So one of these complicated workaround aren't hacks, but this convenient, one line function is the hack? Just gotta figure what's the point of you using the eval. Understanding eval and knowing when it might be useful is way more important. When the code in question is known beforehand (not determined at runtime), theres no reason to Again, JavaScript in a browser doesn't have that problem, because the program is running in the user's own account anyway. If person A crafts a script that gets. Or, if it is, it's evil in the same way that reflection, file/network I/O, threading, and IPC are "evil" in other languages. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Why should you avoid JavaScripts eval function when possible? Most browsers support it, and if you're really in a pinch, you could write a parser for JSON pretty easily. 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. Passing user input to eval() is a security risk, but also each invocation of eval() creates a new instance of the JavaScript interpreter. Prone to Injection Attack: Consider you have created a code that uses the value entered by the no need use eval for work with arguments at all. Put code into a textarea then press eval button. So the more we can do to throw a question mark over eval, the better. Using eval() on untrusted code can open a program up to several different injection attacks. example coming from the network) that has been tampered with. Javascript: How to use eval() safely - Stack Overflow Many of the URL params are created in the format {myParam}. Everyone says eval is EVIL (as in bad) and slow (as I have found), but I can't really do anything else - the server simply pulls the data out the database and pushes to the browser. Otherwise XSS wouldn't be a security vulnerability. Is Linux swap still needed with Ubuntu 22.04. Therefore, you can use eval() to avoid it. Why is using the JavaScript eval function a bad idea? I read the developer doc and this worked as a direct replacement in my application. All of this is said in all generality. In the final act, how to drop clues without causing players to feel "cheated" they didn't find them sooner? What are some examples of open sets that are NOT neighborhoods? This is a common antipattern when dealing with a JSON response from an Ajax request. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. When debugging in Chrome (v28.0.1500.72), I found that variables are not bound to closures if they are not used in a nested function that produces the closure. It's not that eval is evil (it's in Lisp, so it must be good) it's simply a sign of a hack - you need something to work and you forced it. Connect and share knowledge within a single location that is structured and easy to search. BTW: Prototype.js calls eval directly five times (including in evalJSON() and evalResponse()). Are throat strikes much more dangerous than other acts of violence (that are legal in say MMA/UFC)? Similarly, using eval() securely is possible, but it is so hard in practice that it is discouraged. Lateral loading strength of a bicycle wheel. Where custom validation at runtime can be made without re-deploying your services. It looks like eval() in this case is no more evil than any other function that could possibly save you some time. Its also important to remember that passing strings to setInterval(), setTimeout(), how to give credit for a picture I modified from a scientific article? As others have pointed out, if you want to quickly convert the arguments object into an array, you can do so with this expression: You could use that instead of the var args = [i1, i2 lines above. Eval is complementary to compilation which is used in templating the code. By templating I mean that you write a simplified template generator that What should be chosen as country of visit if I take travel insurance for Asian Countries, What does skinner mean in the context of Blade Runner 2049. I could do the same as I am doing here on the server but that just shifts the burden higher up the chain. How Did Old Testament Prophets "Earn Their Bread"? Mainly, it's a lot harder to maintain and debug. How to maximize the monthly 1:1 meeting with my boss? Thanks for contributing an answer to Stack Overflow! The use of Just for example - to parse returning value (JSON like, server defined strings, etc.) Do I have to spend any movement to do so? Eval: run a code string - The Modern JavaScript Tutorial In production code, consider it a last resortand even then, try something elsebecause eval is difficult to control and thus dangerous. debugability, and certainly performance that should not be overlooked. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. How could the Intel 4004 address 640 bytes if it was only 4-bit? There is no reason not to use eval() as long as you can be sure that the source of the code comes from you or the actual user. So the json string "{foo:alert('XSS')}" would not pass since alert('XSS') is not a proper value. It could be a powerful construct but is often misused. As it is, you are just wrong; #3 holds true for many scenarios. Only if your server-side security sucks. This function takes an arbitrary string and executes it as JavaScript code. javascript - Why is it important to never use `eval` in conjunction The concern comes in when you're using it to process input that you don't control. Why is eval() unsafe, wouldn't a user on a modern browser run whatever is evaluated on it anyway? eval isn't always evil. There are times where it's perfectly appropriate. However, eval is currently and historically massively over-used by people What alternatives do I have? @PaulBrewczynski, the security problem appears when user A saves his part of code to be, In the 3+ years since I answered this, my understanding of what happens has, let's say, deepened. For example, a program running as administrator/root would never want to eval() user input, because that input could potentially be "rm -rf /etc/important-file" or worse. Is the difference between additive groups and multiplicative groups just a matter of notation? eval () is a dangerous function, which executes the code it's passed with the privileges of the caller. eval() isn't evil. Or, if it is, it's evil in the same way that reflection, file/network I/O, threading, and IPC are "evil" in other languages. If If you were to ask if it's suitable to use eval() in PHP however, the answer is NO, unless you whitelist any values which may be passed to your eval statement. Unfortunately it is rejected by CSP (default-src 'self') just like eval in Firefox. Now they can make a bogus browser extension that can tape into that eval and steal data again. was there in Java's native code. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. In those cases that lead to vulnerabilities and many other things. You can't stop someone with tools like Firebug if they want to mess with stuff obviously but that is what server-side validation is about. nczonline.net/blog/2013/06/25/eval-isnt-evil-just-misunderstood. Congrats, you've opened pandora's box for user data to be sent to whoever. Unfortunately there are a lot of uncooperative people working on JavaScript and it's implementations in browsers so you'll be forced to use eval() in JavaScript, I've never had to use it in PHP. Is there an easier way to generate a multiplication table? Two points come to mind: Security (but as long as you generate the string to be evaluated yourself, this might be a non-issue) Performance: until t If there is user input that goes to the server, then comes back to the client, and that code is being used in eval without being sanitized. This assumes that there is a single user. let value = eval('let i = 0; ++i'); alert( value); // 1. Re "Code injection - Again, JavaScript in a browser doesn't have that problem," & " Also, if you're running in the browser then code injection is a pretty minor risk, I believe." So to do this I figured I'm gonna basically need a formula. WebViewed 7k times. Why a kite flying at 1000 feet in "figure-of-eight loops" serves to "multiply the pulling effect of the airflow" on the ship to which it is attached? globals automatically. For debugging/testing an idea before implementing it the proper way. As I mentioned, if EVAL would not exist, attackers have many tools to hack into your server irrespective of your browser's EVAL capability. If your server-side security is solid enough for anyone to attack from anywhere, you should not worry about EVAL. Security (but as long as you generate the string to be evaluated yourself, this might be a non-issue), Performance: until the code to be executed is unknown, it cannot be optimized. its better to use the browsers built-in methods to parse the JSON response to make It screams out to me "The author gave up on good programming design and just found something that worked". I won't attempt to refute anything said heretofore, but i will offer this use of eval() that (as far as I know) can't be done any other way. Asking for help, clarification, or responding to other answers. In fact, there are very few good use cases for running If it takes 0.2 seconds to compile 2000 lines of JavaScript, what is my performance degradation if I eval four lines of JSON? use a library from JSON.org. For browsers that dont support JSON.parse() natively, you can :-) The security issues are essentially a server-side problem because, now, with tool like Firebug, you can attack any JavaScript application. And if not, why do so many languages implement it? question is known beforehand (not determined at runtime), theres no reason to use I used it once while pentesting a site - we wrote a small php script that decrypts and executes cryptographically signed payloads from non-logged HTTP data sources on the fly. It's like a goto. There are some good articles on this if you search. : associative-array notation (obj["prop"] is the same as obj.prop), closures, object-oriented techniques, functional techniques - use them instead. This is one of good articles talking about eval and how it is not an evil: But I found one example where it should be used: For example, I have a function that constructs a general google.maps.ImageMapType object for me, but I need to tell it the recipe, how should it construct the tile URL from the zoom and coord parameters: Eval is useful for code generation when you don't have macros. For quick hacks, no problem because it's a handy quick-out. When creating/testing code segments