why we should not use eval in javascriptwhy we should not use eval in javascript

For example, I might set my username to: In general, #1 is true for quite a lot, if not most function calls. javascript eval() and security - Stack Overflow Just updating the above code.. --hi there!-- needs to be in quotes as it is a string. You are simply making an array from your function 8 arguments, removing the ones that are less than or equal to zero. If you allow (third-party) code to enter you page, it can order things on behalf of your customer, or change their gravatar, or whatever they could do through your site. Would a passenger on an airliner in an emergency be forced to evacuate? But with that caveat it has a clear place in interop situations and is far from "evil". I'm writing some JavaScript code to parse user-entered functions (for spreadsheet-like functionality). What is the best way to visualise such data? Heres some of the reasons to avoid using it: Malicious code: invoking eval can crash a computer. https://jsfiddle.net/CynderRnAsh/qux1osnw/. Everything loaded into the browser is subject to manipulation and should be treated as such. If you're interested, here's my use-case of eval, which I'd love to change: This does not seem to be the case in Chrome v108, which runs the V8 JS engine, same as Node. Plot multiple lines along with converging dotted line. I can't imagine any good reason you'd want to do that though. Then get the function's .toString(). From a pragmatic standpoint, there's no benefit to using an eval() in a situation where things can be done otherwise. Of course I didn't mention that at the time because I didn't want to influence the answers! Still, it does remind me of an example I had in mind when I submitted the question, which was to do with allowing a user to specify a mathematical formula to be evaluated, similar to spreadsheet functionality. How do I distinguish between chords going 'up' and chords going 'down' when writing a harmony? benefit because the code evaluated in new Function() will be running in a local function I'd like to take a moment to address the premise of your question - that eval() is " evil ". The word " evil ", as used by programming language peo Reasons to Avoid Using eval() Malicious code: invoking eval can crash a computer. In some languages such as java, the ability to introspect reduces or eliminates the need to use java's eval. that lead to vulnerabilities and many other things. There are mainly 4 reasons why eval () method should be avoided: 1. There is a small potential "Every major browser now has a built in console why would they bother to use an eval statement?" Initial code and most proposed solutions doesn't return result by traditional way. When you know what the danger is, and when you're taking the appropriate precautions. The advice, "The hyperscript is generated as a string first ()" Makes more sense to do all code generation in the build phase, write the resulting hyperscript code to a separate executable (.js) file, then deploy that file to test and production. This code should be made to use the arguments array that every Javascript function has access to. WebWhy Should We Not Use the eval() in JavaScript ? JS for all it's flaws is really flexible, and allows a lot of room to make it flexible. Server-side JavaScript could have that problem. That opened my eyes a lot! It's the suggested alternative here: What are the Alternatives to eval in JavaScript? Once your code grows beyond 'hello world' proportions, it quickly becomes impossible to prove you are not leaking user input into, https://github.com/getify/You-Dont-Know-JS/blob/master/scope%20%26%20closures/ch2.md#eval. You can see how the code generation was achieved if you're curious here. How Did Old Testament Prophets "Earn Their Bread"? How Did Old Testament Prophets "Earn Their Bread". Why did Kirk decide to maroon Khan and his people instead of turning them over to Starfleet? This might work with multimethods, for example, once the the particular arguments in use are determined. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. http://www.nczonline.net/blog/2013/06/25/eval-isnt-evil-just-misunderstood/. While it's certainly ideal to have pre-made scripts included in your page that your WASM code can invoke directly, sometimes it's not practicable and you need to pass in dynamic Javascript from a Webassembly language like C# to really accomplish what you need to do. Lateral loading strength of a bicycle wheel. Connect and share knowledge within a single location that is structured and easy to search. Its also important to remember that passing strings to setInterval(), setTimeout(), Is there ever a good reason to use eval()? - Stack Overflow But if the Engine finds an eval(..) in the code, it essentially has to assume that all its awareness of identifier location may be invalid, because it cannot know at lexing time exactly what code you may pass to eval(..) to modify the lexical scope, or the contents of the object you may pass to with to create a new lexical scope to be consulted. Any recommendation? What are some examples of open sets that are NOT neighborhoods? So to do this I figured I'm gonna basically need a formula. What's it called when a word that starts with a vowel takes the 'n' from 'an' (the indefinite article) and puts it on the word? What are the Alternatives to eval in JavaScript? - Stack Why should you avoid the JavaScript eval() function? I like compact generator way more than endless list of functions upon functions. When the code in The eval function is a powerful and easy way to dynamically generate code, so what are the caveats? @Sean McMillan: I want to believe you, but if someone is going to intercept and change javascript going to. I was looking for this answer and came to this post. Well, use HTTPS, then. How do I make the first letter of a string uppercase in JavaScript? How can I specify different theory levels for different atoms in Gaussian? What's the main benefit of using eval() in JavaScript? Are there good reasons to minimize the number of keywords in a language? Lottery Analysis (Python Crash Course, exercise 9-15). In any case, javascript itself is not for real