why we should not use eval in javascript

The basic rule of security is that all input is evil, but in case of JavaScript even JavaScript itself could be evil, because in JavaScript you can overwrite any function and you just can't be sure you're using the real one, so, if a malicious code start before you, you can't trust any JavaScript built-in function :D, If you REALLY need it (80% of the time eval is NOT needed) and you're sure of what you' re doing, just use eval (or better Function ;) ), closures and OOP cover the 80/90% of the case where eval can be replaced using another kind of logic, the rest is dynamically generated code (for example, if you're writing an interpreter) and as you already said evaluating JSON (here you can use the Crockford safe evaluation ;) ). 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? Generating X ids on Y offline machines in a short time period without collision. 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. Not the answer you're looking for? As we can see, the string passed to eval for execution is dynamic. Otherwise XSS wouldn't be a security vulnerability. rev2023.7.5.43524. But 99.9% of the uses of eval that I stumble across are not needed (not including setTimeout stuff). Let's just say that SOME eval'd code executes more slowly. (no line numbers, etc. My example should be a good reason of when not to use eval. Performance of EVAL can be increased by using the following method; instead of executing the script, you must return a function. What's the main benefit of using eval() in JavaScript? That's assuming that this formula is arbitrary. Even if it's a trusted source, I don't use it, because the code returned by JSON might be garbled, which could at best do something wonky, at worst, expose something bad. 11 Answers Sorted by: 18 function StrippedExample (i1, i2, i3, i4, i5, i6, i7, i8) { var args = [i1, i2, i3, i4, i5, i6, i7, i8]; // put values in an array this.i = []; for (var i=0,j=0 Mainly, it's a lot harder to maintain and debug. What's the main benefit of using eval() in JavaScript? Connect and share knowledge within a single location that is structured and easy to search. Why should you avoid JavaScripts eval function when possible? True, I didn't know about how eval() worked in perl. Non-anarchists often say the existence of prisons deters violent crime. Connect and share knowledge within a single location that is structured and easy to search. So to do this I figured I'm gonna basically need a formula. Which appears to work fine, but what I'm concerned about is the safety of this code. How can I remove a specific item from an array in JavaScript? I'm not au fait with all the ins and outs of Javascript. For browsers that dont support JSON.parse() natively, you can Why are lights very bright in most passenger trains, especially at night? eval is rarely the right choice. A question about eval() in javascript why is it evil and how can I accomplish the same thing without using it? Only if your server-side security sucks. This code should be made to use the arguments array that every Javascript function has access to. Not too bad if your users are only on your page shortly, but it can be a problem for webapp's. eval isn't necessary for setTimeout. That doesn't mean it's not a great option. The evaled code is executed in the current lexical environment, so it can see outer variables: let a = 1; function f() { let a This might work with multimethods, for example, once the the particular arguments in use are determined. Lottery Analysis (Python Crash Course, exercise 9-15). But the question isn't about the uses of eval() but lets just say some how the string you running with eval() is affected by a malicious party. http://blogs.popart.com/2009/07/javascript-injection-attacks/ Eval is only good for generating some templates to do complex string processing based on something that is not used in advance. So when to not use eval()? 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. 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? Plot multiple lines along with converging dotted line. use a library from JSON.org. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Is there a non-combative term for the word "enemy"? Lottery Analysis (Python Crash Course, exercise 9-15), For a manual evaluation of a definite integral. Offhand thought: eval is good for implementing a poor man's expression compiler, or things like that. 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? In your specific context, eval() might be 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. function takes an arbitrary string and executes it as JavaScript code. The other objection is that it's firing up a full JavaScript parser (and so in theory costly), but frankly unless you're doing this hundreds of thousands of times in a tight loop, it's not going to matter. In the final act, how to drop clues without causing players to feel "cheated" they didn't find them sooner? You can also use Function() {}, but be careful when using these on the server unless you want users to take over your server hahahah. @cHao For example, if you have a large Game-Application (5-10MB Javascript), its better to build first a simple fast-loading AJAX-Preloader (1kb), which loads the large Main-Script, while displaying a Loading-Bar or something similar. 