eval is PERFECT! As long as you control the input into eval, it's safe to use it. Eval (and similar) functions are last-resorts in most languages that contain them, but sometimes it's necessary. If a hacker can write script on your system, then you are pretty much screwed. @AkashKava, A string can originate with one user-agent, be stored in a database, and then served to another browser which. In fact nothing on the machine that eval can give access to. For instance, you're making a toy calculator, and you want to work on the gui first, so you just use eval to do the "back-end" work in the background. When debugging in Chrome (v28.0.1500.72), I found that variables are not bound to closures if they are not used in a nested function that produces Your server is reading the swagger file that you have created. evaluators. That second parameter in JSON.stringify lets you put a callback to run that you can check via typeof if it's a function. No javascript engine cannot find and eval in the code with 100% guarantee. This is such general advice that it could be applied to literally any block of code that exists. It's just that, Re "There is no reason not to use eval() as long as you can be sure that the source of the code comes from you or the actual user." Lottery Analysis (Python Crash Course, exercise 9-15). If that is not the case, This means just some simple on page calculations will not harm anything. Is there ever a situation where using eval() is justified (except perl)? Longpoke. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The hyperscript is generated as a string first and before returning it, eval() it to turn it into executable code. Needing to use eval usually indicates a problem in your design. This never happened in history of browsing, can you show us an example? rev2023.7.5.43524. Writing code for imitating a browser is not difficult. You can use it, but it makes it harder to find problems and harder on the people who may need to make changes later. I saw people advocate to not use eval, because is evil, but I saw the same people use Function and setTimeout dynamically, so they use eval under the hoods :D, BTW, if your sandbox is not sure enough (for example, if you're working on a site that allow code injection) eval is the last of your problems. 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). VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV. I believe it's because it can execute any JavaScript function from a string. Client-side security is straight nonsense. From a pragmatic standpoint, there's no benefit to using an eval() in a situation where things can be done otherwise. whenever user-supplied data is used to generate the script. Use it for things that have no other way. At the end you will be running malicious code. It's also a dull, rusty substitute for hygienic macros. While there may be numerous instances where you can accomplish what you need to accomplish by concatenating a script together and running it on the fly, you typically have much more powerful and maintainable techniques at your disposal: associative-array notation (obj["prop"] is the same as obj.prop), closures, object-oriented techniques, functional techniques - use them instead. Developers use AI tools, they just dont trust them (Ep. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. When and how is javascript eval() vulnerable to injection? I wonder why. eval is rarely the right choice. You can get rid of j, for example: is equivalent. For example: if you use eval server-side and a mischievous user How can I convert a string to boolean in JavaScript? Browser consoles do not by themselves allow one user to run code in another users browser so they are irrelevant when deciding whether it is worth protecting against code injection. eval is not evil if running on the client, even if using unsanitized input crafted by the client. 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? @MikeSamuel, eval can execute code in other user's browser, I havent heard this, have you tried this? Why do most languages use the same token for `EndIf`, `EndWhile`, `EndFunction` and `EndStructure`? Any time you use eval you need to sanity-check what you're doing, because chances are you could be doing it a better, safer, cleaner way. Without that the user has to wait for the whole Application to load without any visually feedback. What are the Alternatives to eval in JavaScript? - Stack I can't imagine any good reason you'd want to do that though. @SammieFox There are other (and better) ways of doing this, most notably, The answer is dangerous advice; too many developers have a false sense of being in control. I always used "document.getElementById" to access the DOM; ironically, I only did it at the time because I didn't have a clue how objects worked in JavaScript ;-), agree. Most JSON libraries do not, in fact use eval under the hood, exactly to protect against the security risks. In truth, I agree with him too. Why isn't Summer Solstice plus and minus 90 days the hottest in Northern Hemisphere? rev2023.7.5.43524. And this is where you need to get really real. This code should be made to use the arguments array that every Javascript function has access to. Initial code and most proposed solutions doesn't return result by traditional way. 