[erlang-questions] Two beautiful programs - or web programming made easy

Edmond Begumisa <>
Mon Feb 14 21:38:58 CET 2011


You've outlined a nice list of the top security concerns of *every*  
web-developer generating dynamic content from the client-side (probalby a  
good chunk websites uploaded since 2005.)

What I still don't get is why you find the client-side generation of  
dynamic content from static js files acceptable while using eval for the  
same you find completely unacceptable. I don't see how XSS, CSRF, SQL  
Injection, and all the things you list are more unmanageable from static  
js that generates content vs streamed js that generates content.

- Edmond -

On Mon, 14 Feb 2011 23:43:57 +1100, Frédéric Trottier-Hébert  
<> wrote:

> On 2011-02-14, at 03:35 AM, Joe Armstrong wrote:
>>
>> Ok so "separation of concerns" is good but having different notations  
>> for expressing the concerns
>> is crazy- to make a web thing that interacts with a server you need to  
>> learn something like
>>
>>         HTML
>>         Javascript
>>         CSS
>>         PHP
>>         MySQL
>>
>> And to be able to configure Apache and MySQL - other combinations are  
>> possible.
>
> I can agree with that. To have a functional website, you do need to know  
> a lot of different technologies. The web evolved organically and each  
> part of the problem space had its own solution developed over time.
>
>>
>> Then you have to split the flow of control to many places.
>>
>> All of this is crazy madness. There should be *one* notation that is  
>> powerful enough to express all
>> these things. In the browser is seems sensible to forget about css and  
>> html only use Javascript
>> The only communication with the browser should be by sending it  
>> javascript.
>
> There should, but there isn't. The truth here is that most programmers  
> are awful at design. In any somewhat large setup, your backend  
> programmers, your designers and your integrators (the guys just handling  
> HTML, and CSS, maybe some Javascript) are not necessarily the same  
> person.
>
> Right now the ring of web technologies is divided in a way that makes it  
> somewhat simple to have different people from different background and  
> knowledges to work on different part of your software. It makes sense to  
> have the designer or integrator to be able to change the look and feel  
> of a website without having to play in your code and maybe mess up  
> database queries. Modern template engines in fact try to forbid all  
> kinds of seriously side-effecting code (like DB queries) from happening  
> in the templates.
>
> There should be no worry for your guy working in Javascript that he'd  
> not need to suddenly learn Erlang to be able to debug your application.
> Then again, this separation of concerns allows specialists to work on  
> their speciality with more ease. It makes things somewhat simpler in  
> larger organisations, but quite painful for one-man operations. I'll  
> tell you that it makes a lot of sense when you know all of the tools in  
> the toolkit though :)
>
>> How you generate the javascript is irrelevant - by hand or by program -  
>> who cares. If you make it by
>> program the chances are that it's right.
>>
> Yes and no. Generated javascript is nearly as old as the language --  
> many, many .NET apps had that kind of things. Some editors like  
> Dreamweaver could generate JS for you. One of the problem with this is  
> that it was often pure garbage, or it wouldn't work in all browsers  
> uniformly. If you can manage to generate and capture complex behaviours  
> in a compliant manner, all the better. I have myself lost much hope with  
> regards to that though.
>
>> Security is orthogonal to this - send encrypted js over the wire and  
>> make sure your key-rings are secure
>> this is a completely different problem.
>
> This is only transmission security. Encryption has nothing to do with  
> Cross-Site Scripting (XSS, where some user is able to run arbitrary JS  
> in your page for you and ends up stealing information), Cross Site  
> Request Forgery (CSRF, where the attacker uses the fact your application  
> is forgetting about things like the origin of the queries to hijack the  
> client's session in their place. This is related to Same Origin Policy  
> issues and not easy to handle), SQL injection, overwriting some  
> parameters because you don't fetch them in the right order server-side  
> (see problems with the $_REQUEST variable in PHP), etc.
>
> 1. XSS
> XSS is, as mentioned above, the ability to run abritrary JS on a page.  
> This is the risky thing with your eval.  