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? Not without reason, it is frowned upon by masters like Douglas Crockford. Program where I earned my Master's is changing its name in 2023-2024. No javascript engine cannot find and eval in the code with 100% guarantee. access dynamic properties is better and simpler: Using eval() also has security implications, because you might be executing code (for To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Caching f will certainly improve the speed. Which indirectly brings me to that second reason -- accessing the names of objects.. We aren't talking about a specific application, but reasons not to use. Done and no way to stop me because when eval executes, the browser gives it highest authority. I want to key log everything you type on that site and send it to me? Connect and share knowledge within a single location that is structured and easy to search. When the code in It does this by parsing a handlebars template and converting it to hyperscript which is subsequently used by virtual-dom. Thanks for catching that, Brian. Asking for help, clarification, or responding to other answers. Is there an easier way to generate a multiplication table? What should be chosen as country of visit if I take travel insurance for Asian Countries. What does "use strict" do in JavaScript, and what is the reasoning behind it? If you're constructing it entirely from built-in components, it's not very dangerous. One reasonable use is if you have an interpreted language that you've built on top of another language, but you still want to provide some sort of "escape hatch" to allow people to get back to functions that are provided by the underlying language. Is the executive branch obligated to enforce the Supreme Court's decision on affirmative action? question is known beforehand (not determined at runtime), theres no reason to use I have a little bit of code that looks just like this: FireBug profiler claims that second longest function is eval(), taking up to nearly 6% of the run time. Isn't already opened? You are simply making an array from your function 8 arguments, removing the ones that are less than or equal to zero. Provided the thing you're executing eval() on has come from a reasonable source, you're fine. But you shouldnt be afraid to use it when you have a case where A json string should always be tested against the json grammar before using it in eval(). Regarding security, using eval or not will hardly make any difference. Eval is somewhat like a blackbox to the compiler, and JavaScript needs to be able to predict code ahead of time (to some extent) in order to safely and correctly apply performance optimisations and scoping. - You are way off the mark. In other words, in the pessimistic sense, most of those optimizations it would make are pointless if eval(..) is present, so it simply doesn't perform the optimizations at all. Javascript eval() function not working properly - Stack Overflow code for injection attacks Debugging can be more challenging Why isn't Summer Solstice plus and minus 90 days the hottest in Northern Hemisphere? To learn more, see our tips on writing great answers. Does the DM need to declare a Natural 20? Injected code will run (in the browser) as if it came from your site, letting it do any sort of shenanigan that the user could do manually. If you want the user to input some logical functions and evaluate for AND the OR then the JavaScript eval function is perfect. Are you suggesting that code-injection in the browser is not a problem? Is there an easier way to generate a multiplication table? (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.). code for injection attacks, Debugging can be more challenging In case of JSON, it is more or less hard to tamper with the source, because it comes from a web server you control. JavaScript eval() function - Scaler Topics +1 For suggesting an alternative to eval(). Isn't using eval slower than just parsing the JSON? I understand that eval ing Client-side security is straight nonsense. However, there are specific cases where an eval SHOULD be used. If the code is dynamically generated at runtime, theres often a better way to By templating I mean that you write a simplified template generator that generates useful template code which increases development speed. I'm always trying to discourage from using eval. How to maximize the monthly 1:1 meeting with my boss? That opened my eyes a lot! 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. Really what I'm saying is that. Why do most languages use the same token for `EndIf`, `EndWhile`, `EndFunction` and `EndStructure`? @Sean - Both JQuery and Prototype use eval (JQuery uses it via new Function). For example an achievement service with following metadata, Direct injection of object/values thru literal string in a json, useful for templating texts, Can be use as a comparator, say we make rules how to validate quest or events in CMS. With power comes great responsibility. Even Crockford's explanation for 'eval is evil' is weak. python - Why is using 'eval' a bad practice? - Stack Overflow You can see how the code generation was achieved if you're curious here. It's also safe in this scenario because you have complete control over what gets passed in. eval() at all. 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. The browsers garbage collection has no idea if the code that's eval'ed can be removed from memory so it just keeps it stored until the page is reloaded. See here for some more info: eval. eval() = evil. Developers use AI tools, they just dont trust them (Ep. Is javascript eval really that big of a security threat? In some languages such as java, the ability to introspect reduces or eliminates the need to use java's eval. Is there any function which can transform string into expression? Just updating the above code.. --hi there!-- needs to be in quotes as it is a string. At that point, it becomes unsafe because it's a full JavaScript parser but people sometimes try to use it as just an expression evaluator (for instance, when parsing JSON from a source they don't control). A question about eval() in javascript why is it evil and how can I accomplish the same thing without using it? 