security (server-side is obviously the main defense). Improper use of eval opens up your As my display name, which can come from a database and which is not hardcoded. Otherwise XSS wouldn't be a security vulnerability. Looking for advice repairing granite stair tiles, Name of a movie where a guy is committed to a hospital because he sees patterns in everything and has to make gestures so that the world doesn't end. If your server-side security is solid enough for anyone to attack from anywhere, you should not worry about EVAL. So one of these complicated workaround aren't hacks, but this convenient, one line function is the hack? The following code is equivalent, and it will work for any arbitrary number of arguments: One alternative to to pass an array to your function, instead of individual arguments: If you really need to pass in separate arguments, you can access them using your arguments array, which is an object that acts like an array (though it's not really; not all Array methods work on it) that exposes all arguments that have been passed in to your function; they do not even need to be declared in this case, but it's good form to include a comment indicating what sorts of arguments you are expecting for users of your code: If you're guaranteed to only have 8 elements, then you could use 8 in place of inArray.length or arguments.length; I decided to use the more general version in my examples in case that was helpful to you. Plus you might forget to save creating confusion when testing. rev2023.7.5.43524. Simple as that. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Can be errors in the code and break up things in the service, if not fully tested. In truth, I agree with him too. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Raw green onions are spicy, but heated green onions are sweet. The only instance when you should be using eval() is when you need to run dynamic JS on the fly. And if not, why do so many languages implement it? BTW: Prototype.js calls eval directly five times (including in evalJSON() and evalResponse()). If you want to know more: Either you write a compiler (which saves rather than executes the code being generated) or you write an interpreter (where each instruction has a pre-compiled implementation). that came from Ajax call. If it's really needed eval is not evil. Most browsers support it, and if you're really in a pinch, you could write a parser for JSON pretty easily. XSS has been in the top 3 vulns on OWASP's top 10 list for several years running. What should be chosen as country of visit if I take travel insurance for Asian Countries. @T.J.Crowder Ahhhh now I see. (no line numbers, etc.) eval'd code executes slo That includes people writing JavaScript tutorials, unfortunately, and in some cases this can indeed have security consequences - or, more often, simple bugs. Just build a basic scaffolding webpage with textareas and an eval button. Obviously you should always sanitize the input, as to have some control over what your code consumes. eval('al' + 'er' + 't(' + '"hi there!"' Mainly, it's a lot harder to maintain and debug. Why is this? It's also a dull, rusty substitute for hygienic macros. We have browser consoles, script extensions, etc Every single piece of code sent to the client is optional for the client to execute. Rust smart contracts? Turns out that since objects in Javascript are fully dynamic, a property access in Javascript is comparable to introspection in other languages, where you can access and refer to names created on the fly. Why is using the JavaScript eval function a bad idea? rev2023.7.5.43524. If I trick you into clicking a link that goes to that site with my query string attached, I've now executed my code on your machine with full permission from the browser. How could the Intel 4004 address 640 bytes if it was only 4-bit? In some cases, the performance impact can even affect other code outside eval. Eval is not needed even for JSON parsing. And by generate I mean include from an external source (a file, a website, an 'agent') as well as create on the fly inside the program. I'd like to take a moment to address the premise of your question - that eval() is "evil". @AkashKava, I don't understand your question. As for performance, you'll have to weight that against ease of coding. However, for all the reasons mentioned above, you shouldn't use it for your own code, where you have complete control - it's just not necessary, and better-off relegated to the 'tricky JavaScript hacks' shelf. Is the executive branch obligated to enforce the Supreme Court's decision on affirmative action? This function takes an arbitrary string and executes it as JavaScript code. On to your specific case. So if you're looking to run some JavaScript code where it might not otherwise be allowed (Myspace, I'm looking at you) then eval() can be a useful trick. Arguably, it's not dangerous if the input comes from the. Even ways that appear to require it do not. A question about eval() in javascript why is it evil and how can I accomplish the same thing without using it? +1 for the idea, but you have a bug here: Declaring one variable for each property of. For me the evil is not a performance or even a security issue (well, indirectly it's both). And the reason you would want to generate code, aside from the obvious examples of external modules and evaluation sites, is usually to dynamically reference the names of objects and properties in code. Not too bad if your users are only on your page shortly, but it can be a problem for webapp's. @AkashKava What you're failing to realize, is that if I submit javascript in my comment box, and that javascript makes it to the database. Eval can be used to substitute missing metaprogramming features, like templates. It's generally only an issue if you're passing eval user input. Done and no way to stop me because when eval executes, the browser gives it highest authority. My example should be a good reason of when not to use eval. eval isn't necessary for setTimeout. where? How can we compare expressive power between two Turing-complete languages? If your code is compiled, then this is a big hit, because you need to call a possibly-heavy compiler in the middle of run-time. Are there any potential issues that may occur with the way I'm using eval()? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. The word "evil", as used by programming language people, usually means "dangerous", or more precisely "able to cause lots of harm with a simple-looking command". That second parameter in JSON.stringify lets you put a callback to run that you can check via typeof if it's a function. first of all EVAL is not to be blamed for poorly written server side code. Not the answer you're looking for? However, it's not unheard of for Chrome to get strange performance boosts in certain areas of the runtime from version to version. If it takes 0.2 seconds to compile 2000 lines of JavaScript, what is my performance degradation if I eval four lines of JSON? Having parsed the formula I could convert it into JavaScript and run eval() on it to yield the result. Can you tell me a use case where you can't do these instead of eval? If the data is comming from your server and its something that you, the developer has generated, there is no harm in using eval(). You can see how the code generation was achieved if you're curious here. True, I didn't know about how eval() worked in perl. And this is where you need to get really real. Is the difference between additive groups and multiplicative groups just a matter of notation? This may become more of an issue as the next generation of browsers come out with some flavor of a JavaScript compiler. It does this by parsing a handlebars template and converting it to hyperscript which is subsequently used by virtual-dom. Also note that eval() is much slower than other specialized JSON etc. This eliminates 99% of cases where eval is used, across the board in all languages and contexts. Non-anarchists often say the existence of prisons deters violent crime. evaluators. For example, you could also say there's always ways to avoid object oriented programming. code for injection attacks Debugging can be more challenging When doing alot of testing and tweaking on the code the minor extra steps can really add up. That is: dynamical (or more accurately) programmically-created object names (as opposed to values). international train travel in Europe for European citizens. But you shouldnt be afraid to use it when you have a case where Once again, you have failed to understand concept of poor server side security. When you parse a JSON structure with a parse function (for example, jQuery.parseJSON), it expects a perfect structure of the JSON file (each property name is in double quotes). @JeffWalden, great comment. Why is it better to control a vertical/horizontal than diagonal? But also be aware that JavaScript uses JIT compilation and this works very poorly with eval. Also, if you're running in the browser then code injection is a pretty minor risk, I believe. What can I use instead of eval in JavaScript? ITExpertly.com What are the security issues with "eval ()" in JavaScript? If your webserver isn't authenticated via HTTPS, then you could suffer some sort of man-in-the-middle attack where another host intercepts the request and sends its own data. But security-wise, why not just parse? To give an all-too-typical example, to set the colour of an element with an id stored in the variable 'potato': If the authors of the kind of code above had a clue about the basics of how JavaScript objects work, they'd have realised that square brackets can be used instead of literal dot-names, obviating the need for eval: which is much easier to read as well as less potentially buggy. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Rust smart contracts? But saying that you can control anything in javascript is like saying you own gravity, @T.J.Crowder In those cases though, there is no reason to use. What actually happens is a new execution context is created. Exactly why you, "eval is not evil if running on the client, even if using unsanitized input crafted by the client" This is not true. That doesn't mean it's not a great option. How to maximize the monthly 1:1 meeting with my boss? How can I validate an email address in JavaScript? Eval: run a code string - The Modern JavaScript Tutorial Every major browser now has a built-in console which your would-be hacker can use with abundance to invoke any function with any value - why would they bother to use an eval statement - even if they could? I can't change the database layout since everything hooks into those 8 variables and is a massive undertaking. A little old thread here, but from what I've read-- not claiming I traced it back myself-- JSON.parse in fact eval's it's input in the final stage. See. It really doesn't add anything to this question; in particular, it doesn't help anyone coming here determine whether or not their particular usage is problematic or not. Behind the scenes, JavaScript still has to evaluate and execute See here for some more info: eval. I recently wrote a library called Hyperbars which bridges the gap between virtual-dom and handlebars. A more likely scenario is that you are eval'ing scripts that have undergone slight modification each time and as such could not be cached. Component Development for the Java Platform. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, apparently new fcuntion(a) is 67% slower than eval(a) on chrome, for me new functions(a) is 80% slower latest chrome on osx. However, I've always shied away from using eval() if I can avoid it because it's evil (and, rightly or wrongly, I've always thought it is even more evil in JavaScript, because the code to be evaluated might be changed by the user). BUT: when eval() is used inside a function that causes a closure, ALL the variables of outer functions are bound to the closure, even if they are not used at all. Program where I earned my Master's is changing its name in 2023-2024. You see lots of people saying eval() is evil and they have no idea why except that they read it somewhere. 