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Client Side Security Fallacies
Never rely solely on information you receive from untrusted sources

One of the most laughable aspects of client/server* systems is client side based security access restrictions. What I mean by this is when credentials and actions are not checked and restricted on the server side of the equation, only on the client side, which can ALWAYS be bypassed.

To briefly explain why it is basically insane to trust a client computer; ANY multimedia, software, data, etc that has touched a person’s computer is essentially now their property. Once something has been on or through a person’s computer, the user can make copies, modify it, and do whatever the heck they want with it. This is how the digital world works. There are ways to help stop copying and modification, like hashes and encryption, but most of the ways in which things are implemented nowadays are quite fallible. There may be, for example, safeguards in place to only allow a user to use a piece of software on one certain computer or for a certain amount of time (DRM [Digital Rights Management]), but these methods are ALWAYS bypassable. The only true security comes by not letting information which people aren’t supposed to have access to cross through their computer, and keeping track of all verifiable factual information on secure servers. A long time ago at an IGDA [International Game Developers Association] meeting (I only ever went to the one unfortunately :-\), I learned an interesting truth that hadn’t occurred to me before from the lecturer. That is, that companies that make games and other software [usually] know it will sooner or later be pirated/cracked**. The true intention of software DRM is to make it hard enough to crack to discourage the crackers into giving up, and to make it take long enough so that hopefully people stop waiting for a free copy and go ahead and buy it. By the time a piece of software is cracked (if it takes as long as they hope), the companies know the majority of the remainder of the people usually wouldn’t have bought it anyways. Now I’m done with the basic explanation of client side insecurities, back to the real reason for this post.

While it is actually proper to program safeguards into client side software, you can never rely on it for true security. Security measures should always be duplicated in both client and server software. There are two reasons off the top of my head for implementing security access restrictions into the client side of software. The first is to help remove strain on servers. There is no point in asking a server if something is valid when the client can immediately confirm that it isn’t. The second reason is for speed. It’s MUCH quicker if a client can detect a problem and instantly inform the user than having to wait for a server to answer, though this time is usually imperceptible to the user, it can really add up.

So I thought I’d give a couple of examples of this to help you understand more where I’m coming from. This is a very big problem in the software industry. I find exploitable instances of this kind of thing on a very regular basis. However, I generally don’t take advantage of such holes, and try to inform the companies/programmers if they’ll listen. The term for this is white hat hacking, as opposed to black hat.

First, a very basic example. Let’s say you have a folder on your website “/PersonalPictures” that you wanted to restrict access to with a password. The proper way to do it would be to restrict access to the whole folder and all files in it on the server side, requiring a password be sent to the server to view the contents of each file. This is normally done through Apache httpd (the most utilized web server software) with an “.htaccess” file and the mod_auth (authentication) module. The improper way to do it would be a page that forwarded to the “hidden” section with a JavaScript script like the following.

if(prompt('Please enter the password')=='SecretPassword')

The problem with this code is two fold (besides the fact it pops up a request window :-) ). First, the password is exposed in plain text to the user. Fortunately, passwords are usually not as easy to find as this, but I have found passwords in web pages and Flash code before with some digging (yes, Flash files (and Java!) are 100% decompilable to their original source code, sans comments). The second problem is that once the person goes to the URL “/PersonalPictures”, they can get back there and to all files inside it without the password, and also give it freely to others (no need to mention the fact that the URL is written in plain text here, as it’s the same as with the password). This specific problem with JavaScript was much more prevalent in the old day when people ran their web pages through free hosting sites like Geocities (now owned and operated by Yahoo) which didn’t allow for proper password protection.

This kind of problem is still around on the web, though it morphed with the times into a new form. Many server side scripts I have found across the Internet assume their client side web pages can take care of security and ignore the necessary checks in the server scripts. For example, very recently I was on a website that only allowed me to add a few items to a list. The way it was done is that there was a form with a textbox that you submitted every time you wanted to add an entry to the list. After submitting, the page was reloaded with the updated list. After you added the maximum allowed number of items to the list, when the page refreshed, the form to add more was gone. This is incredibly easy to bypass however. The normal way to do this would be to just send the modified packets directly to the server with whatever information you want in it. The easier method would be to make your own form submission page and just submit to the proper URL all you want. The Firebug extension for Firefox however makes this kind of thing INCREDIBLY easy. All that needs to be done is to add an attribute to the form to send the requests to a new window “<form action=... method=... target=_blank>”, so the form is never erased/overwritten and you can keep sending requests all you want. Using Firebug, you can also edit the values of hidden input boxes for this kind of thing.

AJAX (Asynchronous JavaScript and XML - A tool used in web programming to send and receive data from a server without having to refresh a page) has often been lampooned as insecure for this kind of reason. In reality, the medium itself is not insecure at all; it’s just how people use it.

As a matter of fact, the majority of my best and most fun Ragnarok hacking was done with these methods. I just monitored the packets that came in and out of the system, reverse engineered how they were all structured, then made modifications and resent them myself to see what I could do. With this, I was able to do things like (These should be most of the exploits; listed in descending order of usefulness & severity):

  • Duplicate items
  • Crash the server (It was never fixed AFAIK, but I stopped playing 5+ years ago. I just put that it was fixed on my site so people wouldn’t look for it ^_^; )
  • Warp to any map from any warp location (warp locations are only supposed to link to 1 other map)
  • Spoof your name during chats (so you could pretend someone else was saying something - Ender’s game, anyone? ^_^)
  • Use certain skills of other classes (I have up pictures of my swordsman using merchant skills to house a selling shop)
  • Add skills points to an item on your skill tree that is not yet available (and use it immediately)
  • Warp back to save point without dying
  • Talk to NPCs on a map from any location on that map, and sometimes from other maps (great for selling items when in a dungeon)
  • Attack with weapons much quicker than was supposed to be allowed
  • Use certain skills on creatures from any location on a map no matter how far they are
  • Equip any item in any spot (so you could equip body armor on your head slot and get much more free armor defense points)
  • Run commands on your party/guild and in chat rooms as if you were the leader/admin
  • Rollback a characters stat’s to when you logged on that session (part of the dupe hack)
  • Bypass text repetition, length, and curse filters
  • Find out user account names

The original list is here; it should contain most of what I found. I took it down very soon after putting it up (replacement here) because I didn’t want to explicitly screw the game over with people finding out about these hacks (I had a lot of bad encounters with the company that ran the game, they refused to acknowledge or fix existing bugs when I reported them). There were so many things the server didn’t check just because the client wasn’t allowed to do them naturally.

Here are some very old news stories I saved up for when I wrote about this subject:

Just because you don’t give someone a way to do something doesn’t mean they won’t find a way.

*A server is a computer you connect to and a client is the connecting computer. So all you people connecting to this website are clients connecting to my web server.
**“Cracked” usually means to make a piece of software usable when it is not supposed to be, bypassing the DRM

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