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. When doing alot of testing and tweaking on the code the minor extra steps can really add up. My experience is with PHP & Python, and the general consensus seems to be that if you want to use eval, don't. example of javascript's eval not being evil? However, if your input isnt from the user, is there any real danger?" Making statements based on opinion; back them up with references or personal experience. All you have to do is make a HTTP request to the server with your favourite user agent string. If you created or sanitized the code you eval, it is never evil. first of all EVAL is not to be blamed for poorly written server side code. Rust smart contracts? The attacks we are trying to avoid are when user provided values get saved, then later placed into javascript and eval'd. How can we compare expressive power between two Turing-complete languages? If you are looking for the basics of eval() look here: Longpoke. We have browser consoles, script extensions, etc Every single piece of code sent to the client is optional for the client to execute. JS for all it's flaws is really flexible, and allows a lot of room to make it flexible. As it is, you are just wrong; #3 holds true for many scenarios. Why do most languages use the same token for `EndIf`, `EndWhile`, `EndFunction` and `EndStructure`? eval is rarely the right choice. @Prestaul: Since the supposed attacker can just use whatever developer tool to change the JavaScript in the client, why do you say Eval() opens up your code to injection attacks? Eval adds to maintenance hell. How do I remove a property from a JavaScript object? I am building a little game and I've gotten to the point where I need to calculate data in the tips of abilities which is unique to each individual unit. In production code, consider it a last resortand even then, try something elsebecause eval is difficult to control and thus dangerous. Untrusted client-side JavaScript code can't do that much anyway. EDIT: by the way, I wouldn't suggest (for all the security reasons pointed out heretofore) that you base you object names on user input. Any recommendation? 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. This assumes that there is a single user. 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 Find centralized, trusted content and collaborate around the technologies you use most. It is quite useful, though, and used with moderation can add a lot of good functionality. eval is evil if running on the server using input submitted by a client that was not created by the developer or that was not sanitized by the developer. But with that caveat it has a clear place in interop situations and is far from "evil". If you absolutely must The JavaScript Engine has a number of performance optimizations that it performs during the compilation phase. BTW: Prototype.js calls eval directly five times (including in evalJSON() and evalResponse()). Its also important to remember that passing strings to setInterval(), setTimeout(), 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. @AkashKava, A string can originate with one user-agent, be stored in a database, and then served to another browser which. At the end you will be running malicious code. JavaScript has lots of metaprogramming power using stuff like changing prototypes, obj[member], Proxy, json.parse, window, decorator functions(adverbs) where newf = decorator(oldf), higher order function like Array.prototype.map(f), passing arguments to other functions, keyword arguments via {}. Unless you are 100% sure that the code being evaluated is from a trusted source (usually your own application) then it's a surefire way of exposing your system to a cross-site scripting attack. javascript - Why is it important to never use `eval` in conjunction @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. Most browsers support it, and if you're really in a pinch, you could write a parser for JSON pretty easily. jQuery has utilized the native JSON.parse() since 1.4 (way back in 1/2010)! At the time I commented that wasn't the case. When another user views that comment (that I put javascript in), eval will take that javascript when it is rendered, and evaluate it using the interpreter, causing my embedded javascript to execute on the other user's browser. 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(). Good to know that. When and how is javascript eval() vulnerable to injection? Code injection is a problem when one user can enter code that is then run in another user's browser. However, experienced programmers often have a better architecture in their code, and eval() will rarely be required or even thought about due to this better architecture. If your server-side security is solid enough for anyone to attack from anywhere, you should not worry about EVAL. Avoid it, As Crockford himself might say "This kind of statement tends to generate irrational neurosis. This is the best use I've seen of eval() so far. Connect and share knowledge within a single location that is structured and easy to search. I was looking for this answer and came to this post. WebWhy Should We Not Use the eval() in JavaScript ? So you may do something like this. How can we compare expressive power between two Turing-complete languages? Asking for help, clarification, or responding to other answers. The client can run any arbitrary code they want to, even if the developer did not code it; This is true not only for what is evaled, but the call to eval itself. 