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. Behind the scenes, JavaScript still has to evaluate and execute Why should you avoid the JavaScript eval() function? Would a passenger on an airliner in an emergency be forced to evacuate? @Qix - running that test on my browser (Chrome 53) shows. Developers use AI tools, they just dont trust them (Ep. This answer completely ignores the risks of, If you have complete control over what you're passing to. Server-side JavaScript could have that problem. Maybe I use sh and perl too much, but I've never seen anyone treat eval with the disdain that goto gets. python - Why is using 'eval' a bad practice? - Stack Overflow How Did Old Testament Prophets "Earn Their Bread". 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. If you created or sanitized the code you eval, it is never evil. rev2023.7.5.43524. There are mainly 4 reasons why eval () method should be avoided: 1. Use jsfiddle and show the world a real world example where it can cause harm. Not the answer you're looking for? Passing user input to eval() is a security risk, but also each invocation of eval() creates a new instance of the JavaScript interpreter. This can I'm always trying to discourage from using eval. Only during testing, if possible. Why are lights very bright in most passenger trains, especially at night? http://www.nczonline.net/blog/2013/06/25/eval-isnt-evil-just-misunderstood/. But security-wise, why not just parse? 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. eval() makes sense. If you're constructing it entirely from built-in components, it's not very dangerous. 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. This isn't related much to eval() function but this article has pretty good information: rev2023.7.5.43524. How do I include a JavaScript file in another JavaScript file? For anything non-trivial, implement a sublanguage. This looks like it could be refactored so that eval() isn't necessary - tileURLexpr is just a template so some judicious use of replace() would do the job. Of course I didn't mention that at the time because I didn't want to influence the answers! Developers use AI tools, they just dont trust them (Ep. 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). The hyperscript is generated as a string first and before returning it, eval() it to turn it into executable code. I would go as far as to say that it doesn't really matter if you use eval() in javascript which is run in browsers.*(caveat). That doesn't mean it's not a great option. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval. But with that caveat it has a clear place in interop situations and is far from "evil". Overvoltage protection with ultra low leakage current for 3.3 V. Does a Michigan law make it a felony to purposefully use the wrong gender pronouns? In some cases, the performance impact can even affect other code outside eval. 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. I believe it's because it can execute any JavaScript function from a string. Using it makes it easier for people to inject rogue code into the appl If you generated javascript code and wanted to immediately execute it (let's say for performance benefits over direct interpretation), that would be a use case for eval. This answer does not cover the security implications well enough. How to check whether a string contains a substring in JavaScript? Is there ever a good reason to use eval()? - Stack Overflow My example should be a good reason of when not to use eval. @plodder - Where are you getting your info? In all other cases I would go great lengths to ensure user supplied data conforms to my rules before feeding it to eval(). 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). Why is using the JavaScript eval function a bad idea? OTOH: man-in-the-middle is not the typical attack scenario for the garden variety web app, whereas i.e. Poor cookies validation or poor ACL implementation on the server causes most attacks. The performance of eval() isn't an issue in a situation like this too because you only need to interpret the generated string once and then reuse the executable output many times over. Not the answer you're looking for? On the server side eval is useful when dealing with external scripts such as sql or influxdb or mongo. A recent Java vulnerability, etc. So you'd like to read the URLs and then convert them to template strings without having to do complex replacements because you have many endpoints. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Is eval in Javascript considered safe if not using variable code? @AkashKava, I don't understand your question. How can we compare expressive power between two Turing-complete languages? But you shouldnt be afraid to use it when you have a case where @akkishore, I will appreciate if you come up with a real life example that supports your over stated statements. The kind of stuff that, in LISP, would call for a macro. Can you suggest how I might evaluate a function-local dynamic variable name without eval? @Brad: Yeah, use cases are very few and far between. Does this change how I list it on my CV? To learn more, see our tips on writing great answers. Caching f will certainly improve the speed. Performance - eval() runs the interpreter/compiler. However, while some caching of compiled scripts may happen this will only be limited to scripts that are eval'd repeated with no modification. Why do most languages use the same token for `EndIf`, `EndWhile`, `EndFunction` and `EndStructure`? To pass functions through JSON, there is a way to do it without eval. So use it wisely is you are using it. Code injection is a problem when one user can enter code that is then run in another user's browser. If the code is dynamically generated at runtime, theres often a better way to Be very careful. But with the help of eval and a little helper function it gets a much better look: importable might look like (this version doesn't support importing concrete members). The real harm is beliving everything you read. JSON.Stringify without quotes on properties? How Did Old Testament Prophets "Earn Their Bread"? Can you tell me a use case where you can't do these instead of eval? Unless you let eval() a dynamic content (through cgi or input), it is as safe and solid as all other JavaScript in your page. It's generally only an issue if you're passing eval user input. It's faster than switching back and forth between your text editor and browser. Once again, you have failed to understand concept of poor server side security. When you know what the danger is, and when you're taking the appropriate precautions. I recently wrote a library called Hyperbars which bridges the gap between virtual-dom and handlebars. But also be aware that JavaScript uses JIT compilation and this works very poorly with eval. As for performance, you'll have to weight that against ease of coding. Eval is used when you need to 'generate' and execute code. @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. Their username, their user id in the database, their e-mail address, etc. Using it makes it easier for people to inject rogue code into the application. Eval is evil - Why we should not use eval in JavaScript So, when is it OK to use something dangerous? Connect and share knowledge within a single location that is structured and easy to search. Thanks for contributing an answer to Stack Overflow! All such unnecessary uses of eval add to a maintenance hell. 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. First of all, the browser invokes the entire script in a sandbox. Mainly, it's a lot harder to maintain and debug. It's like a goto . You can use it, but it makes it harder to find problems and harder on the peop Code injection is a problem when one user can enter code that is then run in another user's browser. (But then, someone who /really/ knew what they were doing would say: which is more reliable than the dodgy old trick of accessing DOM elements straight out of the document object.). 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. What is the best way to visualise such data? At the time I commented that wasn't the case. JavaScript eval() Method - W3Schools jQuery uses it in parseJSON (via Function constructor). How can we compare expressive power between two Turing-complete languages? @JeffWalden, great comment. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. I have found eval() in this particular situation the exact opposite of evil. How can I specify different theory levels for different atoms in Gaussian? Really what I'm saying is that. This eliminates 99% of cases where eval is used, across the board in all languages and contexts. What are the pros and cons of allowing keywords to be abbreviated? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Never_use_eval! But 99.9% of the uses of eval that I stumble across are not needed (not including setTimeout stuff). Why a kite flying at 1000 feet in "figure-of-eight loops" serves to "multiply the pulling effect of the airflow" on the ship to which it is attached? 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. When doing alot of testing and tweaking on the code the minor extra steps can really add up. In case of JSON, it is more or less hard to tamper with the source, because it comes from a web server you control. 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). How can we compare expressive power between two Turing-complete languages? I have written a framework, where developers don't use EVAL, but they use our framework and in turn that framework has to use EVAL to generate templates. Find centralized, trusted content and collaborate around the technologies you use most. Improper use of eval opens up your The only instance when you should be using eval() is when you need to run dynamic JS on the fly. 4 parallel LED's connected on a breadboard, Book about a boy on a colony planet who flees the male-only village he was raised in and meets a girl who arrived in a scout ship. Javascript Eval is evil but in certain situations only? 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 Even ways that appear to require it do not. Lateral loading strength of a bicycle wheel, Open Konsole terminal always in split view. 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. It does this by parsing a handlebars template and converting it to hyperscript which is subsequently used by virtual-dom.