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). Regarding security, using eval or not will hardly make any difference. I think eval executes at runtime, or, at the least it is quirky. Only during testing, if possible. Why security is an issue if client anyway could do with our code anything he/she wants ? Untrusted client-side JavaScript code can't do that much anyway. If someone has the time to test if memory leaks can be produced by that, please leave me a comment below. eval isn't always evil. How Did Old Testament Prophets "Earn Their Bread"? Is there a non-combative term for the word "enemy"? 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned. eval is rarely the right choice. I recently wrote a library called Hyperbars which bridges the gap between virtual-dom and handlebars. As long as the JSON itself contains no data a user has uploaded, there is no major drawback to use eval. Lottery Analysis (Python Crash Course, exercise 9-15), For a manual evaluation of a definite integral. With eval (), malicious code can run inside your application without permission. whenever user-supplied data is used to generate the script. Therefore, I submit to you that there cannot be one correct answer to this question: There is a very good reason to use eval if you are programming in Perl, and All such unnecessary uses of eval add to a maintenance hell. What should be chosen as country of visit if I take travel insurance for Asian Countries. Find centralized, trusted content and collaborate around the technologies you use most. At that point, it becomes unsafe because it's a full JavaScript parser but people When and how is javascript eval() vulnerable to injection? "If your server-side security is solid enough for anyone to attack from anywhere, you should not worry about EVAL." Let's say you're using eval on a query string. jQuery has utilized the native JSON.parse() since 1.4 (way back in 1/2010)! If it doesn't use double quotes around property names, it might be a string representation of an object literal, but it is not, See article by Nikolas Zakas - "eval() isnt evil, just misunderstood", @vitmalina From Zakas' article: "This can be dangerous if youre taking user input and running it through eval(). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Generate random string/characters in JavaScript, Accessing an object property with a dynamically-computed name, Why is my variable unaltered after I modify it inside of a function? WebDec 2, 2009 at 14:59 4 I believe it was from an article comparing python and lisp than I learned about eval. For example: if However, I've always shied away from using eval () if I can avoid it because it's evil (and, rightly or wrongly, I've always thought it is even more evil in JavaScript, From what I understand, you're generating the strings yourself, so assuming you're careful not to allow a string like "rm -rf something-important" to be generated, there's no code injection risk (but please remember, it's very very hard to ensure this in the general case). Have the user enter that separately and then you can use if/then logic to ultimately use the correct operator. Asking for help, clarification, or responding to other answers. Unfortunately it is rejected by CSP (default-src 'self') just like eval in Firefox. (about javascript and performance, certainly Steve Yegge's presentation). What's the main benefit of using eval() in JavaScript? It is my opinion that if you're parsing the formula, you might as well compute the result during the parse rather than run another parser (the one inside eval()). Where custom validation at runtime can be made without re-deploying your services. Some of these boil down to being able to essentially statically analyze the code as it lexes, and pre-determine where all the variable and function declarations are, so that it takes less effort to resolve identifiers during execution. eval() isn't evil. There's probably other ways to code this, and probably ways to optimize it, but this is done longhand and without any bells and whistles for clarity sake to illustrate a use of eval that really doesn't have any other alternatives. All you have to do is make a HTTP request to the server with your favourite user agent string. What is the best way to visualise such data? The statement "eval is evil" is a reasonably good guideline. 586), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Temporary policy: Generative AI (e.g., ChatGPT) is banned. jQuery uses it in parseJSON (via Function constructor). As I mentioned, if EVAL would not exist, attackers have many tools to hack into your server irrespective of your browser's EVAL capability. Find centralized, trusted content and collaborate around the technologies you use most. The evaled code is executed in the current lexical environment, so it can see outer variables: let a = 1; function f() { let a What I like to point out here is, that eval() must not necessarily refer to the native eval() function. @Justin - if the protocol is compromised, well, typically the initial page load would have been sent over that same protocol, and then it's a moot point because the client is already as compromised as it can possibly be. rev2023.7.5.43524. who serves wrong string? No javascript engine cannot find and eval in the code with 100% guarantee. I'm not au fait with all the ins and outs of Javascript. If you can trust the server 100% from which the data arrives at the client, it's not a real problem at all (talking about security issues with eval). Thanks for contributing an answer to Stack Overflow! I'm talking about JS that you download asynchronously from the server And 9 times of 10 you could easily avoid doing that by refactoring. Let's get real folks: Every major browser now has a built-in console which your would-be hacker can use with abundance to invoke any function with WebDo NOT use eval () Executing JavaScript from a string is an BIG security risk. I could do the same as I am doing here on the server but that just shifts the burden higher up the chain. Is there an easier way to generate a multiplication table? The JavaScript Engine has a number of performance optimizations that it performs during the compilation phase. The performance of eval() isn't an issue in a situation like this because you only need to interpret the generated string once and then reuse the executable output many times over. For anything non-trivial, implement a sublanguage.