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. See for yourself: @Tomas the irony there being that json2.js uses eval to parse JSON, You're not addressing the issue of code that uses eval being difficult to debug. definitely try this as a drop in replacement. Ability of one user to inject code that can run in another's browser is a major issue. globals automatically. Using the console to edit many lines of code? Don't buy it.". For (a stupid) example, if you're writing a Brainfuck compiler, you'll probably want to construct a function that performs the sequence of instructions as a string, and eval it to return a function. Thanks @Benjie. Unanticipated effects of those evals are legion. I'd like to take a moment to address the premise of your question - that eval() is "evil". What are the security issues with "eval ()" in JavaScript? Have the user enter that separately and then you can use if/then logic to ultimately use the correct operator. you're right, maybe it is not quite as language-agnostic as I thought. One thing to keep in mind is that you can often use eval() to execute code in an otherwise restricted environment - social networking sites that bl 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). Improper use of eval opens up your As far as client script goes, I think the issue of security is a moot point. For example: if For example: if you use eval server-side and a mischievous user Lottery Analysis (Python Crash Course, exercise 9-15). Javascript: How to use eval() safely [duplicate]. The security issues are the most well known. 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. With eval (), malicious code can run inside your application without permission. "Every major browser now has a built in console why would they bother to use an eval statement?" @plodder - Where are you getting your info? 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. 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. Besides the possible security issues if you are executing user-submitted code, most of the time there's a better way that doesn't involve re-parsing the code every time it's executed. This is a common antipattern when dealing with a JSON response from an Ajax request. 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. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. It's not always a bad idea. Doesn't seem practical. Obviously you should always sanitize the input, as to have some control over what your code consumes. 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. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. I have found eval() in this particular situation the exact opposite of evil. Is there an eval() alternative for this expression? Is the difference between additive groups and multiplicative groups just a matter of notation? 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. You can access the virtual file created by Chrome for your evaled coded by adding the, Hmm, guess I got lucky when I was first learning JavaScript. JavaScript eval() Method - W3Schools Lastly, related to executing code, one might use eval to increase performance -- instead of a multi level conditional or property access that determines which code to run or which object to use, one might create a minimal code snippet that might have to be executed hundreds of thousands of times, eval it to a function, and then just call that function. All testing tools mock browsers anyway; if an attacker want to harm you, EVAL is their last resort. For anything non-trivial, implement a sublanguage. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Heres some of the reasons to avoid using it: Malicious code: invoking eval can crash a computer. What conjunctive function does "ruat caelum" have in "Fiat justitia, ruat caelum"? Question of Venn Diagrams and Subsets on a Book. @MikeSamuel, eval can execute code in other user's browser, I havent heard this, have you tried this? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Never_use_eval, developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/. 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. I read the developer doc and this worked as a direct replacement in my application. That is: dynamical (or more accurately) programmically-created object names (as opposed to values). Do large language models know what they are talking about? So my answer is: 'eval is suitable when you are writing perl 5 and sh'. 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. 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. WebDec 2, 2009 at 14:59 4 I believe it was from an article comparing python and lisp than I learned about eval. One way to validate your script is keep the hash of your scripts somewhere safe, so you can check them before running. Performance - eval() runs the interpreter/compiler. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval. Why schnorr signatures uses H(R||m) instead of H(m)? This is one of good articles talking about eval and how it is not an evil: Understanding eval and knowing when it might be useful is way more important. This may become more of an issue as the next generation of browsers come out with some flavor of a JavaScript compiler. Plus you might forget to save creating confusion when testing. Note this is a very simple example. nczonline.net/blog/2013/06/25/eval-isnt-evil-just-misunderstood. As for performance, you'll have to weight that against ease of coding. As my display name, which can come from a database and which is not hardcoded. I wonder why. 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, that came from Ajax call. This answer completely ignores the risks of, If you have complete control over what you're passing to. 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 real harm is beliving everything you read. eval is very dangerous if any of the expression is supplied by the user. Why is it better to control a vertical/horizontal than diagonal? Eval is complementary to compilation which is used in templating the code. In all other cases I would go great lengths to ensure user supplied data conforms to my rules before feeding it to eval().
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