> http://en.wikipedia.org/wiki/Cross-site_scripting contains many details  
> on understanding the related issues. It's not always a simple matter of  
> escaping. Some more advanced attacks even rely on string encoding to  
> make sure your escaping fails. See  
> http://www.governmentsecurity.org/forum/index.php?showtopic=18105.
>
> 2. CSRF
> CSRF is a tricky thing. Because HTTP doesn't support sessions, over the  
> years, the guys from Netscape (back then) or Opera (or whoever) ended up  
> using Cookies to share data on every query. What happens there is that  
> on every query the browser sends to a server, it also packages the  
> cookies neatly in the headers -- no matter what page you were on when  
> they were sent. The issue here is that the server might not check from  
> what page the call is coming from.
>
> Basically, if twitter had an URL call such as  
> http://twitter.com/tweet/add?message=SomeMessageHere that would  
> automatically add a tweet from your account and I put that link in an  
> image tag on some site, every time you would load that image, you would  
> automatically make a call to the server, your browser sending in your  
> cookies and making it look like YOU actually made that call, even if you  
> didn't know. In this case, the request is especially easy to do because  
> twitter would be using GET parameters to have side-effects on the  
> server. By forcing people into using POST, you can make things harder,  
> but not impossible.
>
> One way to work again POST is using a fake website -- let's say I use  
> learnyousomeerlang.com. On my own site, I'll be putting a fake  
> javascript form inside an iframe (so that the page doesn't refresh when  
> submitted) and have the script automatically send in the POST form. Now  
> I send the link to my trick page over twitter and everyone who clicks on  
> it from there will be guaranteed to have their session open and sending  
> in data. I've in fact used this trick to have the site owner at my old  
> job to close his own admin account on his own website so he could  
> realise the importance of the threat.
>
> How can you solve this one? Well there are a few ways -- for one you  
> could check the HTTP referrer, but that won't work everywhere -- if you  
> expect calls from flash, it doesn't always send these elements of the  
> HTTP header. In the case of HTTPS, depending on how you handle things,  
> the header might not always be sent either so you can't know for sure.  
> Better than that, if I'm using the <img> trick on your own website (on  
> twitter, for twitter users), the domain will be the same, without you  
> being able to check for anything.
>
> The only foolproof way to do this is to use what they call 'tokens':  
> each call you make to the server has to have a unique piece of data that  
> the server knows about that can prove that the call you just made comes  
> from you, but also from your own forms on your own websites. These  
> tokens should have an expiration time and be hidden from plain view,  
> submitted automatically with any form. If you don't have this, your  
> application might not be safe.
>
> This has *nothing* to do with encryption, and everything to do with not  
> understanding the potential threats of the web correctly. It is an  
> application-level issue, much like XSS is. And it's pretty damn  
> important.
>
> 3. SQL injection is a different beast, where you do not properly escape  
> the parameters of a request going to the database, letting your run  
> arbitrary DB calls. Erlang with Mnesia doesn't have to worry about that,  
> but Erlang with any SQL has to, even if you end up using QLC (it depends  
> on the library at the back in this case though, and is generally safe  
> enough). http://en.wikipedia.org/wiki/Sql_injection has sufficient  
> details.
>
> 4.You have to consider that sometimes these attacks are combined  
> together to be able to really do damage.
>
> I haven't even covered using weak hashing for passwords, bad security  
> policies on cookies, opening files on dynamic paths without filtering  
> the input, etc.
>
> Web application security is not a joke and it's certainly not easy. It's  
> a very serious thing and most developers get it wrong at one point or  
> another. Wordpress got it wrong, Twitter got it wrong, facebook got it  
> wrong, Google got it wrong, and so on, even though they're supposed to  
> be leaders in the field. Most of them got it wrong more than once too.  
> This is why I kind of support a 'paranoid' line of thought.
>
> --
> Fred Hébert
> http://www.erlang-solutions.com
>


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