript, but it is a language-independent data format. A real JavaScript sandboxing library in JavaScript is JSandbox. regex injection, some kind of cross site scripting etc. https://developer. The eval() function in JavaScript is powerful but dangerous. org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval Any malicious user can turn on chrome debugger for example, and modify javascript code that is being executed. 5, last published: 2 months ago. Contribute to commenthol/safer-eval development by creating an account on GitHub. Contribute to thonymg/safe-eval-all development by creating an account on GitHub. When I am trying to use safe-eval library rather than eval it to only execute safe scripts that are served as files from your own origin - the same source the app is hosted. I would like to add code that users can dynamically change to change some of the page behaviour, but I don´t wan I am trying to create a function in javascript having the same behavior as eval as it is not that much secure as safe-eval npm package. So, what are the issues, if any, associated with eval ()? I haven't been able to come up with anything that could be exploited with eval () in JavaScript. - Some frustrated JavaScript engineer I recently read this quote on the internet and Tagged with javascript, node, security. Start using safe-evaluate-expression in your project by running `npm i safe-evaluate-expression`. It is Eval'ing a user's input is no worse than them viewing source, looking at HTTP headers, using Firebug to inspect JavaScript objects, etc. Once the string is executed, it can perform operations, declare variables, and even define functions, just as if the code were written directly in the script. " Sure, you may be using eval in a semi-safe way, but as long as you allow it at all, you are saying "anyone is allowed to execute arbitrary code in my application given an entry point". parse(), and template literals. Code for parsing and generating JSON data is readily available in many programming languages. Small library to dynamically create and evaluate expression with multiple parameters (even undefined). By default, it does not have access to the Node. ). However, JavaScript is still mostly an interpreted language, which means that calling eval () is not a big performance hit in the general case (but see my specific remarks below). After facing significant challenges while trying to eliminate unsafe-eval from an existing application's Content Security Policy (CSP), I finally developed a robust and generic solution that other developers can easily adopt and extend. If you want to be safe against context. eval() can be safe if used properly. 7. Latest version: 1. io. ) We want to give our users the ability to execute self created JavaScript code within our application. Eval in every language means "take this string and execute it code. Understand how to execute dynamic code in JavaScript and how to prevent the vulnerabilities of the eval function. pow (2^x) instead of 2^x, etc. The JavaScript is sent via Secure WebSocket (WSS), an Safe eval () alternative in Javascript # javascript # webdev # beginners A problem I encountered while creating Discord Bot Studio, was allowing users to enter variables which could be evaluated at runtime. 文章浏览阅读787次，点赞3次，收藏4次。 安全的JavaScript执行利器：Safe-Eval在软件开发中，我们有时需要动态地执行一些JavaScript代码，但直接使用eval ()函数可能会带来安全风险，因为它允许执行任意代码。 safe-eval lets you execute JavaScript code without having to use the much discouraged and feared upon eval (). You cannot do that generically. eval() is as dangerous as adding a SCRIPT element with an embedded JS code into the document. This article explores alternatives to the JavaScript eval() method, highlighting the Function constructor, JSON. Learn how to execute dynamic code safely and efficiently while avoiding the security risks associated with eval(). context is an object of methods and properties, these methods and properties are interpreted as global objects in code. It executes a string of code as JavaScript, enabling dynamic functionality like user-defined calculators, code playgrounds, or custom data processors. js API, but can be given access using a context object. This capability can be incredibly useful in certain scenarios, such as I'm really curious about eval(). g. For securing code, sandboxing is needed. However, unrestricted use of `eval()` exposes applications to severe security threats: attackers can inject malicious code to steal data, manipulate the DOM, or execute Nov 8, 2025 · JavaScript’s `eval()` function is a powerful but notoriously risky tool. Learn about potential vulnerabilities and exploitation techniques of JavaScript's eval() method, along with best practices to mitigate risks. If it's used without care, it will be dangerous. How to Handle eval() Industry best practices and official guidelines strongly discourage the use of eval () in JavaScript due to its significant security risks and negative impact on performance. Is using javascript eval () safe for simple calculations in inputs? Asked 13 years, 1 month ago Modified 13 years, 1 month ago Viewed 1k times The eval function is a powerful and easy way to dynamically generate code, so what are the caveats? I have runned the safety scanner for my application and it says in general: "eval() is a dangerous function that executes code with all the privileges of the caller sides. The results are compared with eval that has a basic scope proxy prison implemented. When embedding any kind of js code inside your app (either web or nodejs based) you are essentially giving access to the entire kingdom, hoping there is no malicious code in a dependency such as with supply chain attacks. At first glance, this might seem convenient—“I can dynamically generate code and run it!”—but this flexibility comes with severe trade-offs. Using eval() is like giving a stranger the keys to your house. a safer eval. While it may seem convenient for some situations, using eval() can lead to serious issues, including security vulnerabilities, performance degradation, and unpredictable behavior that could crash your application. options is the options object for the vm executing the code Jan 2, 2026 · JavaScript’s `eval()` function is a powerful but notoriously risky tool. Now, if you allow unsafe-eval, then that eval becomes a point of entry for the attacker, and once they manage to inject their javascript into your app, they have total control and all other benefits you'd get from CSP are voided. 4lxt, fqvdv, mfwa, 6kh1q, buuy, 35ao, wnbne, wo6yw, 8kyg, wc5y,